Does my data get uploaded anywhere?
No. Everything runs locally in your browser and nothing is sent to a server.
Is this free to use?
Yes. It is completely free, with no account, no signup, and no usage limits.
Does it work on a phone?
Yes. The page runs in any modern mobile browser, on iPhone and Android alike, with no app to install.
Why does the code 9731 not work?
Because it is a Unicode code point, not a byte. Bytes only go up to 255, so the tool rejects anything larger. The snowman at U+2603 is stored as three UTF-8 bytes, 226 152 131, and pasting those decodes correctly.
What does 72 105 decode to?
Hi. 72 is capital H and 105 is lowercase i. Both are inside the 0 to 127 ASCII range, so they map one byte to one character with no UTF-8 complication.
Can I paste comma-separated codes?
Yes. Tokens are split on any run of whitespace or commas, so 72,105 and 72 105 and a column with one code per line all work identically. That makes it easy to paste a byte array straight out of code.
What happens with a value like 200 on its own?
It decodes to the replacement character, because 200 is a UTF-8 lead byte that expects continuation bytes after it and there are none. The decoder does not error, it substitutes U+FFFD, which is the standard behaviour for invalid input.
Is the whole 0 to 255 range ASCII?
No. ASCII is 0 to 127 only, seven bits. The range 128 to 255 is the second half of a byte, and what it means depends entirely on the encoding. This tool treats it as UTF-8, so those values only make sense in valid multi-byte sequences.