Decimal to Hex Converter: Exact, Any Length

1. Type or paste the decimal number. Spaces and underscores between digits are ignored, and a leading minus sign is kept.
2. Read the hexadecimal result at the top. Turn on Upper case if you need FF rather than ff.
3. Add a 0x prefix or a # yourself when you paste the result into code or a stylesheet; the converter gives the bare digits so they fit any notation.
4. For a negative number that should be stored as a fixed-width integer, read the Two's complement row with a width of 8, 16, 32 or 64 bits. Each group of four bits is one hex digit.

Divide the number by 16 and write down the remainder as a hex digit, using a to f for 10 to 15. Then divide the quotient again until it is 0, and read the remainders from last to first. For 755: 755 ÷ 16 = 47 remainder 3, 47 ÷ 16 = 2 r 15 (f), 2 ÷ 16 = 0 r 2. Reading upwards gives 2f3.

The page uses arbitrary-precision integers rather than floating point, so values past 2 to the power of 53, the limit for ordinary JavaScript and spreadsheet numbers, still convert exactly. The 39-digit number 340,282,366,920,938,463,463,374,607,431,768,211,455 becomes thirty-two f digits with no rounding. Inputs of up to 10,000 digits are accepted.

Two hex digits make one byte. The number 4,294,967,295 is ffffffff, four bytes with every bit set. Colours in CSS use one byte per channel, so the decimal channel values 255, 136 and 0 become ff, 88 and 00, or #ff8800.