diff --git a/lib/localiser/localisation/calibration.ex b/lib/localiser/localisation/calibration.ex
new file mode 100644
index 0000000..0174bd7
--- /dev/null
+++ b/lib/localiser/localisation/calibration.ex
@@ -0,0 +1,86 @@
+defmodule Localiser.Localisation.Calibration do
+  @moduledoc false
+
+  # Returns [{rssi, is_outlier}] using Tukey IQR fences on the full sample list.
+  def classify_outliers([]), do: []
+
+  def classify_outliers(samples) do
+    {lo, hi} = iqr_fences(samples)
+    Enum.map(samples, fn rssi -> {rssi, rssi < lo or rssi > hi} end)
+  end
+
+  # Live hint: is this single reading an outlier relative to the samples collected so far?
+  # Returns false when fewer than 5 existing samples (not enough signal).
+  def outlier?(_reading, existing) when length(existing) < 5, do: false
+
+  def outlier?(reading, existing) do
+    {lo, hi} = iqr_fences(existing)
+    reading < lo or reading > hi
+  end
+
+  # OLS regression for RSSI = A - 10n * log10(d).
+  # stages :: [%{distance: float, mean_rssi: float}]
+  # Requires at least 2 stages with distinct distances.
+  # Returns {:ok, {rssi_ref :: integer, path_loss_exp :: float}} or {:error, :insufficient_data}.
+  def least_squares(stages) when length(stages) < 2, do: {:error, :insufficient_data}
+
+  def least_squares(stages) do
+    points = Enum.map(stages, fn %{distance: d, mean_rssi: rssi} ->
+      {:math.log10(d), rssi}
+    end)
+
+    xs = Enum.map(points, &elem(&1, 0))
+    ys = Enum.map(points, &elem(&1, 1))
+
+    x_bar = mean(xs)
+    y_bar = mean(ys)
+
+    cov_xy = xs |> Enum.zip(ys) |> Enum.reduce(0.0, fn {x, y}, acc ->
+      acc + (x - x_bar) * (y - y_bar)
+    end)
+
+    var_x = Enum.reduce(xs, 0.0, fn x, acc -> acc + (x - x_bar) * (x - x_bar) end)
+
+    if var_x == 0.0 do
+      {:error, :insufficient_data}
+    else
+      beta = cov_xy / var_x
+      a = y_bar - beta * x_bar
+
+      path_loss_exp = -beta / 10.0
+      rssi_ref = round(a)
+
+      {:ok, {rssi_ref, path_loss_exp}}
+    end
+  end
+
+  # --- Private ---
+
+  defp iqr_fences(samples) do
+    sorted = Enum.sort(samples)
+    n = length(sorted)
+
+    q1 = percentile(sorted, n, 0.25)
+    q3 = percentile(sorted, n, 0.75)
+    iqr = q3 - q1
+
+    {q1 - 1.5 * iqr, q3 + 1.5 * iqr}
+  end
+
+  defp percentile(sorted, n, p) do
+    idx = p * (n - 1)
+    lo = floor(idx)
+    hi = ceil(idx)
+
+    if lo == hi do
+      Enum.at(sorted, lo) * 1.0
+    else
+      frac = idx - lo
+      Enum.at(sorted, lo) * (1 - frac) + Enum.at(sorted, hi) * frac
+    end
+  end
+
+  defp mean(list) do
+    Enum.sum(list) / length(list)
+  end
+end
diff --git a/lib/localiser/localisation/filter/particle.ex b/lib/localiser/localisation/filter/particle.ex
index be97509..7b63282 100644
--- a/lib/localiser/localisation/filter/particle.ex
+++ b/lib/localiser/localisation/filter/particle.ex
@@ -8,7 +8,7 @@ defmodule Localiser.Localisation.Filter.Particle do
   @default_velocity_noise     0.3    # σ m/s per update (constant velocity model)
   @default_max_speed          2.0    # m per update cap (constant velocity model)
   @default_likelihood_model   :laplacian
-  @default_sensor_sigma       3.0    # σ (Gaussian) or b (Laplacian) in metres
+  @default_sensor_sigma       1.0    # σ (Gaussian) or b (Laplacian) in metres
   @default_weight_floor       1.0e-6
   @default_injection_fraction 0.03
   @default_resample_threshold 0.5
diff --git a/lib/localiser/localisation/sensor/server.ex b/lib/localiser/localisation/sensor/server.ex
index 618483e..23436fe 100644
--- a/lib/localiser/localisation/sensor/server.ex
+++ b/lib/localiser/localisation/sensor/server.ex
@@ -5,12 +5,19 @@ defmodule Localiser.Localisation.Sensor.Server do
 
