Chmod Calculator

Each permission has a value — read 4, write 2, execute 1 — and each digit is the sum for one audience: owner, group, others, in that order. rwxr-xr-x is (4+2+1)(4+1)(4+1) = 755. Once you see the digits as sums, every permission string reads instantly.

644 (rw-r--r--) for regular files: you edit, everyone reads. 755 (rwxr-xr-x) for anything that must be entered or run — directories and scripts — because execute on a directory means permission to enter it. That pair covers the vast majority of a web server's filesystem.

It grants every user on the system write and execute — meaning any compromised account or process can modify your file and have those modifications run. When 777 'fixes' something, the real issue is almost always ownership (fix with chown) rather than permissions; 755 plus correct ownership solves it without the hole.

The leading digit holds special bits: setuid (4), setgid (2), and sticky (1). They're power tools with security implications — setuid executables run as their owner regardless of who launches them — and worth understanding before using, which is why this calculator sticks to the everyday three digits.