Chmod Calculator

How to use it

1. Check read / write / execute for each of owner, group and others — each checkbox is worth 4, 2 and 1.
2. Add special bits when you need them: setuid (4), setgid (2) or sticky (1), shown as the fourth octal digit.
3. Read the result as an octal number (for chmod 644 file) and as a symbolic string (as ls -l shows it).
4. Use the presets row to jump to common modes like 755, 644 or 600.

Common use cases

• Web server files — 644 for files and 755 for directories is the usual starting point before locking down.
• Key material — private keys want 600 so only the file owner can read them.
• Shared directories — 775 plus group ownership for teams that need to write together.
• Learning permissions — toggle one bit at a time and watch how octal digits and the symbolic string move together.

Why it stays private

The calculator is pure local logic: checkboxes, arithmetic and text output. Nothing is sent, so it also works on machines where you would rather not install anything to inspect permissions.

FAQ

Q: What does the uppercase S or T mean? A: A special bit is set while its execute bit is not. rwSr-xr-x has setuid but no owner-executable bit — usually a configuration mistake worth checking.

Q: Why does setgid on a directory matter? A: On directories, setgid makes new files inherit the directory's group instead of the creator's primary group, which is how shared team folders keep consistent ownership.

Q: Do I need the extra first digit for normal permissions? A: No. chmod 644 and chmod 0644 are the same; the four-digit form is only needed when a special bit is set, and even then many shells accept three digits plus a separate u+s style command.