What does chmod 755 mean?

The owner may read, write and execute; group and others may read and execute but not write. It is the usual mode for directories and executable scripts.

What is the difference between 644 and 755?

The execute bit. 644 is for ordinary files that shouldn't be run; 755 adds execute, which files need to be run as programs and directories need to be entered.

Why does SSH reject my private key?

Because it is readable by others. SSH requires 600 (rw-------) on private keys and refuses to use anything more permissive.

What is the sticky bit?

The leading 1 in a mode like 1777. On a shared directory such as /tmp, it means only a file's owner can delete it, even though everyone can write to the directory.

Chmod Permission Calculator

Convert octal or symbolic chmod modes into readable Unix permissions.

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What is the Chmod Permission Calculator?

A chmod calculator converts between the octal notation of Unix file permissions (755) and their symbolic form (rwxr-xr-x), showing what the owner, group and other users are each allowed to do with a file.

Unix permissions are nine bits in three groups of three: read, write and execute for the owner, the group and everyone else. Each group is one octal digit, because read is 4, write is 2 and execute is 1 — so 7 is all three, 6 is read plus write, 5 is read plus execute.

That gives the two modes you type most often. 644 (rw-r--r--) is the standard for a file: the owner can edit it, everyone else can read it. 755 (rwxr-xr-x) is the standard for a directory or a script: the owner can modify, everyone can enter or execute. On a directory the execute bit means 'may traverse into', which is why a directory with 644 is unusable.

A fourth leading digit sets the special bits: 4 for setuid (run as the file's owner), 2 for setgid (run as the group, or inherit the group on new files in a directory) and 1 for the sticky bit, which on /tmp stops users deleting each other's files. Setuid on a writable binary is a privilege-escalation finding, not a configuration choice.

Chmod Permission Calculator at a glance

Bit values: read 4 · write 2 · execute 1
644: rw-r--r-- — the normal file default
755: rwxr-xr-x — directories and executables
Special bits: 4 setuid · 2 setgid · 1 sticky (leading digit)

How to use the Chmod Permission Calculator

1Enter an octal mode such as 755, or a symbolic string such as rwxr-xr-x.
2Read the equivalent notation and the per-class permission breakdown.
3Check the special-bit digit if the mode has four digits.

When you would reach for it

Working out the right mode for a deploy script or an uploaded file.
Auditing an SSH key or config file that a service refuses to load.
Explaining permission bits during code review or onboarding.

Chmod Permission Calculator — frequently asked questions

Last reviewed 25 July 2026 · questions or corrections: contact@decoder.tools