diff --git a/library/base64.c b/library/base64.c
index 16c254d..8c99538 100644
--- a/library/base64.c
+++ b/library/base64.c
@@ -85,15 +85,16 @@
         return( 0 );
     }
 
-    n = ( slen << 3 ) / 6;
+    n = slen / 3 + ( slen % 3 != 0 );
 
-    switch( ( slen << 3 ) - ( n * 6 ) )
+    if( n > ( SIZE_T_MAX - 1 ) / 4 )
     {
-        case  2: n += 3; break;
-        case  4: n += 2; break;
-        default: break;
+        *olen = SIZE_T_MAX;
+        return( MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL );
     }
 
+    n *= 4;
+
     if( dlen < n + 1 )
     {
         *olen = n + 1;