This is plain decoding of a trusted string: utf16 decode if (unit <= 0xD7FF || unit >= 0xE000) /* one unit */; else /* two unit

This is plain decoding of a trusted string: utf16 decode if (unit <= 0xD7FF || unit >= 0xE000) /* one unit */; else /* two units */ utf-8 decode if (unit < 0xE0) { if (unit < 0xC0) /* one unit */ else /* two units */ } else if (unit < 0xF0) /* three units */ else /* four units */ UTF-16 will take one comparison for all characters up to 0xD7FF; UTF-8 normally takes two. Same when encoding: utf16 encode if (value <= 0xFFFF) /* one unit */ else /* two units */ utf8 encode if (value <= 0x7FF) { if (value <= 0x7F) /* one unit */ else /* two units */ } else if (value <= 0xFFFF) /* three units */ else /* four units */ (We can tune UTF-8 it to take one comparison for ASCII if we expect mostly ASCII.) Things get even more complex when we are to read an untrusted string. In UTF-8 we have five byte types, invalid bytes, conditionally invalid bytes, and must recognize surrogates (as errors). In UTF-16 all 16-bit units are valid and there are only three unit types: character or surrogate, high or low. I once wrote a decoder for UTF-8/16 and here's my stats: UTF-8 : 13 byte types, 13 states, 10 actions UTF-16LE : 3 byte types, 4 states, 5 actions