   alias Localiser.Domain.Sensors
   alias Localiser.Domain.Schema.{Sensor, SensorCalibration}
+  alias Localiser.Localisation.Calibration
   alias Localiser.MQTT.Connection, as: MQTTConnection
 
   @default_rssi_ref -59
   @default_path_loss_exp 2.0
+  @default_samples 30
 
-  # mode: :ok | {:calibrating, buffer :: [integer()], target :: pos_integer()}
+  # mode:
+  #   :ok
+  #   {:calibration_mode, completed_stages}
+  #   {:calibrating_stage, distance, samples, completed_stages}
+  #
+  # completed_stages :: [%{distance: float, mean_rssi: float, readings: [{rssi, is_outlier}]}]
   defstruct [:sensor_id, :sensor_db_id, :floor_x, :floor_y, :rssi_ref, :path_loss_exp, mode: :ok]
 
   def start_link({sensor, room}) do
@@ -21,28 +28,42 @@ defmodule Localiser.Localisation.Sensor.Server do
     {:via, Registry, {Localiser.Registry, {:sensor, sensor_id}}}
   end
 
-  # Returns %{sensor_id, floor_x, floor_y, distance, rssi, tx_power} for a raw RSSI reading.
-  # tx_power is the beacon-advertised expected RSSI at 1 m (nil if not available).
   def measure(sensor_id, rssi, tx_power \\ nil) do
     GenServer.call(via(sensor_id), {:measure, rssi, tx_power})
   end
 
-  # Returns true if the sensor is currently collecting calibration samples.
+  # Returns true only when a stage is actively collecting (used by RSSI.Buffer).
   def calibrating?(sensor_id) do
     GenServer.call(via(sensor_id), :calibrating?)
   end
 
-  # Feeds a raw RSSI value into the calibration buffer.
+  # Returns a JSON-serialisable snapshot of the current calibration state for channel join.
+  def calibration_state(sensor_id) do
+    GenServer.call(via(sensor_id), :calibration_state)
+  end
+
+  # Feeds a raw RSSI value into the active stage buffer. Ignored between stages.
   def calibration_reading(sensor_id, rssi) do
     GenServer.cast(via(sensor_id), {:calibration_reading, rssi})
   end
 
-  # Starts calibration mode. sample_target: number of RSSI samples to collect.
-  def begin_calibration(sensor_id, sample_target \\ 50) do
-    GenServer.cast(via(sensor_id), {:begin_calibration, sample_target})
+  # Puts the sensor into calibration mode (between-stages). Returns :ok.
+  def begin_calibration_mode(sensor_id) do
+    GenServer.call(via(sensor_id), :begin_calibration_mode)
   end
 
-  # Aborts an in-progress calibration without saving.
+  # Starts collecting samples for a given distance. Returns {:ok, samples_needed} or {:error, reason}.
+  def start_stage(sensor_id, distance) do
+    GenServer.call(via(sensor_id), {:start_stage, distance})
+  end
+
+  # Runs OLS regression over completed stages and saves the result. Requires >= 2 stages.
+  # Returns {:ok, %{rssi_ref: integer, path_loss_exp: float}} or {:error, reason}.
+  def finish_calibration(sensor_id) do
+    GenServer.call(via(sensor_id), :finish_calibration)
+  end
+
+  # Aborts calibration from any state, discarding all stages.
   def abort_calibration(sensor_id) do
     GenServer.cast(via(sensor_id), :abort_calibration)
   end
@@ -84,37 +105,144 @@ defmodule Localiser.Localisation.Sensor.Server do
 
