Chmod Calculator

Calculate chmod permissions without memorising the octal table

Unix file permissions are simple in principle and fiddly in practice. Every file and directory carries three sets of permissions — for the owner, the group and everyone else — and each set can allow reading, writing and executing. The chmod command expresses that as a three-digit octal number like 755, which is fast to type once you know it and baffling until you do. This calculator turns the guesswork into a grid: tick the boxes for the access you want and read off the octal value, the symbolic rwxr-xr-x string and a ready-to-run command. Type an octal number instead and the grid fills in, so it works as both a builder and a decoder.

How the numbers work

Each permission has a value: read is 4, write is 2 and execute is 1. Add together the ones you want for a single class and you get that class's digit, from 0 (no access) to 7 (full access). Read plus write is 6; read plus execute is 5; all three is 7. Line up the digits for owner, group and other and you have the full mode — 640 means the owner can read and write, the group can read, and others get nothing. The symbolic notation says the same thing with letters, which is why chmod output and ls -l listings both show strings like -rw-r--r--.

The modes you actually reach for

A handful of values cover most real work. Use 644 for ordinary files that should be world-readable but only owner-writable, such as configs and web assets. Use 755 for directories and for scripts or binaries that need the execute bit. Use 600 for private files like SSH keys and environment files that no one else should read, and 700 for private directories. Reach for 664 and 775 only when a shared group genuinely needs write access, and treat 777 as a smell rather than a solution — it is almost always a sign that ownership, not permission, is the real problem.

Permissions, ownership and the wider toolkit

Mode is only half the story: chmod sets what each class may do, while chown decides who the owner and group actually are. A file that stubbornly refuses access often needs the right owner, not looser bits. When you are scripting these changes, the Cron Expression Builder helps schedule the jobs that run them, and the SQL Snippet Library is handy when the same deploy script also touches a database. For encoding the file contents you are protecting, the Base64 Encoder & Decoder and Hash Generator round out a typical shell-scripting session.

A note on the execute bit

The execute bit means two different things depending on what it is attached to. On a regular file it controls whether the file can be run as a program or script. On a directory it controls whether you can enter the directory and access the files inside it — without execute on a folder, even readable files within become unreachable. That is why directories are almost always 755 or 700 rather than 644: dropping the execute bit on a folder locks everyone, including the owner, out of its contents.

Further reading

This calculator pairs with a short series on permissions: chmod 755 vs 644 compares the two most common modes, chmod -R covers changing a whole directory tree safely and octal vs symbolic notation explains the two ways to write a mode.