diff --git a/lib/phoenix/socket/pool_supervisor.ex b/lib/phoenix/socket/pool_supervisor.ex
index 4f2d30421f..ae2646d752 100644
--- a/lib/phoenix/socket/pool_supervisor.ex
+++ b/lib/phoenix/socket/pool_supervisor.ex
@@ -107,15 +107,13 @@ defmodule Phoenix.Socket.PoolDrainer do
         end
       end)
 
-    rounds = div(total, size) + 1
+    rounds = div(total + size - 1, size)
 
     for {pids, index} <-
           collection |> Stream.concat() |> Stream.chunk_every(size) |> Stream.with_index(1) do
-      count = if index == rounds, do: length(pids), else: size
-
       :telemetry.execute(
         [:phoenix, :socket_drain],
-        %{count: count, total: total, index: index, rounds: rounds},
+        %{count: length(pids), total: total, index: index, rounds: rounds},
         %{
           endpoint: endpoint,
           socket: name,
diff --git a/test/phoenix/socket/pool_drainer_test.exs b/test/phoenix/socket/pool_drainer_test.exs
new file mode 100644
index 0000000000..c6105d6e8e
--- /dev/null
+++ b/test/phoenix/socket/pool_drainer_test.exs
@@ -0,0 +1,91 @@
+defmodule Phoenix.Socket.PoolDrainerTest do
+  use ExUnit.Case, async: true
+
+  alias Phoenix.Socket.{Broadcast, PoolDrainer, PoolSupervisor}
+
+  Application.put_env(:phoenix, __MODULE__.Endpoint, server: false)
+
+  defmodule Endpoint do
+    use Phoenix.Endpoint, otp_app: :phoenix
+  end
+
+  setup_all do
+    start_supervised!(Endpoint)
+    :ok
+  end
+
+  # Each test drains its own pool, so tests cannot affect each other.
+  setup do
+    name = :"#{__MODULE__}.Pool#{System.unique_integer([:positive])}"
+    start_supervised!({PoolSupervisor, {Endpoint, name, 2}})
+
+    id = {__MODULE__, name}
+    :telemetry.attach(id, [:phoenix, :socket_drain], &__MODULE__.handle_event/4, {self(), name})
+    on_exit(fn -> :telemetry.detach(id) end)
+
+    %{name: name}
+  end
+
+  def handle_event(_event, measurements, %{socket: name}, {parent, name}) do
+    send(parent, {:drain, measurements})
+  end
+
+  def handle_event(_event, _measurements, _metadata, _config), do: :ok
+
+  # Starts connections, drains them in batches and waits for the drainer to stop.
+  defp drain(name, connections, opts) do
+    parent = self()
+
+    for key <- 1..connections//1 do
+      connection = fn -> receive(do: (msg -> send(parent, {:drained, key, msg}))) end
+      spec = %{id: key, start: {Task, :start_link, [connection]}, restart: :temporary}
+      {:ok, _pid} = PoolSupervisor.start_child(%{endpoint: Endpoint, handler: name}, key, spec)
+    end
+
+    drainer = Keyword.merge([batch_interval: 1, log: false], opts)
+    {:ok, pid} = PoolDrainer.start_link({Endpoint, name, drainer: drainer})
+    GenServer.stop(pid)
+  end
+
+  describe "terminate/2" do
+    test "notifies every connection that the socket is draining", %{name: name} do
+      drain(name, 3, batch_size: 2)
+
+      for key <- 1..3 do
+        assert_receive {:drained, ^key, %Broadcast{event: "phx_drain"}}
+      end
+    end
+
+    test "last batch is smaller when total is not a multiple of batch size", %{name: name} do
+      drain(name, 5, batch_size: 2)
+
+      assert_receive {:drain, %{index: 1, rounds: 3, count: 2, total: 5}}
+      assert_receive {:drain, %{index: 2, rounds: 3, count: 2, total: 5}}
+      assert_receive {:drain, %{index: 3, rounds: 3, count: 1, total: 5}}
+      refute_receive {:drain, _}
+    end
+
+    test "reports no extra round when total is a multiple of batch size", %{name: name} do
+      drain(name, 4, batch_size: 2)
+
+      assert_receive {:drain, %{index: 1, rounds: 2, count: 2, total: 4}}
+      assert_receive {:drain, %{index: 2, rounds: 2, count: 2, total: 4}}
+      refute_receive {:drain, _}
+    end
+
+    test "does not wait for another interval after the last batch", %{name: name} do
+      interval = 5_000
+      started_at = System.monotonic_time(:millisecond)
+
+      drain(name, 4, batch_size: 4, batch_interval: interval)
+
+      assert System.monotonic_time(:millisecond) - started_at < interval
+    end
+
+    test "does nothing when there are no connections", %{name: name} do
+      drain(name, 0, batch_size: 2)
+
+      refute_receive {:drain, _}
+    end
+  end
+end