Calculate Unix file permissions. Click to toggle read/write/execute for owner, group and other. Shows octal and symbolic notation.
| Owner | Group | Other | |
|---|---|---|---|
| Read | |||
| Write | |||
| Execute |
The Chmod Calculator makes Unix file permission management straightforward by letting you toggle read, write, and execute bits for owner, group, and other via a visual checkbox grid. It instantly shows both the octal value (e.g. 755) and the symbolic notation (e.g. rwxr-xr-x), along with a plain-English description of what the permissions mean. You can also type an octal code directly to reverse-engineer any permission set.
Unix file permissions are stored as a 9-bit bitmask divided into three groups of three: owner, group, and others. Each group's read (4), write (2), and execute (1) bits are summed to produce a single digit between 0 and 7, giving the familiar three-digit octal notation like 755 or 644. The calculator reads the state of each checkbox, performs this bitwise addition in JavaScript, and instantly renders both the octal code and the symbolic rwxr-xr-x-style notation without any server involvement.
The calculator is a reliable reference tool, but always double-check critical permissions before applying them to a live server - especially for configuration files, SSH keys, and web-accessible directories. As a general rule, web server files should be 644 (owner read/write, everyone else read-only) and directories 755, while private keys like ~/.ssh/id_rsa must be 600. The plain-English description the tool shows for each combination helps you verify you have selected exactly the right access level.
Octal notation (e.g. chmod 755) specifies absolute permissions by replacing all existing bits at once with a three-digit number derived from binary arithmetic. Symbolic notation (e.g. chmod u+x) modifies permissions relative to their current state using operators like +, -, and = with letters for user (u), group (g), others (o), and all (a). The calculator displays both forms so you can use whichever syntax is appropriate for your script or terminal workflow.
Standard chmod calculators focus on the three basic permission groups, but Unix also has a fourth octal digit for special bits: setuid (4000), setgid (2000), and the sticky bit (1000). Setuid makes an executable run with the file owner's privileges, setgid forces new files in a directory to inherit the directory's group, and the sticky bit prevents non-owners from deleting files in a shared directory like /tmp. If you need to set these, prepend the appropriate digit to your octal code, e.g. chmod 1755 for a sticky executable directory.
The command-line chmod utility applies permissions directly to files on a live system, while this calculator is purely a reference tool for building and understanding permission values before running any command. It is especially helpful for developers who work with Unix permissions infrequently and want to verify the exact meaning of a value like 664 or 700 before applying it. You can also use it in reverse - type an octal code and see exactly what each bit allows - making it a fast sanity-check before modifying sensitive server files.