diff --git a/http.go b/http.go
index 1b2b92c..eb487dd 100644
--- a/http.go
+++ b/http.go
@@ -274,7 +274,7 @@ func readBodyFixedSize(r *bufio.Reader, n int, buf []byte) ([]byte, error) {
 	bufLen := len(buf)
 	bufCap := bufLen + n
 	if cap(buf) < bufCap {
-		b := make([]byte, bufLen, bufCap)
+		b := make([]byte, bufLen, round2(bufCap))
 		copy(b, buf)
 		buf = b
 	}
@@ -349,3 +349,16 @@ func parseChunkSize(r *bufio.Reader) (int, error) {
 	}
 	return n, nil
 }
+
+func round2(n int) int {
+	if n <= 0 {
+		return 0
+	}
+	n--
+	x := uint(0)
+	for n > 0 {
+		n >>= 1
+		x++
+	}
+	return 1 << x
+}
diff --git a/http_test.go b/http_test.go
index 6740ec2..c618b65 100644
--- a/http_test.go
+++ b/http_test.go
@@ -10,6 +10,25 @@ import (
 	"time"
 )
 
+func TestRound2(t *testing.T) {
+	testRound2(t, 0, 0)
+	testRound2(t, 1, 1)
+	testRound2(t, 2, 2)
+	testRound2(t, 3, 4)
+	testRound2(t, 4, 4)
+	testRound2(t, 5, 8)
+	testRound2(t, 7, 8)
+	testRound2(t, 8, 8)
+	testRound2(t, 9, 16)
+	testRound2(t, 0x10001, 0x20000)
+}
+
+func testRound2(t *testing.T, n, expectedRound2 int) {
+	if round2(n) != expectedRound2 {
+		t.Fatalf("Unexpected round2(%d)=%d. Expected %d", n, round2(n), expectedRound2)
+	}
+}
+
 func TestResponseReadTimeout(t *testing.T) {
 	resp := &Response{}