Chmod Calculator

What this does

Calculates Unix/Linux file permissions in both octal (644) and symbolic (rw-r--r--) form. Toggle the read/write/execute checkboxes for owner, group, and other (world) — both representations update live. Or paste an octal value and the checkboxes flip to match.

The output also gives you the exact chmod command to run, plus a recursive find variant that’s safer than chmod -R for mixed file/directory trees (you almost always want different perms on directories vs files — typically 755 on dirs and 644 on files).

Why two formats exist

Octal (644, 755, 777) is compact but opaque if you don’t have the bit math memorized. Symbolic (rwxr-xr-x) is readable but verbose and harder to type. Both encode the same nine bits — read/write/execute for owner, group, and other — plus three special bits (setuid, setgid, sticky).

The math: read = 4, write = 2, execute = 1. Add them up per role. Owner can read+write = 4+2 = 6. Group can read = 4. Other can read = 4. Result: 644.

What 644, 755, 600, 700, 777 actually mean

• 644 — owner reads/writes, everyone else reads only. Default for files: source code, configs, HTML.
• 755 — owner full access, everyone else can read and execute. Default for directories and executables.
• 600 — owner reads/writes, no one else has any access. SSH private keys, secrets.
• 700 — owner full access, no one else has any access. SSH config dirs (~/.ssh).
• 777 — everyone has full access. Almost always wrong. Use only for /tmp and shared scratch directories. If you’re typing this to “fix permissions”, you almost certainly want 755 or 600.
• 400 — owner read-only. Useful for read-only secrets the owner shouldn’t accidentally modify.

Special bits, briefly

• setuid (4xxx): when an executable runs, it runs as the file’s owner instead of the user invoking it. passwd uses this so non-root users can edit /etc/shadow. Dangerous on writable binaries.
• setgid (2xxx): on executables, runs as the file’s group. On directories, new files inherit the directory’s group instead of the creator’s primary group. Common on shared team directories.
• sticky (1xxx): only the file’s owner can delete or rename files in the directory, even if others have write access. /tmp is the canonical example — you can write to /tmp but you can’t delete other users’ files.

Common mistakes this tool catches

• Confusing 644 with 0644. They’re the same. The leading zero just denotes octal in C-style notation. Either works in chmod.
• Octal 777 vs 7777. Plain 777 is rwxrwxrwx; 7777 adds setuid + setgid + sticky on top. Big difference.
• chmod -R 777 on a project directory. Makes every script executable and every config world-writable. Use the find variant: find . -type f -exec chmod 644 {} \; then find . -type d -exec chmod 755 {} \;.

Privacy

The tool runs entirely in your browser. No values leave your machine.

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