diff --git a/.dialyzer_ignore b/.dialyzer_ignore
index a13cca82..2dbafa78 100644
--- a/.dialyzer_ignore
+++ b/.dialyzer_ignore
@@ -1,5 +1,5 @@
 lib/mint/tunnel_proxy.ex:49
-lib/mint/http1.ex:915
+lib/mint/http1.ex:927
 lib/mint/unsafe_proxy.ex:173
 lib/mint/unsafe_proxy.ex:198
 test/support
diff --git a/lib/mint/core/conn.ex b/lib/mint/core/conn.ex
index 12af3b2c..11ae1b6a 100644
--- a/lib/mint/core/conn.ex
+++ b/lib/mint/core/conn.ex
@@ -62,4 +62,6 @@ defmodule Mint.Core.Conn do
   @callback put_proxy_headers(conn(), Mint.Types.headers()) :: conn()
 
   @callback put_log(conn(), boolean()) :: conn()
+
+  @callback request_body_window(conn(), Types.request_ref()) :: non_neg_integer() | :infinity
 end
diff --git a/lib/mint/http.ex b/lib/mint/http.ex
index 71c1c6ff..bb37b22d 100644
--- a/lib/mint/http.ex
+++ b/lib/mint/http.ex
@@ -623,6 +623,10 @@ defmodule Mint.HTTP do
 
   This function always returns an updated connection to be stored over the old connection.
 
+  When streaming a body of arbitrary size, use `request_body_window/2` to learn
+  how many bytes you can send right now without violating HTTP/2 flow control,
+  then split your body accordingly before passing each chunk to this function.
+
   For information about transfer encoding and content length in HTTP/1, see
   `Mint.HTTP1.stream_request_body/3`.
 
@@ -1065,6 +1069,75 @@ defmodule Mint.HTTP do
   @impl true
   def put_proxy_headers(conn, headers), do: conn_apply(conn, :put_proxy_headers, [conn, headers])
 
+  @doc """
+  Returns the request body flow-control window for the streaming request
+  identified by `request_ref`.
+
+  The semantics differ by protocol:
+
+    * In HTTP/2, returns `min(connection_window, stream_window)` — the maximum
+      number of body bytes that can be sent right now without violating flow
+      control. Exceeding this value in a single `DATA` frame would close the
+      connection with a `FLOW_CONTROL_ERROR`. See `Mint.HTTP2.get_window_size/2`
+      for the underlying primitives.
+
+    * In HTTP/1, returns `:infinity`. HTTP/1 has no application-level
+      flow-control mechanism: any amount of body data is protocol-valid.
+
+  The value returned reflects only the protocol-level flow-control
+  constraint. It does not account for the operating-system socket send
+  buffer: under either protocol, `stream_request_body/3` can still block
+  when that buffer fills up. To bound this behavior, configure
+  `send_timeout` on the socket via `:transport_opts` when establishing the
+  connection (see `Mint.HTTP.connect/4`).
+
+  Raises `ArgumentError` if `request_ref` is not associated with an active
+  streaming request.
+
+  ## Examples
+
+  Streaming a binary body in chunks that respect the protocol window:
+
+      defp stream_body(conn, ref, "") do
+        Mint.HTTP.stream_request_body(conn, ref, :eof)
+      end
+
+      defp stream_body(conn, ref, body) do
+        conn
+        |> Mint.HTTP.request_body_window(ref)
+        |> send_body_chunk(conn, ref, body)
+      end
+
+      defp send_body_chunk(0, conn, ref, body) do
+        with {:ok, conn} <- wait(conn, ref) do
+          stream_body(conn, ref, body)
+        end
+      end
+
+      defp send_body_chunk(window, conn, ref, body) do
+        chunk_size = min(window, byte_size(body))
+        <<chunk::binary-size(chunk_size), rest::binary>> = body
+
+        with {:ok, conn} <- Mint.HTTP.stream_request_body(conn, ref, chunk) do
+          stream_body(conn, ref, rest)
+        end
+      end
+
+      defp wait(conn, ref) do
+        # Wait for the server to refill the request body window with a
+        # WINDOW_UPDATE frame. The concrete implementation depends on the
+        # socket mode and other context.
+      end
+
+  Note that `min(:infinity, n) == n` thanks to Erlang term ordering, so the
+  same loop works on HTTP/1 (each iteration sends the entire remaining body in
+  a single chunk) and on HTTP/2 (each iteration sends at most the current
+  flow-control window).
+  """
+  @doc since: "1.8.0"
+  @impl true
+  def request_body_window(conn, ref), do: conn_apply(conn, :request_body_window, [conn, ref])
+
   ## Helpers
 