   @impl true
   def handle_call(:calibrating?, _from, state) do
-    {:reply, match?({:calibrating, _, _}, state.mode), state}
+    {:reply, match?({:calibrating_stage, _, _, _}, state.mode), state}
   end
 
   @impl true
-  def handle_cast({:begin_calibration, target}, state) do
+  def handle_call(:calibration_state, _from, state) do
+    snapshot = case state.mode do
+      :ok ->
+        %{status: "idle"}
+
+      {:calibration_mode, completed} ->
+        %{
+          status: "calibration_mode",
+          samples_needed: samples_needed(),
+          completed_stages: Enum.map(completed, &render_stage/1)
+        }
+
+      {:calibrating_stage, distance, samples, completed} ->
+        %{
+          status: "stage_active",
+          distance: distance,
+          samples_needed: samples_needed(),
+          stage_progress: {length(samples), samples_needed()},
+          completed_stages: Enum.map(completed, &render_stage/1)
+        }
+    end
+
+    {:reply, snapshot, state}
+  end
+
+  @impl true
+  def handle_call(:begin_calibration_mode, _from, state) do
     MQTTConnection.publish("localiser/sensor/#{state.sensor_id}/cmd", ~s({"action":"calibrate_start"}))
-    {:noreply, %{state | mode: {:calibrating, [], target}}}
+    broadcast_calibration(state.sensor_id, {:calibration_mode_entered, state.sensor_id, samples_needed()})
+    {:reply, :ok, %{state | mode: {:calibration_mode, []}}}
   end
 
   @impl true
-  def handle_cast(:abort_calibration, %{mode: {:calibrating, _, _}} = state) do
-    MQTTConnection.publish("localiser/sensor/#{state.sensor_id}/cmd", ~s({"action":"calibrate_stop"}))
-    {:noreply, %{state | mode: :ok}}
+  def handle_call({:start_stage, _distance}, _from, %{mode: {:calibrating_stage, _, _, _}} = state) do
+    {:reply, {:error, :already_active}, state}
   end
 
-  def handle_cast(:abort_calibration, state), do: {:noreply, state}
+  def handle_call({:start_stage, _distance}, _from, %{mode: :ok} = state) do
+    {:reply, {:error, :not_in_calibration_mode}, state}
+  end
+
+  def handle_call({:start_stage, distance}, _from, %{mode: {:calibration_mode, completed}} = state) do
+    n = samples_needed()
+    broadcast_calibration(state.sensor_id, {:stage_started, state.sensor_id, distance, length(completed)})
+    {:reply, {:ok, n}, %{state | mode: {:calibrating_stage, distance, [], completed}}}
+  end
 
   @impl true
-  def handle_cast({:calibration_reading, rssi}, %{mode: {:calibrating, buffer, target}} = state) do
-    buffer = [rssi | buffer]
+  def handle_call(:finish_calibration, _from, %{mode: {:calibration_mode, completed}} = state)
+      when length(completed) >= 2 do
+    case Calibration.least_squares(completed) do
+      {:ok, {rssi_ref, path_loss_exp}} ->
+        sensor_struct = %Sensor{id: state.sensor_db_id, sensor_id: state.sensor_id}
 
