MMaxTools

chmod Calculator

Convert Unix permissions between symbolic and numeric modes.

The chmod Calculator converts Unix file permissions between symbolic form (rwxr-xr-x) and numeric form (755). Tick read/write/execute per user class and the octal mode, symbolic string and full chmod command appear instantly.

Unix permissions confuse everyone at first: read=4, write=2, execute=1 per class (user, group, other), summed per class into one digit — 644 for normal files, 755 for executables and directories, 600 for secrets. The calculator makes the mapping visible and includes the special bits (setuid 4, setgid 2, sticky 1).

Presets cover the everyday modes, and the numeric result pastes straight into your terminal command. Everything computes locally in your browser.

user

group

other

Special

Rarely needed — leave None for normal files.

chmod command

chmod 0644 file

symbolic: rw-r--r--

Numeric mode

0644

Read=4, write=2, execute=1 per class (user/group/other), summed to one digit each. The sticky bit (t) applies to shared directories; setuid/setgid (s) change who executes a program as.

How to use the chmod Calculator

  1. Tick read/write/execute for user, group and other.
  2. Optionally set a special bit (setuid, setgid, sticky).
  3. Read the numeric mode and symbolic string.
  4. Copy the chmod command.
  5. Use the presets for common cases like 644 and 755.

Frequently asked questions

What do the numbers mean?

Each class gets one digit: read=4 + write=2 + execute=1. 755 means user has 7 (rwx), group 5 (r-x), other 5 (r-x). The first digit (when present) holds special bits.

What is the sticky bit used for?

On shared directories like /tmp it lets users create files but only delete their own — the 't' in drwxrwxrwt. Setuid/setgid make executables run with the file owner's or group's privileges.

When should I use 644 vs 755?

644 (rw-r--r--) for normal files — readable by everyone, writable by you. 755 (rwxr-xr-x) for directories and executables — everyone can traverse or run them. Never 777 on shared systems.

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