diff --git a/quic/core/quic_sent_packet_manager.cc b/quic/core/quic_sent_packet_manager.cc
index 0d6c862..f7e91b9 100644
--- a/quic/core/quic_sent_packet_manager.cc
+++ b/quic/core/quic_sent_packet_manager.cc
@@ -1504,6 +1504,13 @@
   return QuicTime::Delta::Infinite();
 }
 
+QuicByteCount QuicSentPacketManager::GetAvailableCongestionWindowInBytes()
+    const {
+  QuicByteCount congestion_window = GetCongestionWindowInBytes();
+  QuicByteCount bytes_in_flight = GetBytesInFlight();
+  return congestion_window - std::min(congestion_window, bytes_in_flight);
+}
+
 std::string QuicSentPacketManager::GetDebugState() const {
   return send_algorithm_->GetDebugState();
 }
diff --git a/quic/core/quic_sent_packet_manager.h b/quic/core/quic_sent_packet_manager.h
index 7b127b5..22067ff 100644
--- a/quic/core/quic_sent_packet_manager.h
+++ b/quic/core/quic_sent_packet_manager.h
@@ -285,6 +285,11 @@
     return send_algorithm_->GetCongestionWindow();
   }
 
+  // Returns the difference between current congestion window and bytes in
+  // flight. Returns 0 if bytes in flight is bigger than the current congestion
+  // window.
+  QuicByteCount GetAvailableCongestionWindowInBytes() const;
+
   QuicBandwidth GetPacingRate() const {
     return send_algorithm_->PacingRate(GetBytesInFlight());
   }
diff --git a/quic/core/quic_sent_packet_manager_test.cc b/quic/core/quic_sent_packet_manager_test.cc
index 02ae04c..bf141e4 100644
--- a/quic/core/quic_sent_packet_manager_test.cc
+++ b/quic/core/quic_sent_packet_manager_test.cc
@@ -4719,6 +4719,23 @@
             kMinTrustedInitialRoundTripTimeUs);
 }
 
+TEST_F(QuicSentPacketManagerTest, GetAvailableCongestionWindow) {
+  SendDataPacket(1);
+  EXPECT_EQ(kDefaultLength, manager_.GetBytesInFlight());
+
+  EXPECT_CALL(*send_algorithm_, GetCongestionWindow())
+      .WillOnce(Return(kDefaultLength + 10));
+  EXPECT_EQ(10u, manager_.GetAvailableCongestionWindowInBytes());
+
+  EXPECT_CALL(*send_algorithm_, GetCongestionWindow())
+      .WillOnce(Return(kDefaultLength));
+  EXPECT_EQ(0u, manager_.GetAvailableCongestionWindowInBytes());
+
+  EXPECT_CALL(*send_algorithm_, GetCongestionWindow())
+      .WillOnce(Return(kDefaultLength - 10));
+  EXPECT_EQ(0u, manager_.GetAvailableCongestionWindowInBytes());
+}
+
 }  // namespace
 }  // namespace test
 }  // namespace quic