-    if length(buffer) >= target do
-      finalize_calibration(buffer, state)
+        case Sensors.add_calibration(sensor_struct, %{
+               rssi_ref: rssi_ref,
+               path_loss_exp: path_loss_exp,
+               calibrated_at: DateTime.utc_now()
+             }) do
+          {:ok, _} ->
+            Logger.info("[Sensor.Server] Calibration finished for #{state.sensor_id}: rssi_ref=#{rssi_ref} n=#{path_loss_exp}")
+            MQTTConnection.publish("localiser/sensor/#{state.sensor_id}/cmd", ~s({"action":"calibrate_stop"}))
+            broadcast_calibration(state.sensor_id, {:calibration_finished, state.sensor_id, rssi_ref, path_loss_exp})
+            Phoenix.PubSub.broadcast(Localiser.PubSub, "sensors", {:calibration_complete, state.sensor_id, rssi_ref, path_loss_exp})
+            result = %{rssi_ref: rssi_ref, path_loss_exp: path_loss_exp}
+            {:reply, {:ok, result}, %{state | rssi_ref: rssi_ref, path_loss_exp: path_loss_exp, mode: :ok}}
+
+          {:error, reason} ->
+            Logger.error("[Sensor.Server] Failed to save calibration for #{state.sensor_id}: #{inspect(reason)}")
+            {:reply, {:error, :save_failed}, state}
+        end
+
+      {:error, reason} ->
+        {:reply, {:error, reason}, state}
+    end
+  end
+
+  def handle_call(:finish_calibration, _from, %{mode: {:calibration_mode, _}} = state) do
+    {:reply, {:error, :insufficient_stages}, state}
+  end
+
+  def handle_call(:finish_calibration, _from, %{mode: {:calibrating_stage, _, _, _}} = state) do
+    {:reply, {:error, :stage_active}, state}
+  end
+
+  def handle_call(:finish_calibration, _from, state) do
+    {:reply, {:error, :not_in_calibration_mode}, state}
+  end
+
+  @impl true
+  def handle_cast(:abort_calibration, state) do
+    case state.mode do
+      :ok ->
+        {:noreply, state}
+
+      _ ->
+        MQTTConnection.publish("localiser/sensor/#{state.sensor_id}/cmd", ~s({"action":"calibrate_stop"}))
+        broadcast_calibration(state.sensor_id, {:calibration_cancelled, state.sensor_id})
+        {:noreply, %{state | mode: :ok}}
+    end
+  end
+
+  @impl true
+  def handle_cast({:calibration_reading, rssi}, %{mode: {:calibrating_stage, distance, samples, completed}} = state) do
+    n = samples_needed()
+    is_outlier = Calibration.outlier?(rssi, samples)
+    new_samples = [rssi | samples]
+    progress = {length(new_samples), n}
+
+    broadcast_calibration(state.sensor_id, {:calibration_reading, state.sensor_id, rssi, is_outlier, %{stage: progress}})
+
+    if length(new_samples) >= n do
+      classified = Calibration.classify_outliers(new_samples)
+      clean = for {r, false} <- classified, do: r
+
+      mean_rssi = if clean == [] do
+        mean(new_samples)
+      else
+        mean(clean)
+      end
+
+      stage = %{distance: distance, mean_rssi: mean_rssi, readings: classified}
+      new_completed = [stage | completed]
+
+      broadcast_calibration(state.sensor_id, {:stage_complete, state.sensor_id, distance, classified, mean_rssi})
+
+      {:noreply, %{state | mode: {:calibration_mode, new_completed}}}
     else
-      {:noreply, %{state | mode: {:calibrating, buffer, target}}}
+      {:noreply, %{state | mode: {:calibrating_stage, distance, new_samples, completed}}}
     end
   end
 
   def handle_cast({:calibration_reading, _rssi}, state), do: {:noreply, state}
 
-  # Position updated (sensor dragged in layout).
   @impl true
   def handle_info({:sensor_enrolled, %Sensor{sensor_id: sid} = sensor, room}, %{sensor_id: sid} = state) do
     floor_x = (room.x || 0.0) + (sensor.x || 0.0)
@@ -122,45 +250,26 @@ defmodule Localiser.Localisation.Sensor.Server do
     {:noreply, %{state | floor_x: floor_x, floor_y: floor_y}}
   end
 
-  # Ignore PubSub messages not relevant to this server.
   def handle_info(_msg, state), do: {:noreply, state}
 
