CHMOD Calculator
Unix permissions in all three notations at once — nine checkboxes, the octal number and the rwx string — with every edit updating the other two. 755 stops being folklore and becomes something you can read.
Nine bits, three spellings
The permission system is just nine bits: read, write, execute — for owner, group and everybody else. Octal packs each triple into a digit; the symbolic form spells the same bits as letters. Neither is the "real" one, which is why this page keeps all three views live from a single source of truth: click a box and watch the digit move.
The command line at the bottom is ready to paste — chmod with your octal and a file placeholder — because the number is rarely the goal; the changed file is.
Frequently asked questions
How does 755 decode?
Each digit is one audience, each digit is read+write+execute as 4+2+1. So 7 = 4+2+1 = rwx for the owner, and each 5 = 4+1 = r-x for group and others: everyone can read and enter, only the owner writes. 644 is the same logic without execute — the standard for files rather than directories.
Why do directories need execute?
On a directory, x means 'may enter and traverse'. A directory with r but no x lets you list names yet open nothing inside — one of Unix's classic confusions. Hence 755 for directories and 644 for files as the web-server default pair.
When is 777 wrong?
Almost always. It grants every local user write access — on shared hosting that includes other tenants' compromised scripts. If something 'only works with 777', the real problem is the file's owner, and chown is the fix that does not open the door.
What about setuid, setgid and the sticky bit?
They are the optional fourth digit (4755, 2755, 1777) and deliberately out of scope here: each one changes program behaviour, not just access, and copy-pasting them without understanding is how privilege-escalation bugs are born. The nine ordinary bits cover the everyday cases.
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