Chmod Calculator

One input per line. Output is numbered to match the input order.
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Permissions
Owner
Group
Others
Special bits
644Octal
rw-r--r--Symbolic
06444-digit octal

Owner can read and write; group can read; others can read.

A lowercase s or t means the special bit and the execute bit are both set; the uppercase S or T means the special bit is set without execute — which is almost always a mistake worth checking.

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Understand Chmod Calculator

Converts Unix file permissions between octal numbers (755), symbolic notation (rwxr-xr-x), and a checkbox matrix for user, group, and other.

How it works

Permissions are nine bits in three groups of three, one group each for the owner, the group, and everyone else. Within a group, read is 4, write is 2, and execute is 1, and the digit is their sum — so 7 is 4+2+1 = rwx, 5 is 4+1 = r-x, and 6 is 4+2 = rw-. That makes 755 exactly rwxr-xr-x and 644 exactly rw-r--r--. An optional fourth digit in front carries the special bits: setuid is 4, setgid is 2, and the sticky bit is 1, which is why /tmp is 1777 and a shared group directory is often 2775.

When to use it

  • Working out the octal number to pass to `chmod` when you know the rwx string you want, or the reverse when reading `ls -l` output
  • Fixing an SSH key that is refused because it is group-readable — private keys must be 600 and ~/.ssh must be 700
  • Setting up a shared directory where new files should inherit the group, which needs the setgid bit (2775) rather than a plain 775
  • Reading a Dockerfile, Ansible task, or deployment script that hardcodes a mode and checking that it means what the author intended

Watch out for

  • Execute means something different on a directory. On a file it means "run this"; on a directory it means "traverse into it", so a directory with read but not execute lets you list the names and nothing else — 644 on a directory is almost always a mistake.
  • 777 is almost never the fix. It makes the file world-writable, which most web servers and SSH will refuse to trust, and it hides the real problem, which is usually ownership rather than permission.
  • Octal mode is absolute and symbolic mode is relative. `chmod 755` sets all nine bits at once and wipes any special bits you did not include, while `chmod u+x` adds one bit and leaves the rest alone.
  • Your umask silently subtracts from the default. New files start from 666 and directories from 777, minus the umask — so with the common umask 022 you get 644 and 755, not the numbers you may have expected.

Not the right tool for: ACLs, extended attributes, and Windows permissions. Anything set with `setfacl`, SELinux labels, or NTFS ACLs sits alongside these nine bits and is not represented by an octal mode.

Frequently Asked Questions

What does chmod 755 mean?

The owner can read, write and execute; group and others can read and execute but not write. Each digit is the sum of read (4), write (2) and execute (1), so 7 = 4+2+1 and 5 = 4+1. In symbolic notation that is rwxr-xr-x, the usual setting for directories and scripts.

What is the difference between 644 and 755?

644 (rw-r--r--) is for regular files: the owner can edit it, everyone else can only read it. 755 (rwxr-xr-x) adds the execute bit, which is required for scripts to run and for directories to be entered at all. A directory without execute cannot be listed into.

What does a capital S or T in permissions mean?

Lowercase s or t means the special bit and the execute bit are both set. Uppercase S or T means the special bit is set while execute is not — usually a mistake, because a setuid file that cannot be executed does nothing but look alarming in an audit.

How to Use Chmod Calculator

  1. Paste or type your input in the input area above.
  2. The tool processes your input automatically or click Run.
  3. Copy or download the result using the action buttons.
  4. Use Ctrl+Enter to run quickly from the keyboard.