   # Private
 
-  defp finalize_calibration(buffer, state) do
-    rssi_ref = median(buffer)
-    sensor_struct = %Sensor{id: state.sensor_db_id, sensor_id: state.sensor_id}
-
-    case Sensors.add_calibration(sensor_struct, %{
-           rssi_ref: rssi_ref,
-           path_loss_exp: state.path_loss_exp,
-           calibrated_at: DateTime.utc_now()
-         }) do
-      {:ok, _calibration} ->
-        Logger.info("[Sensor.Server] Calibration complete for #{state.sensor_id}: rssi_ref=#{rssi_ref}")
-        MQTTConnection.publish("localiser/sensor/#{state.sensor_id}/cmd", ~s({"action":"calibrate_stop"}))
-        Phoenix.PubSub.broadcast(Localiser.PubSub, "sensors", {:calibration_complete, state.sensor_id})
-        {:noreply, %{state | rssi_ref: rssi_ref, mode: :ok}}
-
-      {:error, reason} ->
-        Logger.error("[Sensor.Server] Failed to save calibration for #{state.sensor_id}: #{inspect(reason)}")
-        {:noreply, %{state | mode: :ok}}
-    end
+  defp broadcast_calibration(sensor_id, message) do
+    Phoenix.PubSub.broadcast(Localiser.PubSub, "calibration:#{sensor_id}", message)
   end
 
-  defp median(list) do
-    sorted = Enum.sort(list)
-    len = length(sorted)
-    mid = div(len, 2)
-
-    if rem(len, 2) == 0 do
-      round((Enum.at(sorted, mid - 1) + Enum.at(sorted, mid)) / 2)
-    else
-      Enum.at(sorted, mid)
-    end
+  defp render_stage(%{distance: d, mean_rssi: r, readings: readings}) do
+    %{distance: d, mean_rssi: r, readings: Enum.map(readings, fn {rssi, outlier} -> %{rssi: rssi, outlier: outlier} end)}
+  end
+
+  defp samples_needed do
+    Application.get_env(:localiser, :calibration_samples, @default_samples)
+  end
+
+  defp mean(list) do
+    Enum.sum(list) / length(list)
   end
 
-  # d = 10 ^ ((rssi_ref - rssi) / (10 * n))
   defp rssi_to_distance(rssi, rssi_ref, path_loss_exp) do
     :math.pow(10.0, (rssi_ref - rssi) / (10.0 * path_loss_exp))
   end
diff --git a/lib/localiser/web/channels/calibration_channel.ex b/lib/localiser/web/channels/calibration_channel.ex
new file mode 100644
index 0000000..d5ad2e5
--- /dev/null
+++ b/lib/localiser/web/channels/calibration_channel.ex
@@ -0,0 +1,53 @@
+defmodule Localiser.Web.Channels.CalibrationChannel do
+  use Phoenix.Channel
+
+  alias Localiser.Domain.Sensors
+  alias Localiser.Localisation.Sensor.Server, as: SensorServer
+
+  @impl true
+  def join("calibration:" <> sensor_id, _params, socket) do
+    case Sensors.get_sensor_by_sensor_id(sensor_id) do
+      nil ->
+        {:error, %{reason: "sensor not found"}}
+
+      _sensor ->
+        Phoenix.PubSub.subscribe(Localiser.PubSub, "calibration:#{sensor_id}")
+        state = SensorServer.calibration_state(sensor_id)
+        {:ok, state, assign(socket, :sensor_id, sensor_id)}
+    end
+  end
+
+  @impl true
+  def handle_info({:calibration_mode_entered, _sensor_id, samples_needed}, socket) do
+    push(socket, "mode_entered", %{samples_needed: samples_needed})
+    {:noreply, socket}
+  end
+
+  def handle_info({:stage_started, _sensor_id, distance, completed_count}, socket) do
+    push(socket, "stage_started", %{distance: distance, completed_stages: completed_count})
+    {:noreply, socket}
+  end
+
+  def handle_info({:calibration_reading, _sensor_id, rssi, is_outlier, %{stage: {cur, total}}}, socket) do
+    push(socket, "reading", %{rssi: rssi, outlier: is_outlier, stage_progress: %{current: cur, total: total}})
+    {:noreply, socket}
+  end
+
+  def handle_info({:stage_complete, _sensor_id, distance, readings, mean_rssi}, socket) do
+    rendered = Enum.map(readings, fn {rssi, outlier} -> %{rssi: rssi, outlier: outlier} end)
+    push(socket, "stage_complete", %{distance: distance, readings: rendered, mean_rssi: mean_rssi})
+    {:noreply, socket}
+  end
+
+  def handle_info({:calibration_finished, _sensor_id, rssi_ref, path_loss_exp}, socket) do
+    push(socket, "finished", %{rssi_ref: rssi_ref, path_loss_exp: path_loss_exp})
+    {:noreply, socket}
+  end
+
+  def handle_info({:calibration_cancelled, _sensor_id}, socket) do
+    push(socket, "cancelled", %{})
+    {:noreply, socket}
+  end
+
+  def handle_info(_msg, socket), do: {:noreply, socket}
+end
diff --git a/lib/localiser/web/channels/sensors_channel.ex b/lib/localiser/web/channels/sensors_channel.ex
index ea0552d..4078b3b 100644
--- a/lib/localiser/web/channels/sensors_channel.ex
+++ b/lib/localiser/web/channels/sensors_channel.ex
@@ -23,8 +23,8 @@ defmodule Localiser.Web.Channels.SensorsChannel do
     {:noreply, socket}
   end
 
