diff --git a/docs/API.md b/docs/API.md
index a81a0e6..7ebc4b7 100644
--- a/docs/API.md
+++ b/docs/API.md
@@ -299,6 +299,8 @@ Requires `--audio-stdin`, `--audio-http`, or another configured stream source to
     "capacity": 131072,
     "buffered": 0.09,
     "bufferedDurationSeconds": 0.27,
+    "highWatermark": 15000,
+    "highWatermarkDurationSeconds": 0.34,
     "written": 890000,
     "underruns": 0,
     "overflows": 0
@@ -368,6 +370,8 @@ The stream uses a lock-free ring buffer (default: 2 seconds at input rate). Buff
     "capacity": 131072,
     "buffered": 0.09,
     "bufferedDurationSeconds": 0.27,
+    "highWatermark": 15000,
+    "highWatermarkDurationSeconds": 0.34,
     "written": 890000,
     "underruns": 0,
     "overflows": 0
@@ -379,5 +383,7 @@ The stream uses a lock-free ring buffer (default: 2 seconds at input rate). Buff
 - **overflows**: Audio arrived faster than DSP consumed (data dropped)
 - **buffered**: Fill ratio (0.0 = empty, 1.0 = full)
 - **bufferedDurationSeconds**: Approximate seconds of audio queued in the buffer (`available` frames divided by the sample rate)
+- **highWatermark**: Highest observed buffer occupancy (frames) since the buffer was created
+- **highWatermarkDurationSeconds**: Equivalent peak time (`highWatermark` frames divided by the sample rate)
 
 When no audio is streaming, the transmitter falls back to the configured tone generator or silence.
diff --git a/internal/audio/stream.go b/internal/audio/stream.go
index 14ceac1..09f6de3 100644
--- a/internal/audio/stream.go
+++ b/internal/audio/stream.go
@@ -24,6 +24,7 @@ type StreamSource struct {
 	Underruns atomic.Uint64
 	Overflows atomic.Uint64
 	Written   atomic.Uint64
+	highWatermark atomic.Int64
 }
 
 // NewStreamSource creates a ring buffer with the given capacity (rounded up
@@ -54,6 +55,7 @@ func (s *StreamSource) WriteFrame(f Frame) bool {
 	s.ring[int(wp)&s.mask] = f
 	s.writePos.Add(1)
 	s.Written.Add(1)
+	s.updateHighWatermark()
 	return true
 }
 
@@ -114,11 +116,14 @@ func (s *StreamSource) Stats() StreamStats {
 	if s.size > 0 {
 		buffered = float64(available) / float64(s.size)
 	}
+	highWatermark := int(s.highWatermark.Load())
 	return StreamStats{
 		Available:               available,
 		Capacity:                s.size,
 		Buffered:                buffered,
 		BufferedDurationSeconds: s.bufferedDurationSeconds(available),
+		HighWatermark:           highWatermark,
+		HighWatermarkDurationSeconds: s.bufferedDurationSeconds(highWatermark),
 		Written:                 s.Written.Load(),
 		Underruns:               s.Underruns.Load(),
 		Overflows:               s.Overflows.Load(),
@@ -131,6 +136,8 @@ type StreamStats struct {
 	Capacity                int     `json:"capacity"`
 	Buffered                float64 `json:"buffered"`
 	BufferedDurationSeconds float64 `json:"bufferedDurationSeconds"`
+	HighWatermark           int     `json:"highWatermark"`
+	HighWatermarkDurationSeconds float64 `json:"highWatermarkDurationSeconds"`
 	Written                 uint64  `json:"written"`
 	Underruns               uint64  `json:"underruns"`
 	Overflows               uint64  `json:"overflows"`
@@ -143,6 +150,19 @@ func (s *StreamSource) bufferedDurationSeconds(available int) float64 {
 	return float64(available) / float64(s.SampleRate)
 }
 
+func (s *StreamSource) updateHighWatermark() {
+	available := s.Available()
+	for {
+		prev := s.highWatermark.Load()
+		if int64(available) <= prev {
+			return
+		}
+		if s.highWatermark.CompareAndSwap(prev, int64(available)) {
+			return
+		}
+	}
+}
+
 // --- StreamResampler ---
 
 // StreamResampler wraps a StreamSource and rate-converts from the stream's
diff --git a/internal/audio/stream_test.go b/internal/audio/stream_test.go
index 43fe0ee..2169e09 100644
--- a/internal/audio/stream_test.go
+++ b/internal/audio/stream_test.go
@@ -221,6 +221,29 @@ func TestStreamSource_StatsBufferedDuration(t *testing.T) {
 	}
 }
 
+func TestStreamSource_StatsHighWatermark(t *testing.T) {
+	rate := 44100
+	s := NewStreamSource(64, rate)
+	for i := 0; i < 12; i++ {
+		s.WriteFrame(NewFrame(0, 0))
+	}
+	for i := 0; i < 5; i++ {
+		s.ReadFrame()
+	}
+	stats := s.Stats()
+	if stats.HighWatermark != 12 {
+		t.Fatalf("expected high watermark 12, got %d", stats.HighWatermark)
+	}
+	expected := float64(stats.HighWatermark) / float64(rate)
+	if math.Abs(stats.HighWatermarkDurationSeconds-expected) > 1e-9 {
+		t.Fatalf("high watermark duration %.9f != %.9f", stats.HighWatermarkDurationSeconds, expected)
+	}
+	if stats.HighWatermark < stats.Available {
+		t.Fatalf("high watermark %d < available %d", stats.HighWatermark, stats.Available)
+	}
+}
+
+
 // --- StreamResampler tests ---
 
 func TestStreamResampler_1to1(t *testing.T) {