Chmod Calculator
Tick the boxes, get octal, symbolic and the command. Includes setuid, setgid and sticky.
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Worked example
755 is owner read/write/execute, everyone else read/execute — the standard for directories and executables. 644 is owner read/write, everyone else read — the standard for regular files.
Unix permissions are three groups of three bits: read, write, execute, for owner, group and other. Each group is one octal digit, which is why 755 is three digits and not a number with a value of seven hundred and fifty-five.
Directories are different
On a directory, read means "list the names", write means "create or delete entries", and execute means "traverse into it". A directory without execute cannot be entered even if you know the exact path — the classic cause of a permission error that looks impossible.