-  def handle_info({:calibration_complete, sensor_id}, socket) do
-    push(socket, "calibration_complete", %{sensor_id: sensor_id})
+  def handle_info({:calibration_complete, sensor_id, rssi_ref, path_loss_exp}, socket) do
+    push(socket, "calibration_complete", %{sensor_id: sensor_id, rssi_ref: rssi_ref, path_loss_exp: path_loss_exp})
     {:noreply, socket}
   end
 
diff --git a/lib/localiser/web/controllers/sensor_controller.ex b/lib/localiser/web/controllers/sensor_controller.ex
index be1e652..19c2d65 100644
--- a/lib/localiser/web/controllers/sensor_controller.ex
+++ b/lib/localiser/web/controllers/sensor_controller.ex
@@ -108,21 +108,39 @@ defmodule Localiser.Web.Controllers.SensorController do
       unauthorized: {"Unauthorized", "application/json", Schemas.Error}
     ]
 
-  operation :calibration_start,
-    summary: "Begin RSSI calibration",
+  operation :calibration_begin,
+    summary: "Enter calibration mode (between-stages)",
     parameters: [id: [in: :path, type: :integer, required: true]],
-    request_body: {"Calibration params", "application/json", Schemas.CalibrationStartParams, required: true},
     responses: [
-      ok: {"Calibration started", "application/json", Schemas.CalibrationStatus},
-      bad_request: {"Missing reference_distance", "application/json", Schemas.Error},
+      ok: {"Calibration mode entered", "application/json", Schemas.CalibrationBeginResponse},
       unauthorized: {"Unauthorized", "application/json", Schemas.Error}
     ]
 
-  operation :calibration_stop,
-    summary: "Abort active calibration",
+  operation :calibration_stage_start,
+    summary: "Start collecting samples for a specific distance",
+    parameters: [id: [in: :path, type: :integer, required: true]],
+    request_body: {"Stage params", "application/json", Schemas.CalibrationStageParams, required: true},
+    responses: [
+      ok: {"Stage started", "application/json", Schemas.CalibrationStageResponse},
+      bad_request: {"Missing or invalid distance", "application/json", Schemas.Error},
+      unprocessable_entity: {"Stage already active", "application/json", Schemas.Error},
+      unauthorized: {"Unauthorized", "application/json", Schemas.Error}
+    ]
+
+  operation :calibration_finish,
+    summary: "Run regression over completed stages and save calibration",
     parameters: [id: [in: :path, type: :integer, required: true]],
     responses: [
-      ok: {"Calibration aborted", "application/json", Schemas.CalibrationStatus},
+      ok: {"Calibration saved", "application/json", Schemas.CalibrationFinishResponse},
+      unprocessable_entity: {"Insufficient stages or stage active", "application/json", Schemas.Error},
+      unauthorized: {"Unauthorized", "application/json", Schemas.Error}
+    ]
+
+  operation :calibration_cancel,
+    summary: "Abort calibration, discard all stages",
+    parameters: [id: [in: :path, type: :integer, required: true]],
+    responses: [
+      ok: {"Calibration cancelled", "application/json", Schemas.CalibrationStatus},
       unauthorized: {"Unauthorized", "application/json", Schemas.Error}
     ]
 
