diff --git a/src/bytes/buffer.go b/src/bytes/buffer.go
index 46ca1d5ad3f..7f9139bca84 100644
--- a/src/bytes/buffer.go
+++ b/src/bytes/buffer.go
@@ -56,6 +56,10 @@ func (b *Buffer) String() string {
 // b.Len() == len(b.Bytes()).
 func (b *Buffer) Len() int { return len(b.buf) - b.off }
 
+// Cap returns the capacity of the buffer's underlying byte slice, that is, the
+// total space allocated for the the buffer's data.
+func (b *Buffer) Cap() int { return cap(b.buf) }
+
 // Truncate discards all but the first n unread bytes from the buffer.
 // It panics if n is negative or greater than the length of the buffer.
 func (b *Buffer) Truncate(n int) {
diff --git a/src/bytes/buffer_test.go b/src/bytes/buffer_test.go
index 75145b05e92..7de17ae47e2 100644
--- a/src/bytes/buffer_test.go
+++ b/src/bytes/buffer_test.go
@@ -231,6 +231,23 @@ func TestMixedReadsAndWrites(t *testing.T) {
 	empty(t, "TestMixedReadsAndWrites (2)", &buf, s, make([]byte, buf.Len()))
 }
 
+func TestCapWithPreallocatedSlice(t *testing.T) {
+	buf := NewBuffer(make([]byte, 10))
+	n := buf.Cap()
+	if n != 10 {
+		t.Errorf("expected 10, got %d", n)
+	}
+}
+
+func TestCapWithSliceAndWrittenData(t *testing.T) {
+	buf := NewBuffer(make([]byte, 0, 10))
+	buf.Write([]byte("test"))
+	n := buf.Cap()
+	if n != 10 {
+		t.Errorf("expected 10, got %d", n)
+	}
+}
+
 func TestNil(t *testing.T) {
 	var b *Buffer
 	if b.String() != "<nil>" {
diff --git a/src/bytes/export_test.go b/src/bytes/export_test.go
index 3b915d5ead8..f61523e60bb 100644
--- a/src/bytes/export_test.go
+++ b/src/bytes/export_test.go
@@ -7,7 +7,3 @@ package bytes
 // Export func for testing
 var IndexBytePortable = indexBytePortable
 var EqualPortable = equalPortable
-
-func (b *Buffer) Cap() int {
-	return cap(b.buf)
-}