diff --git a/src/encoding/base64/base64.go b/src/encoding/base64/base64.go
index 6aa8a15bdc8..87f68970627 100644
--- a/src/encoding/base64/base64.go
+++ b/src/encoding/base64/base64.go
@@ -278,7 +278,7 @@ func NewEncoder(enc *Encoding, w io.Writer) io.WriteCloser {
 // of an input buffer of length n.
 func (enc *Encoding) EncodedLen(n int) int {
 	if enc.padChar == NoPadding {
-		return (n*8 + 5) / 6 // minimum # chars at 6 bits per char
+		return n/3*4 + (n%3*8+5)/6 // minimum # chars at 6 bits per char
 	}
 	return (n + 2) / 3 * 4 // minimum # 4-char quanta, 3 bytes each
 }
@@ -623,7 +623,7 @@ func NewDecoder(enc *Encoding, r io.Reader) io.Reader {
 func (enc *Encoding) DecodedLen(n int) int {
 	if enc.padChar == NoPadding {
 		// Unpadded data may end with partial block of 2-3 characters.
-		return n * 6 / 8
+		return n/4*3 + n%4*6/8
 	}
 	// Padded base64 should always be a multiple of 4 characters in length.
 	return n / 4 * 3
diff --git a/src/encoding/base64/base64_test.go b/src/encoding/base64/base64_test.go
index 0ad88ebb3a7..97aea845ae3 100644
--- a/src/encoding/base64/base64_test.go
+++ b/src/encoding/base64/base64_test.go
@@ -9,8 +9,10 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
 	"reflect"
 	"runtime/debug"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -262,11 +264,12 @@ func TestDecodeBounds(t *testing.T) {
 }
 
 func TestEncodedLen(t *testing.T) {
-	for _, tt := range []struct {
+	type test struct {
 		enc  *Encoding
 		n    int
-		want int
-	}{
+		want int64
+	}
+	tests := []test{
 		{RawStdEncoding, 0, 0},
 		{RawStdEncoding, 1, 2},
 		{RawStdEncoding, 2, 3},
@@ -278,19 +281,30 @@ func TestEncodedLen(t *testing.T) {
 		{StdEncoding, 3, 4},
 		{StdEncoding, 4, 8},
 		{StdEncoding, 7, 12},
-	} {
-		if got := tt.enc.EncodedLen(tt.n); got != tt.want {
+	}
+	// check overflow
+	switch strconv.IntSize {
+	case 32:
+		tests = append(tests, test{RawStdEncoding, (math.MaxInt-5)/8 + 1, 357913942})
+		tests = append(tests, test{RawStdEncoding, math.MaxInt/4*3 + 2, math.MaxInt})
+	case 64:
+		tests = append(tests, test{RawStdEncoding, (math.MaxInt-5)/8 + 1, 1537228672809129302})
+		tests = append(tests, test{RawStdEncoding, math.MaxInt/4*3 + 2, math.MaxInt})
+	}
+	for _, tt := range tests {
+		if got := tt.enc.EncodedLen(tt.n); int64(got) != tt.want {
 			t.Errorf("EncodedLen(%d): got %d, want %d", tt.n, got, tt.want)
 		}
 	}
 }
 
 func TestDecodedLen(t *testing.T) {
-	for _, tt := range []struct {
+	type test struct {
 		enc  *Encoding
 		n    int
-		want int
-	}{
+		want int64
+	}
+	tests := []test{
 		{RawStdEncoding, 0, 0},
 		{RawStdEncoding, 2, 1},
 		{RawStdEncoding, 3, 2},
@@ -299,8 +313,18 @@ func TestDecodedLen(t *testing.T) {
 		{StdEncoding, 0, 0},
 		{StdEncoding, 4, 3},
 		{StdEncoding, 8, 6},
-	} {
-		if got := tt.enc.DecodedLen(tt.n); got != tt.want {
+	}
+	// check overflow
+	switch strconv.IntSize {
+	case 32:
+		tests = append(tests, test{RawStdEncoding, math.MaxInt/6 + 1, 268435456})
+		tests = append(tests, test{RawStdEncoding, math.MaxInt, 1610612735})
+	case 64:
+		tests = append(tests, test{RawStdEncoding, math.MaxInt/6 + 1, 1152921504606846976})
+		tests = append(tests, test{RawStdEncoding, math.MaxInt, 6917529027641081855})
+	}
+	for _, tt := range tests {
+		if got := tt.enc.DecodedLen(tt.n); int64(got) != tt.want {
 			t.Errorf("DecodedLen(%d): got %d, want %d", tt.n, got, tt.want)
 		}
 	}