Chmod Calculator
| Octal | Symbolic | Typical use |
|---|---|---|
| 600 | rw------- | private files (SSH keys) |
| 644 | rw-r--r-- | web pages, config files |
| 655 | rw-r-xr-x | read-only scripts |
| 700 | rwx------ | private directories |
| 755 | rwxr-xr-x | scripts, shared executables |
| 777 | rwxrwxrwx | world-writable (dangerous) |
Every Unix file carries three permission sets (owner, group, others), each with three flags (read = 4, write = 2, execute = 1). Sum the flags per set to get one octal digit, and three digits describe the whole file. This calculator does the addition visually and outputs both the octal number and the symbolic string.
The presets cover the files you touch daily: 644 for web pages (owner writes, everyone reads), 755 for scripts (owner writes, everyone runs), 600 for secrets like SSH keys (only the owner reads, and SSH enforces this).
How to use
- Pick the permission level for each of owner, group and others - the octal and symbolic strings update live.
- Use the presets for the standard file types: 644 for data, 755 for executables, 600 for private files.
- Copy the octal number and use it directly with the chmod command.
Frequently asked questions
What do the three numbers mean in 755?
Owner = 7 (read + write + execute), Group = 5 (read + execute), Others = 5 (read + execute). Each digit is the sum of read (4), write (2) and execute (1) for that subject. 755 is the standard for scripts and executables that everyone should run but only the owner should edit.
Why does SSH refuse my key with wrong permissions?
SSH private keys must be 600 (owner read+write only). If the file is group- or world-readable, ssh considers it compromised and refuses to use it - the error is Permission denied (publickey), not a permission warning. Fix with chmod 600 ~/.ssh/id_rsa.
What is the difference between 644 and 600?
644 = owner reads and writes, group and others read only - the default for web content and documentation. 600 = owner reads and writes, group and others get nothing - required for private keys, credentials, and anything only one person should see. The three zeros at the end are what make 600 private.
Is 777 ever correct?
Almost never. 777 means anyone on the system can read, modify and execute the file - a security hole for anything but temporary shared directories. If a tutorial says to chmod 777, it is skipping the real fix: change the ownership or group instead.