Two's Complement Calculator

How to use

1. 1Pick a bit width: 4, 8, 16, 32, 64, or 128. The width determines the signed range, the sign-bit position, and the padding of the hex and octal output.
2. 2Choose an input mode. Signed decimal accepts negative numbers, Unsigned decimal accepts non-negative numbers, and Binary / hex accepts a raw bit pattern with optional 0b, 0x, or 0o prefixes.
3. 3Type or paste the value. The bit grid, signed value, unsigned value, hex, and octal forms update instantly.
4. 4Click any cell in the bit grid to flip that bit. The sign bit is highlighted in red so you can see how toggling it swings the signed reading.
5. 5Use the copy buttons next to each result to copy a single value, or use Copy summary to grab all four formats at once.

About this tool

Two's Complement Calculator turns any signed integer into the bit pattern a CPU stores for it and decodes any raw bit pattern back to the signed and unsigned values it represents. Three input modes cover the cases that actually come up at a debugger, in a textbook, or in a hex dump. Signed decimal accepts any integer between the chosen width's signed minimum and signed maximum (for 8-bit: -128 to 127; for 32-bit: -2,147,483,648 to 2,147,483,647) and emits the canonical two's-complement bit pattern by computing value AND ((1 << width) - 1) in BigInt. Unsigned decimal accepts non-negative integers up to 2^width - 1 and shows what those bits read as when reinterpreted as a signed integer, which is how a debugger's signed and unsigned views diverge for any value above the sign-bit threshold. Binary, hex, and octal mode parses bit patterns directly: 0b prefixes for binary, 0x for hex, 0o for octal, or a bare 0/1 string treated as binary; underscores, commas, and whitespace are stripped so you can paste a grouped value from a debugger. Six bit widths are supported: 4, 8, 16, 32, 64, and 128, which line up with C's int8_t through __int128 and Rust's i8 through i128. All arithmetic uses BigInt, so 128-bit results are bit-exact rather than rounded through a 64-bit float. Each result shows the full bit pattern grouped into nibbles with bit indices, the signed (two's-complement) value, the unsigned value, the hexadecimal form padded to the natural width, the octal form padded similarly, and the signed and unsigned range for the selected width. The bit grid is interactive: clicking any cell flips that bit and reruns the rest of the conversion, useful for seeing exactly how flipping the sign bit changes the value or how setting bits 8 through 15 changes a 32-bit word. When the input is a negative signed value, a derivation panel shows the classic invert-and-add-one procedure step by step. A reference table at the bottom of the page summarises the signed minimum, signed maximum, and unsigned maximum for every supported width so you can use the page as a quick lookup. Everything runs in your browser; the values you type are never uploaded.

Free to use. Works in your browser. No signup, no login.