@@ -252,24 +270,77 @@ defmodule Localiser.Web.Controllers.SensorController do
     json(conn, %{status: "ok"})
   end
 
-  def calibration_start(conn, %{"id" => id, "reference_distance" => ref_dist}) do
+  def calibration_begin(conn, %{"id" => id}) do
     sensor = Sensors.get_sensor!(id)
-    :ok = SensorServer.begin_calibration(sensor.sensor_id, ref_dist)
-    json(conn, %{status: "calibrating"})
+    :ok = SensorServer.begin_calibration_mode(sensor.sensor_id)
+    json(conn, %{status: "calibration_mode", samples_needed: calibration_samples_needed()})
   end
 
-  def calibration_start(conn, _params) do
+  def calibration_stage_start(conn, %{"id" => id, "distance" => distance})
+      when is_number(distance) and distance > 0 do
+    sensor = Sensors.get_sensor!(id)
+
+    case SensorServer.start_stage(sensor.sensor_id, distance * 1.0) do
+      {:ok, samples_needed} ->
+        json(conn, %{status: "stage_active", distance: distance, samples_needed: samples_needed})
+
+      {:error, :already_active} ->
+        conn
+        |> put_status(:unprocessable_entity)
+        |> json(%{error: "a stage is already active"})
+
+      {:error, :not_in_calibration_mode} ->
+        conn
+        |> put_status(:unprocessable_entity)
+        |> json(%{error: "sensor is not in calibration mode"})
+    end
+  end
+
+  def calibration_stage_start(conn, _params) do
     conn
     |> put_status(:bad_request)
-    |> json(%{error: "reference_distance is required"})
+    |> json(%{error: "distance is required and must be a positive number"})
   end
 
-  def calibration_stop(conn, %{"id" => id}) do
+  def calibration_finish(conn, %{"id" => id}) do
+    sensor = Sensors.get_sensor!(id)
+
+    case SensorServer.finish_calibration(sensor.sensor_id) do
+      {:ok, %{rssi_ref: rssi_ref, path_loss_exp: path_loss_exp}} ->
+        json(conn, %{status: "idle", rssi_ref: rssi_ref, path_loss_exp: path_loss_exp})
+
+      {:error, :insufficient_stages} ->
+        conn
+        |> put_status(:unprocessable_entity)
+        |> json(%{error: "at least 2 completed stages required"})
+
+      {:error, :stage_active} ->
+        conn
+        |> put_status(:unprocessable_entity)
+        |> json(%{error: "cannot finish while a stage is active"})
+
+      {:error, :not_in_calibration_mode} ->
+        conn
+        |> put_status(:unprocessable_entity)
+        |> json(%{error: "sensor is not in calibration mode"})
+
+      {:error, reason} ->
+        conn
+        |> put_status(:unprocessable_entity)
+        |> json(%{error: inspect(reason)})
+    end
+  end
+
+  def calibration_cancel(conn, %{"id" => id}) do
     sensor = Sensors.get_sensor!(id)
     :ok = SensorServer.abort_calibration(sensor.sensor_id)
     json(conn, %{status: "idle"})
   end
 
