chmod Calculator — Octal & Symbolic Permissions | GadgetSurge

About chmod Calculator

Unix file permissions are a three-digit octal number: owner, group, and others. Each digit is the sum of read (4), write (2), and execute (1). Mode 755 means the owner has 7 (rwx), while group and others have 5 (r-x).

Regular files are often 644 (rw-r--r--): owner edits, everyone reads. Directories and executable scripts commonly use 755 so others can traverse or run them. SSH private keys should be 600; OpenSSH prints UNPROTECTED PRIVATE KEY FILE if group or others can read the key.

Mode 777 (rwxrwxrwx) is a frequent but dangerous “fix” for web server permission errors because it allows anyone on the system to modify the file. Prefer the minimum permission that works, usually 644 for files and 755 for directories.

Special bits add a fourth digit: setuid (4) runs a program as the file owner, setgid (2) sets the group on new files in a directory, and sticky (1) on directories like /tmp lets users delete only their own files. Symbolic notation shows these as s/S on execute and t/T for sticky on others.

The execute bit on a directory means permission to enter and traverse it (cd, list contents), not “run” the folder. All calculations run in your browser.

How to Use chmod Calculator

1. Click permission checkboxes for owner, group, and others, or use a preset like 644 or 755.
2. Type an octal mode (755 or 4755) or symbolic string (rwxr-xr-x) — all inputs stay in sync.
3. Review octal, symbolic, and chmod command outputs below the grid.
4. Copy the numeric or symbolic chmod command for your deploy script or server session.

Common Use Cases

• Deploy script permissions: Confirm deploy.sh is 755 before chmod +x on the server.
• SSH key hygiene: Set id_rsa to 600 after copying to ~/.ssh to avoid OpenSSH warnings.
• Shared log directories: Use setgid on a log folder so new files inherit the correct group.
• Read-only secrets: Apply 400 to files that should never be modified by the app user.
• Learning chmod notation: Map between rwx grids and the numeric modes you see in ls -l.
• Documenting runbooks: Copy symbolic chmod u=rwx,g=rx,o=rx commands into infrastructure docs.

Frequently Asked Questions

How do I read chmod 755?

7 = rwx for owner, 5 = r-x for group, 5 = r-x for others. It is the standard mode for directories and executable scripts.

When should I use 644 vs 755?

Use 644 for normal files that should not be executed. Use 755 for directories and anything that must run as a script or binary.

Why does SSH reject my private key?

Keys must not be readable by group or others. Use chmod 600 on id_rsa and chmod 700 on ~/.ssh.

What is the sticky bit on /tmp?

chmod 1777 on /tmp sets sticky so users can create files but only delete their own, even when the directory is world-writable.

What does setuid do?

When set on an executable, the process runs with the file owner’s privileges — for example passwd running as root.

Can I use four-digit octal?

Yes. The leading digit encodes setuid (4), setgid (2), and sticky (1), e.g. 4755 for setuid plus 755 permissions.

Does execute on a directory mean run?

No. Execute on a directory grants traverse access — you need x to cd into it or access paths beneath it.

Is my input uploaded?

No. Permission math runs entirely in your browser.