diff --git a/len.go b/len.go
index 21f4746..cd33d9e 100644
--- a/len.go
+++ b/len.go
@@ -2,6 +2,8 @@ package unibase2n
 
 import "math/bits"
 
+const powtab = "\x00\x04\x03\x00\x02\x00\x00\x00\x01"
+
 // EncodeLen calculate encode length
 //    offset will be appended as til+offset at last
 func (bs Base) EncodeLen(in int) (out, offset int) {
@@ -9,7 +11,7 @@ func (bs Base) EncodeLen(in int) (out, offset int) {
 		return 0, 0
 	}
 	if bits.OnesCount8(bs.bit) == 1 { // 2的幂永无offset (1/2/4/8)
-		return in * (16 >> "\x00\x00\x01\x00\x02\x00\x00\x00\x03"[bs.bit]), 0
+		return in << powtab[bs.bit], 0
 	}
 	if bs.bit == 6 { // 特判 (6) 3*8=4*6
 		out = in / 3 * 4 * 2
@@ -52,3 +54,41 @@ func (bs Base) EncodeLen(in int) (out, offset int) {
 	out += (remain/int(bs.bit) + 1 + 1) * 2
 	return
 }
+
+// DecodeLen calculate decode length
+//    offset must be legal or out will <= 0
+func (bs Base) DecodeLen(in, offset int) (out int) {
+	if in <= 0 || offset < 0 {
+		return 0
+	}
+	if bits.OnesCount8(bs.bit) == 1 { // 2的幂永无offset (1/2/4/8)
+		return in >> powtab[bs.bit]
+	}
+	if bs.bit == 6 { // 特判 (6) 3*8=4*6
+		if offset != 0 {
+			in -= (offset + 1) * 2
+		}
+		return (in>>3)*3 + offset
+	}
+	if bs.bit < 8 { // (3/5/7)
+		if offset != 0 {
+			remain := offset * 8
+			in -= (remain/int(bs.bit) + 1) * 2
+		}
+		return (in>>4)*int(bs.bit) + offset
+	}
+	if bs.bit%2 == 0 { // bit 大于8且可被2整除 (10/12/14)
+		hb := int(bs.bit) >> 1
+		if offset != 0 {
+			remain := offset * 8
+			in -= (remain/int(bs.bit) + 1) * 2
+		}
+		return (in>>4)*hb + offset
+	}
+	// (9/11/13/15)
+	if offset != 0 {
+		remain := offset * 8
+		in -= (remain/int(bs.bit) + 1) * 2
+	}
+	return (in>>5)*int(bs.bit) + offset
+}
diff --git a/len_test.go b/len_test.go
new file mode 100644
index 0000000..ff3afd9
--- /dev/null
+++ b/len_test.go
@@ -0,0 +1,15 @@
+package unibase2n
+
+import "testing"
+
+func TestEnDecodeLen(t *testing.T) {
+	bs := Base{bit: 1}
+	for ; bs.bit < 16; bs.bit++ {
+		for i := 1; i <= 65536; i++ {
+			in := bs.DecodeLen(bs.EncodeLen(i))
+			if i != in {
+				t.Fatal("bit:", bs.bit, "in:", i, "!= out:", in)
+			}
+		}
+	}
+}