Bitmask Calculator

How to use

1. 1Choose Decode a value to break a number into flags, or Encode from flags to build a number from options.
2. 2Pick a flag set (Unix permissions, open() flags, a regex options enum, or generic numbered bits) and a bit width. The width adapts to the chosen set.
3. 3In decode mode, select the input base and type the value (a 0b, 0o, or 0x prefix is detected automatically). In encode mode, tick the flags or bits you want.
4. 4Read the bit layout grid and the value in binary, hex, octal, and decimal, plus the list of set bits with their weights and names.
5. 5Use Copy on any row or Copy summary for the full breakdown. Everything runs in your browser.

About this tool

Bitmask Calculator answers the two questions a generic bitwise calculator does not: given a packed integer, which bits are set and what do they mean, and given a set of options, what single number combines them. The bitwise calculator operates on two operands; this tool operates on one value and a list of named flags, which is the job you actually have when an API, a struct, a permission mode, or a [Flags] enum hands you an opaque number. Decode mode takes a value in binary, octal, decimal, or hex (the base is auto-detected from a 0b, 0o, or 0x prefix) and reports the exact set bit positions, their decimal weights as powers of two, and the named flag each one maps to. Encode mode flips the work around: tick the flags or individual bits you want and the combined value is shown live in all four bases, so you can read off the integer to pass back. A bit layout grid renders the value most-significant bit first with byte boundaries marked and set bits highlighted, so a 32-bit or 64-bit mask is readable at a glance. The value is held as a BigInt, so widths up to 64 bits stay exact where a plain JavaScript number would lose precision past 53 bits. Built-in flag sets cover Unix file permission mode bits (the 12 bits behind chmod, including setuid, setgid, and the sticky bit), common POSIX open() flags, a regular-expression options enum, and a generic numbered bit set for any unlabeled mask, and the bit width adapts to whichever set you choose. The tool also explains the arithmetic: flags are powers of two so combining them with OR is the same as summing their weights, testing a flag is an AND, clearing one is an AND with the complement, and toggling one is an XOR. Everything runs in your browser with BigInt math, so the values and flags you work with are never uploaded.

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