// about this tool

Unix file permissions are stored as a 9-bit mask split into three groups — owner, group, and others — each with three bits for read (4), write (2), and execute (1). Adding the bits gives a single octal digit per group, producing the familiar three-digit notation like 755 or 644. This tool lets you toggle permissions via checkboxes or type an octal value directly, with both representations always in sync.

// when to use

Generate the exact chmod command needed before running it on a server
Decode an octal permission string from ls -l output into human-readable rights
Set secure defaults for web server files (644 for files, 755 for directories)
Explain Unix permission concepts when onboarding junior developers

// faq

What does the execute bit mean on a directory?

On a directory, the execute bit controls whether a user can enter (cd into) the directory and access its contents. Without it, even a directory with read permission will refuse to list files if execute is missing.

What is the difference between 755 and 777?

755 gives the owner full access (rwx) and everyone else read+execute (r-x) — the standard for executables and directories. 777 gives everyone full write access, which is a security risk and rarely the right choice on shared systems.

Why does chmod 644 use three digits, not four?

chmod accepts 3- or 4-digit octal. The optional leading digit controls special bits: setuid (4), setgid (2), and sticky (1). Most files only need the three permission digits. The calculator shows the three-digit form used in everyday practice.

// related tools

// about this tool

// when to use

// faq