🔒 Chmod Calculator

What is Chmod?

Chmod (short for "change mode") is a Unix and Linux command that sets file and directory permissions. Every file on a Unix-based system has three categories of users who may access it: the owner, the group, and others (everyone else). For each category, three types of access can be granted or denied: read, write, and execute. These nine permission bits combine to form the file's access control, and the chmod command is the standard way to modify them from the terminal.

Permissions are represented in two common formats. The numeric (octal) format uses three or four digits, where each digit is the sum of its permission values: read equals 4, write equals 2, and execute equals 1. For example, 755 means the owner has full access (7 = 4+2+1), while the group and others can read and execute but not write (5 = 4+0+1). The symbolic format uses letters: r for read, w for write, x for execute, and a hyphen for denied. So 755 in symbolic form is rwxr-xr-x.

This free chmod calculator lets you work with both formats interactively. Click checkboxes in the permission grid, type a numeric value like 755, or enter a symbolic string like rwxr-xr-x, and all other representations update instantly. The tool also supports special permissions (setuid, setgid, and the sticky bit) via an optional fourth digit. Whether you are configuring a web server, deploying scripts, or managing shared directories, this calculator helps you get the permissions right without memorizing octal codes.

How to Use This Tool

1. Click the checkboxes in the permission grid to toggle read, write, and execute for owner, group, and others. The numeric value, symbolic string, and chmod command update as you click.
2. Alternatively, type an octal value (like 755 or 0644) in the numeric input field. The checkboxes and symbolic display will sync automatically.
3. You can also type a symbolic string (like rwxr-xr-x) directly. The tool parses it and updates everything else to match.
4. Use the preset buttons to load common permission sets quickly. Hover over each preset for a short description of its typical use case.
5. Toggle between file and directory context to see how the type indicator prefix changes (a hyphen for files, d for directories). This does not affect the permission bits themselves but provides accurate ls-style output.
6. Enable special permissions (setuid, setgid, sticky bit) by checking the boxes in the special permissions section. The fourth leading digit will appear in the numeric output.

Key Features

• Bidirectional Sync - Checkboxes, numeric input, and symbolic input are all linked. Changing any one updates the others in real time.
• Special Permissions - Full support for setuid, setgid, and sticky bit, shown as a fourth octal digit and reflected in the symbolic output.
• Common Presets - One-click presets for 755, 644, 600, 777, 700, 666, 444, and 400 cover the most frequently used permission sets.
• Color-Coded Display - Granted permissions appear in green and denied permissions appear in red, making it easy to spot access at a glance.
• Plain English Explanation - A detailed breakdown shows what each user category can and cannot do with the current permission set.
• Ready-to-Use Command - The full chmod command is generated automatically and can be copied to your clipboard in one click.

Frequently Asked Questions

What does chmod 755 mean?

Chmod 755 gives the file owner read, write, and execute permissions (7 = 4+2+1), while the group and others get read and execute permissions only (5 = 4+0+1). This is the standard permission for executable files and directories that need to be accessible to other users on the system.

What is the difference between 644 and 755?

The key difference is the execute bit. With 644, the owner can read and write, and everyone else can only read. With 755, the owner can read, write, and execute, and everyone else can read and execute. Use 644 for regular data files (HTML, CSS, images, configuration files) and 755 for scripts, binaries, and directories.

What are setuid, setgid, and the sticky bit?

These are special permission bits beyond the standard read/write/execute. Setuid (set user ID) causes an executable to run with the permissions of the file owner rather than the user who launched it. Setgid (set group ID) works similarly for the group. The sticky bit, when set on a directory, prevents users from deleting files they do not own, even if they have write access to the directory. The /tmp directory on most Linux systems uses the sticky bit.

Is my data safe when using this tool?

Yes. This chmod calculator runs entirely in your browser using client-side JavaScript. No permission data, filenames, or any other input is sent to a server. You can verify this by using the tool while disconnected from the internet.