Chmod Calculator

Toggle Unix permissions and get octal (755) and symbolic (rwxr-xr-x) values.

The three octal digits are owner, group, and everyone else, and each is a sum of read as 4, write as 2, and execute as 1. Once you see it as bits rather than a number, 755 stops needing to be memorised: 7 is all three, 5 is read and execute without write.

Execute means something different on a directory, and this is where permissions bite. On a file it means runnable; on a directory it means traversable, so a directory that is readable but not executable lets you list the names inside and open none of them.

The values worth knowing by heart are few. 644 for a normal file, 755 for a directory or a script, and 600 for anything holding a secret — an SSH private key with looser permissions is refused outright, which is the one time the system stops you rather than letting you make the mistake.