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Unix Permissions Calculator

Converts a Unix/Linux file mode between octal (755, 0644, 4755) and symbolic (rwxr-xr-x, -rw-r--r--, u=rwx,g=rx,o=rx) notation and explains the read/write/execute bits for owner, group and others plus the setuid, setgid and sticky bits.

When to use

You need the chmod command for a permission string seen in ls -l, or want to know what an octal mode such as 4755 grants.

Do not use when: You need ACLs (getfacl/setfacl), Windows NTFS permissions, or umask arithmetic (subtract the umask bits from 666/777 manually).

Formula

digit = 4·read + 2·write + 1·execute for owner, group, others; special digit = 4·setuid + 2·setgid + 1·sticky; mode = special·512 + owner·64 + group·8 + others

Follows the POSIX mode bits as documented in the GNU coreutils manual; an uppercase S or T in a symbolic string means the special bit is set without the corresponding execute bit.

Inputs

Parameter	Type	Unit	Required	Description
`mode`	string		yes	Octal mode with 3 or 4 digits (755, 0644, 4755, 0o755), a 9-character symbolic string optionally preceded by a file-type character (rwxr-xr-x, -rw-r--r--, drwxr-sr-x), or chmod assignments (u=rwx,g=rx,o=rx).

Outputs

Output	Type	Description
`octal`	string	Three digits, or four when setuid/setgid/sticky are set (e.g. 4755).
`symbolic`	string	Nine characters as shown by ls -l (s/S and t/T mark the special bits).
`owner`	string	Permissions of the file owner.
`group`	string	Permissions of the owning group.
`others`	string	Permissions of everyone else.
`setuid`	boolean	Executable runs with the owner's user id (octal 4000).
`setgid`	boolean	Executable runs with the group id; new files in a directory inherit its group (octal 2000).
`sticky`	boolean	Only a file's owner may delete or rename it inside the directory (octal 1000).
`chmod_command`	string	Equivalent chmod command with the octal mode.
`chmod_symbolic`	string	Equivalent chmod command using u=, g=, o= assignments.
`explanation`	string	How each octal digit is built from read (4), write (2) and execute (1).

Example

755: {"mode":"755"} → {"octal":"755","symbolic":"rwxr-xr-x","owner":"read, write, execute","group":"read, execute","others":"read, execute","setuid":false,"chmod_command":"chmod 755 file","chmod_symbolic":"chmod u=rwx,g=rx,o=rx file"}

rw-r--r--: {"mode":"rw-r--r--"} → {"octal":"644","symbolic":"rw-r--r--","owner":"read, write","group":"read","others":"read","sticky":false,"chmod_command":"chmod 644 file"}

GET https://tttkmbb.com/api/v1/calculate/unix-permissions?mode=755

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/unix-permissions (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/unix-permissions · Markdown: https://tttkmbb.com/developer/unix-permissions.md · JSON definition: https://tttkmbb.com/developer/unix-permissions.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="unix-permissions"
OpenAPI operationId: convert_unix_permissions
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

GNU coreutils manual – chmod: Change access permissions (reference)
GNU coreutils manual – File permissions (mode bits, numeric modes) (reference)

FAQ

What does the leading digit in 4755 mean?

The fourth (leading) digit holds the special bits: 4 = setuid, 2 = setgid, 1 = sticky. 4755 is rwsr-xr-x: everyone can run the program with the owner's privileges, as with /usr/bin/passwd.

What is the difference between 755 and 0755?

None; the leading 0 is the C/shell octal prefix. 0o755 (JavaScript/Python style) is accepted too.

Why does ls show a capital S or T?

A capital letter means the special bit is set but the execute bit in that position is not (e.g. rw-r-Sr-- = 2644), which is usually a mistake.

Related calculators

Number Base Converter — Octal ↔ binary arithmetic behind the mode bits.
Bitwise Operations Calculator — Mode bits are combined with AND/OR masks.