   defp conn_apply(%UnsafeProxy{}, fun, args), do: apply(UnsafeProxy, fun, args)
diff --git a/lib/mint/http1.ex b/lib/mint/http1.ex
index 941e12d0..63c35f43 100644
--- a/lib/mint/http1.ex
+++ b/lib/mint/http1.ex
@@ -667,6 +667,18 @@ defmodule Mint.HTTP1 do
     %{conn | proxy_headers: headers}
   end
 
+  @doc """
+  See `Mint.HTTP.request_body_window/2`.
+  """
+  @doc since: "1.8.0"
+  @impl true
+  def request_body_window(%__MODULE__{streaming_request: %{ref: ref}}, ref), do: :infinity
+
+  def request_body_window(%__MODULE__{}, ref) do
+    raise ArgumentError,
+          "request with request reference #{inspect(ref)} was not found or is not streaming a body"
+  end
+
   ## Helpers
 
   defp decode(:status, %{request: request} = conn, data, responses) do
diff --git a/lib/mint/http2.ex b/lib/mint/http2.ex
index d1b562d0..6f923c4b 100644
--- a/lib/mint/http2.ex
+++ b/lib/mint/http2.ex
@@ -1039,6 +1039,15 @@ defmodule Mint.HTTP2 do
     %{conn | proxy_headers: headers}
   end
 
+  @doc """
+  See `Mint.HTTP.request_body_window/2`.
+  """
+  @doc since: "1.8.0"
+  @impl true
+  def request_body_window(%__MODULE__{} = conn, ref) do
+    min(get_window_size(conn, :connection), get_window_size(conn, {:request, ref}))
+  end
+
   ## Helpers
 
   defp handle_closed(conn) do
diff --git a/lib/mint/unsafe_proxy.ex b/lib/mint/unsafe_proxy.ex
index b6318ffa..5c33a551 100644
--- a/lib/mint/unsafe_proxy.ex
+++ b/lib/mint/unsafe_proxy.ex
@@ -199,4 +199,9 @@ defmodule Mint.UnsafeProxy do
   def put_proxy_headers(%__MODULE__{}, _headers) do
     raise "invalid function for proxy unsafe proxy connections"
   end
+
+  @impl true
+  def request_body_window(%__MODULE__{module: module, state: state}, ref) do
+    module.request_body_window(state, ref)
+  end
 end
diff --git a/test/http_test.exs b/test/http_test.exs
index 70e307c2..28b93988 100644
--- a/test/http_test.exs
+++ b/test/http_test.exs
@@ -1,4 +1,27 @@
 defmodule Mint.HTTPTest do
   use ExUnit.Case, async: true
   doctest Mint.HTTP
+
+  alias Mint.{HTTP, HTTP1.TestServer}
+
+  setup do
+    {:ok, port, server_ref} = TestServer.start()
+    assert {:ok, conn} = HTTP.connect(:http, "localhost", port)
+    assert_receive {^server_ref, server_socket}
+
+    [conn: conn, server_socket: server_socket]
+  end
+
+  describe "request_body_window/2" do
+    test "returns :infinity for an HTTP/1 streaming request", %{conn: conn} do
+      {:ok, conn, ref} = HTTP.request(conn, "GET", "/", [], :stream)
+      assert HTTP.request_body_window(conn, ref) == :infinity
+    end
+
+    test "raises ArgumentError for an unknown request ref", %{conn: conn} do
+      assert_raise ArgumentError, fn ->
+        HTTP.request_body_window(conn, make_ref())
+      end
+    end
+  end
 end
diff --git a/test/mint/http1/conn_test.exs b/test/mint/http1/conn_test.exs
index e354be14..37cf4262 100644
--- a/test/mint/http1/conn_test.exs
+++ b/test/mint/http1/conn_test.exs
@@ -1151,6 +1151,19 @@ defmodule Mint.HTTP1Test do
     {:ok, conn, responses}
   end
 
