Chmod Calculator

The Chmod Calculator converts Unix file permissions between three representations at once — a checkbox grid, symbolic notation (like `rwxr-xr-x`) and octal notation (like `755`) — so changing any one updates the other two instantly. It's the tool for anyone who can recognize `chmod 755` but doesn't want to mentally re-derive what each digit means every time.

It's used for figuring out the correct chmod command before running it on a server (getting file permissions wrong is a classic way to either lock yourself out or accidentally make something world-writable), understanding what a permissions string like `-rwxr-xr--` from an `ls -l` listing actually grants to the file's owner, group and everyone else, and teaching or learning how Unix permissions work by directly seeing how each checkbox maps to a bit in the octal value.

Unix permissions are organized into three groups — owner, group and others — each with three possible permissions: read, write and execute. Each permission maps to a bit value (read = 4, write = 2, execute = 1), and the three bits for a group sum into a single octal digit 0-7, which is why `755` breaks down as owner=7 (4+2+1, full access), group=5 (4+0+1, read and execute), others=5 (4+0+1, read and execute) — the single most common permission setting for scripts and executable files that need to run but shouldn't be edited by anyone else.

Symbolic notation writes the same information as three characters per group — r, w, x if the permission is granted, a dash if not — read left to right as owner, group, then others, so `rwxr-xr--` is immediately readable as 'owner: full access, group: read and execute, others: read only'. A handful of common presets (644 for a typical file, 755 for a typical script or directory, 600 for a private file only its owner can touch) are one click away for the settings that come up constantly. Everything computes instantly in your browser — there's no file involved, just the permission math.