+  defp calibration_samples_needed do
+    Application.get_env(:localiser, :calibration_samples, 30)
+  end
+
   defp render_sensor(sensor) do
     %{
       id:        sensor.id,
diff --git a/lib/localiser/web/router.ex b/lib/localiser/web/router.ex
index d156fee..a89a3bc 100644
--- a/lib/localiser/web/router.ex
+++ b/lib/localiser/web/router.ex
@@ -101,8 +101,10 @@ defmodule Localiser.Web.Router do
     delete "/sensors/:id/place",               SensorController, :unplace
     post   "/sensors/:id/factory_reset",       SensorController, :factory_reset
     post   "/sensors/:id/reconfigure",         SensorController, :reconfigure
-    post   "/sensors/:id/calibration/start",   SensorController, :calibration_start
-    post   "/sensors/:id/calibration/stop",    SensorController, :calibration_stop
+    post   "/sensors/:id/calibration/begin",   SensorController, :calibration_begin
+    post   "/sensors/:id/calibration/stage",   SensorController, :calibration_stage_start
+    post   "/sensors/:id/calibration/finish",  SensorController, :calibration_finish
+    delete "/sensors/:id/calibration",         SensorController, :calibration_cancel
     get    "/sensors/:id/version",             SensorController, :get_version
   end
 end
diff --git a/lib/localiser/web/schemas.ex b/lib/localiser/web/schemas.ex
index 5c8476c..c95df95 100644
--- a/lib/localiser/web/schemas.ex
+++ b/lib/localiser/web/schemas.ex
@@ -140,11 +140,49 @@ defmodule Localiser.Web.Schemas do
     require OpenApiSpex
     OpenApiSpex.schema(%{
       title: "CalibrationStatus", type: :object,
-      properties: %{status: %Schema{type: :string, enum: ["calibrating", "idle"]}},
+      properties: %{status: %Schema{type: :string, enum: ["idle"]}},
       required: [:status]
     })
   end
 
+  defmodule CalibrationBeginResponse do
+    require OpenApiSpex
+    OpenApiSpex.schema(%{
+      title: "CalibrationBeginResponse", type: :object,
+      properties: %{
+        status:         %Schema{type: :string, enum: ["calibration_mode"]},
+        samples_needed: %Schema{type: :integer}
+      },
+      required: [:status, :samples_needed]
+    })
+  end
+
+  defmodule CalibrationStageResponse do
+    require OpenApiSpex
+    OpenApiSpex.schema(%{
+      title: "CalibrationStageResponse", type: :object,
+      properties: %{
+        status:         %Schema{type: :string, enum: ["stage_active"]},
+        distance:       %Schema{type: :number, format: :float},
+        samples_needed: %Schema{type: :integer}
+      },
+      required: [:status, :distance, :samples_needed]
+    })
+  end
+
+  defmodule CalibrationFinishResponse do
+    require OpenApiSpex
+    OpenApiSpex.schema(%{
+      title: "CalibrationFinishResponse", type: :object,
+      properties: %{
+        status:          %Schema{type: :string, enum: ["idle"]},
+        rssi_ref:        %Schema{type: :integer},
+        path_loss_exp:   %Schema{type: :number, format: :float}
+      },
+      required: [:status, :rssi_ref, :path_loss_exp]
+    })
+  end
+
   # ── Request body schemas ────────────────────────────────────────────────────
 
   defmodule SetupParams do
@@ -292,17 +330,17 @@ defmodule Localiser.Web.Schemas do
     })
   end
 
-  defmodule CalibrationStartParams do
+  defmodule CalibrationStageParams do
     require OpenApiSpex
     OpenApiSpex.schema(%{
-      title: "CalibrationStartParams", type: :object,
+      title: "CalibrationStageParams", type: :object,
       properties: %{
-        reference_distance: %Schema{
+        distance: %Schema{
           type: :number, format: :float,
-          description: "Known tag-to-sensor distance in metres"
+          description: "Known tag-to-sensor distance in metres (must be > 0)"
         }
       },
-      required: [:reference_distance]
+      required: [:distance]
     })
   end
 
diff --git a/lib/localiser/web/user_socket.ex b/lib/localiser/web/user_socket.ex
index cbfd0ef..9f63423 100644
--- a/lib/localiser/web/user_socket.ex
+++ b/lib/localiser/web/user_socket.ex
@@ -4,6 +4,7 @@ defmodule Localiser.Web.UserSocket do
   channel "particles:*",     Localiser.Web.Channels.ParticlesChannel
   channel "rooms:occupancy", Localiser.Web.Channels.RoomChannel
   channel "sensors",         Localiser.Web.Channels.SensorsChannel
+  channel "calibration:*",   Localiser.Web.Channels.CalibrationChannel
 
   @impl true
   def connect(%{"token" => token}, socket, _connect_info) do