+  describe "request_body_window/2" do
+    test "returns :infinity for an active streaming request", %{conn: conn} do
+      {:ok, conn, ref} = HTTP1.request(conn, "GET", "/", [], :stream)
+      assert HTTP1.request_body_window(conn, ref) == :infinity
+    end
+
+    test "raises if no request is currently streaming a body", %{conn: conn} do
+      assert_raise ArgumentError, ~r/was not found or is not streaming a body/, fn ->
+        HTTP1.request_body_window(conn, make_ref())
+      end
+    end
+  end
+
   @mint_user_agent "mint/#{Mix.Project.config()[:version]}"
   defp mint_user_agent, do: @mint_user_agent
 end
diff --git a/test/mint/http2/conn_test.exs b/test/mint/http2/conn_test.exs
index 27215664..f9143eca 100644
--- a/test/mint/http2/conn_test.exs
+++ b/test/mint/http2/conn_test.exs
@@ -1772,6 +1772,82 @@ defmodule Mint.HTTP2Test do
         HTTP2.get_window_size(conn, {:request, make_ref()})
       end
     end
+
+    test "request_body_window/2 returns the minimum of connection and request window sizes",
+         %{conn: conn} do
+      {conn, ref} = open_request(conn, :stream)
+
+      send_window = HTTP2.request_body_window(conn, ref)
+      conn_window = HTTP2.get_window_size(conn, :connection)
+      request_window = HTTP2.get_window_size(conn, {:request, ref})
+
+      assert send_window == min(conn_window, request_window)
+    end
+
+    test "request_body_window/2 decreases after streaming body data", %{conn: conn} do
+      {conn, ref} = open_request(conn, :stream)
+
+      initial_send_window = HTTP2.request_body_window(conn, ref)
+      assert initial_send_window > 0
+
+      body_chunk = "hello"
+      {:ok, conn} = HTTP2.stream_request_body(conn, ref, body_chunk)
+
+      assert HTTP2.request_body_window(conn, ref) == initial_send_window - byte_size(body_chunk)
+    end
+
+    test "request_body_window/2 raises if the request is not found", %{conn: conn} do
+      assert_raise ArgumentError, ~r/request with request reference .+ was not found/, fn ->
+        HTTP2.request_body_window(conn, make_ref())
+      end
+    end
+
+    @tag server_settings: [initial_window_size: 5]
+    test "streaming a body larger than the window using request_body_window/2 in a loop",
+         %{conn: conn} do
+      {conn, ref} = open_request(conn, :stream)
+
+      assert_recv_frames [headers(stream_id: stream_id)]
+
+      body = "0123456789ABCDE"
+
+      # First chunk: window is 5, so we send 5 bytes.
+      assert HTTP2.request_body_window(conn, ref) == 5
+      <<chunk1::binary-size(5), rest1::binary>> = body
+      {:ok, conn} = HTTP2.stream_request_body(conn, ref, chunk1)
+
+      assert HTTP2.request_body_window(conn, ref) == 0
+
+      assert_recv_frames [data(stream_id: ^stream_id, data: ^chunk1, flags: flags1)]
+      assert flags1 == set_flags(:data, [])
+
+      # Server replenishes the stream window so we can send more.
+      {:ok, conn, []} =
+        stream_frames(conn, [window_update(stream_id: stream_id, window_size_increment: 5)])
+
+      assert HTTP2.request_body_window(conn, ref) == 5
+      <<chunk2::binary-size(5), rest2::binary>> = rest1
+      {:ok, conn} = HTTP2.stream_request_body(conn, ref, chunk2)
+
+      assert_recv_frames [data(stream_id: ^stream_id, data: ^chunk2)]
+
+      # Final replenishment for the remaining bytes plus :eof.
+      {:ok, conn, []} =
+        stream_frames(conn, [
+          window_update(stream_id: stream_id, window_size_increment: byte_size(rest2))
+        ])
+
+      assert HTTP2.request_body_window(conn, ref) == byte_size(rest2)
+      {:ok, conn} = HTTP2.stream_request_body(conn, ref, rest2)
+      {:ok, _conn} = HTTP2.stream_request_body(conn, ref, :eof)
+
+      assert_recv_frames [
+        data(stream_id: ^stream_id, data: ^rest2),
+        data(stream_id: ^stream_id, data: "", flags: end_flags)
+      ]
+
+      assert end_flags == set_flags(:data, [:end_stream])
+    end
   end
 
   describe "settings" do