diff --git a/bitmap_opt.go b/bitmap_opt.go
index a36431c..91188aa 100644
--- a/bitmap_opt.go
+++ b/bitmap_opt.go
@@ -666,3 +666,17 @@ func (dst *Bitmap) CompareNumKeys(src *Bitmap) int {
 		return 0
 	}
 }
+
+func (ra *Bitmap) LenInBytes() int {
+	if ra == nil {
+		return 0
+	}
+	return len(ra.data) * 2
+}
+
+func (ra *Bitmap) capInBytes() int {
+	if ra == nil {
+		return 0
+	}
+	return cap(ra.data) * 2
+}
diff --git a/bitmap_opt_test.go b/bitmap_opt_test.go
index aa5c569..7bb2555 100644
--- a/bitmap_opt_test.go
+++ b/bitmap_opt_test.go
@@ -924,6 +924,59 @@ func TestCompareNumKeys(t *testing.T) {
 	})
 }
 
+func TestLenBytes(t *testing.T) {
+	t.Run("non-nil bitmap", func(t *testing.T) {
+		bm := NewBitmap()
+
+		for _, x := range []int{1, 1 + maxCardinality, 1 + maxCardinality*2} {
+			bm.Set(uint64(x))
+
+			require.Equal(t, len(bm.ToBuffer()), bm.LenInBytes())
+		}
+	})
+
+	t.Run("empty bitmap", func(t *testing.T) {
+		bm := NewBitmap()
+
+		// real length is greater then 0, though ToBuffer() returns empty slice
+		require.Less(t, 0, bm.LenInBytes())
+	})
+
+	t.Run("nil bitmap", func(t *testing.T) {
+		var bm *Bitmap
+
+		require.Equal(t, 0, bm.LenInBytes())
+	})
+}
+
+func TestCapBytes(t *testing.T) {
+	t.Run("non-nil bitmap", func(t *testing.T) {
+		bm := NewBitmap()
+
+		for _, x := range []int{1, 1 + maxCardinality, 1 + maxCardinality*2} {
+			bm.Set(uint64(x))
+
+			// ToBuffer() sets cap to len, real cap is >= than buffer's one
+			require.LessOrEqual(t, cap(bm.ToBuffer()), bm.capInBytes())
+			require.LessOrEqual(t, bm.LenInBytes(), bm.capInBytes())
+		}
+	})
+
+	t.Run("empty bitmap", func(t *testing.T) {
+		bm := NewBitmap()
+
+		// real cap is greater than 0, though ToBuffer() returns empty slice
+		require.Less(t, 0, bm.capInBytes())
+		require.LessOrEqual(t, bm.LenInBytes(), bm.capInBytes())
+	})
+
+	t.Run("nil bitmap", func(t *testing.T) {
+		var bm *Bitmap
+
+		require.Equal(t, 0, bm.capInBytes())
+	})
+}
+
 func TestMergeToSuperset(t *testing.T) {
 	run := func(t *testing.T, bufs [][]uint16) {
 		containerThreshold := uint64(math.MaxUint16 + 1)