What's the difference between chmod 755 and 777?

With 755, only the owner can write — group and others can just read and execute. With 777, everyone gets write access too. Use 755 for directories and executables in production, and reserve 777 for a debugging session you'll undo right after.

What chmod should I use for SSH private keys?

600 (rw-------). SSH refuses to use a key that's readable by group or others and shows a "Permissions too open" error. The `~/.ssh/` directory itself should be 700, or SSH will reject the key even if the file mode is correct.

Does chmod -R apply the same permissions to files and directories?

Yes, and that's usually a mistake — files rarely need the execute bit unless they're actual programs. Run `find . -type d -exec chmod 755 {} \;` and `find . -type f -exec chmod 644 {} \;` separately instead of one recursive chmod.

What are SUID, SGID, and the sticky bit?

SUID (4000) makes an executable run with the file owner's privileges instead of the caller's. SGID (2000) does the same for the group, or makes new files inherit a directory's group automatically. Sticky bit (1000) restricts deletion so each user can only remove their own files — the classic example is `/tmp`.

CHMOD Calculator

Visual chmod calculator with security warnings. Set read/write/execute permissions, get octal and symbolic output instantly. Includes umask, SUID/SGID/sticky bit.

Updated May 2026

How to Use This Chmod Calculator

Three input modes, one ready-to-paste command

1. Check the boxes or type a value

Toggle read/write/execute for owner, group, and others — or type octal (`755`) or symbolic (`rwxr-xr-x`) directly.

2. Watch for the security warning

Risky values like 777 or 666 trigger an automatic warning with a safer alternative.

3. Copy the ready-to-run command

"Copy Command" copies `chmod 755 [file]` ready to paste straight into your terminal.

What This Chmod Calculator Does

More than octal-to-symbolic conversion

Automatic security warnings

Flags risky permissions (777, 666, world-writable) and suggests the correct alternative on the spot.

Three input modes

Visual checkboxes, octal (755, 4755), or symbolic notation (rwxr-xr-x) — pick whichever you think in.

Special permission bits

SUID, SGID, and the sticky bit in the 4th digit, each with a plain-language explanation.

Built-in umask calculator

See how your system's umask masks the default permissions for new files and directories.

Permission Examples

Common octal values and what they're actually for

Octal / Symbolic

Typical use

644 — rw-r--r--

Web files: .html, .css, images

755 — rwxr-xr-x

Directories, shell scripts, executables

600 — rw-------

Private SSH keys, credentials

777 — rwxrwxrwx

⚠️ Security risk — avoid in production

When You'll Reach for This Tool

The scenarios that come up most for people using a chmod calculator

Web server files

644 lets the server read the file but never overwrite it, even if the app itself gets exploited.

Private SSH keys

chmod 600 ~/.ssh/id_rsa — SSH refuses to use a key that's readable by group or others.

Upload directories

755, with access control handled at the application level — never 777.

wp-config.php and password-bearing configs

640 or 600 — never 644, since the file holds your database credentials in plain text.

Common Mistakes

Using 777 to "fix" a permission error

On shared hosting, every other account on the server can then read and overwrite your files.

chmod -R applying execute to files, not just directories

Apply 755 to directories and 644 to files separately with find -type d / -type f instead of one blanket -R.

Forgetting that `~/.ssh` itself also needs 700

Fixing the private key's permissions alone isn't enough — the whole directory needs the correct mode too.

Why Use This Chmod Calculator

The automatic security warning is the real difference: most chmod calculators only convert octal to symbolic without ever flagging that the value you picked opens a hole.

It covers SUID, SGID, sticky bit, and umask — not just the three basic digits — useful when you need to understand why a permission is set, not just copy a number.

Frequently Asked Questions

It grants read, write, and execute access to the owner, the group, AND everyone else on the system. On shared hosting, any other account can overwrite your files; on a public server, a compromised process gets a free path to write anywhere. Use it only for temporary debugging, then revert immediately.

References

chmod — Linux man page

The official POSIX specification for chmod, with every flag and the full symbolic syntax.

File-system permissions — Wikipedia

A complete overview of the Unix permission model, including special bits and ACLs.