chmod Calculator
Calculate and convert Unix file permissions in octal and symbolic notation, in your browser.
Owner: read, write, execute. Group: read, execute. Others: read, execute.
What it's for
Unix chmod permissions calculator
All Unix systems
Works for Linux, macOS, FreeBSD, and any Unix-based system. Octal and symbolic.
100% private
Calculation happens in your browser. No data is sent to any server.
Detailed breakdown
See exactly which permission maps to each bit for owner, group, and others.
Instant
Real-time conversion as you type. No signup, no waiting.
How it works
Three steps, no hassle
Enter the octal or symbolic value
Type a chmod value like 755 or rwxr-xr-x. The tool auto-detects the format.
Instant conversion
See the equivalent representation in octal, symbolic, and a breakdown by owner, group, and others.
Copy and apply
Copy the ready-to-use chmod command for your terminal or server.
FAQ
Got questions?
chmod (change mode) is a Unix command that modifies the access permissions of files and directories. It was part of the original AT&T Unix system in the 1970s. It controls who can read, write, or execute a file, divided into three categories: owner (user), group, and others.
755 (rwxr-xr-x) is used for directories and executable files: the owner can read, write, and execute; group and others can only read and execute. 644 (rw-r--r--) is used for regular files: the owner can read and write; group and others can only read. On web servers, PHP or HTML files are typically 644 and directories 755.
The execute bit (x) on a file means it can be run as a program. On a directory, it means you can enter it (cd). Without the execute bit on a directory, you cannot list or access its contents even if you have read permission.
These are special bits beyond rwx. Setuid (4000): when the file is executed, it runs with the owner's permissions instead of the launching user's (used in sudo, passwd). Setgid (2000): the file runs with the owning group's permissions; on directories, new files inherit the group. Sticky bit (1000): on directories (like /tmp), only the owner can delete their own files even if others have write permission.
Standard configuration for Apache or Nginx web servers: directories 755, static files (HTML, CSS, JS, images) 644, sensitive config files (.env, config.php) 600 (owner only), executable scripts 700 if they must be private. Never use 777 (all permissions for everyone) in production.
Unix permissions history (AT&T 1970s), octal vs symbolic notation, Linux file system security
The Unix permissions system was designed by Ken Thompson and Dennis Ritchie at Bell Labs in the late 1960s and early 1970s as part of the original Unix operating system. The three-level model (owner, group, others) and three operations (read, write, execute) was a deliberate design decision for a multi-user time-sharing system. This model was formalized in the POSIX.1 standard (IEEE Std 1003.1) in 1988 and is now universal across all Unix-derived systems, including Linux, macOS, Android, and BSD systems.
Octal notation (like 755 or 644) arises naturally from the binary representation of permissions: each octal digit represents exactly 3 bits (read=4, write=2, execute=1). So 7 = 111 in binary = rwx, 5 = 101 = r-x, 4 = 100 = r--. This correspondence makes octal notation more compact than symbolic (rwxr-xr-x) and preferred in scripts and configs. Symbolic notation, on the other hand, is more human-readable and is what ls -l displays.
Linux file system security depends heavily on correct permission configuration. Privilege escalation attacks frequently exploit files with overly permissive permissions (especially 777) or misconfigured setuid. chmod is not just an administration tool: it is a critical line of defense. Security audits (CIS Benchmarks, for example) always include file permission checks. In container environments (Docker, Kubernetes), file permissions remain relevant even though the isolation model adds additional layers.