chmod Calculator

Remembering whether 644 means rw-r--r-- or rwxr-xr-x is the kind of trivia that fades the moment you don't use it for two weeks.

Click checkboxes for each permission (read, write, execute) on each scope (owner, group, others) and the octal value updates as you go. Or paste an octal like 755 and see the symbolic equivalent. Special bits (setuid, setgid, sticky) are supported — those are the four-digit modes like 1755 or 4755.

The symbolic notation chmod accepts (u+x, go-w, a=rx) is also generated, since some teams prefer that form in scripts because it's more obviously what it does. The default umask context is shown so you can predict what permissions a newly created file will have.

The octal notation packs three permission bits into each digit. Read = 4, write = 2, execute = 1, summed for each scope. So 7 = read+write+execute (rwx), 6 = read+write (rw-), 5 = read+execute (r-x), 4 = read-only (r--). Three digits give you owner, group, others — 755 means owner has rwx, group has r-x, others have r-x. The fourth (leading) digit covers special bits: setuid (4), setgid (2), sticky (1). 1755 means rwxr-xr-x with the sticky bit set.

The pain this exists to address: most developers learn 644 and 755 as magic numbers and stop. Then a CI script needs setgid for shared directories, or an SSH key needs 600, and the choice between 'guess and check' or 'read the chmod manpage' is a productivity tax. The calculator skips that — click what you want, paste what you have, see the equivalence. The bidirectional translation matters because a lot of legacy advice still gives octal codes without explaining them.

Worked example: you've cloned a repository where some scripts won't execute. You see permissions of -rw-r--r-- on the file (mode 644). You need execute permission for the owner. Mode 744 adds owner-execute (-rwxr--r--) but doesn't help if the script needs to be runnable by your CI user too. Mode 755 (-rwxr-xr-x) gives owner full and everyone else read+execute, which is the standard for executable scripts. The calculator shows you both options and the difference.

Where the model breaks: NTFS and APFS have richer permission models than the Unix octal triple supports. macOS works via the Unix layer for command-line use, but Finder shows additional ACLs that chmod can't represent. WSL on Windows fakes Unix permissions on top of NTFS, with quirks around files created from Windows side. The octal model is correct for ext4, xfs, btrfs, zfs, hfs+; it's a leaky abstraction on the others. Don't trust the calculator's output as final on those filesystems without verifying with stat or getfacl.