chmod Calculator

Calculate Unix file permissions with octal notation and symbolic representation

What is it and how does it work?

A chmod calculator works out Unix file permissions and shows them in both forms developers use: the three-digit octal number (like 755) and the symbolic representation (like rwxr-xr-x). On Linux and macOS, every file and folder has permissions controlling who can read, write and execute it, split across three groups — the owner, the group and everyone else. Translating between the checkboxes you think in and the octal number the chmod command wants is exactly the small calculation this tool removes.

The key idea is that each permission has a value: read is 4, write is 2 and execute is 1, and you add them up for each of the three groups to get one digit. So rwx (read, write, execute) is 4+2+1 = 7, and r-x (read and execute) is 4+1 = 5, which is how 755 means full access for the owner and read-plus-execute for everyone else. A calculator lets you tick the boxes and read off the number, or enter a number and see exactly what access it grants — useful for setting permissions correctly and for understanding what an existing 644 or 777 actually allows.

Common use cases

• Setting permissions for chmod: tick read, write and execute for owner, group and others to get the octal number to use.
• Decoding an existing value: enter a number like 644 to see exactly which permissions it grants to whom.
• Avoiding over-permissive files: check what 777 really means so you do not give everyone full access by mistake.

Frequently asked questions

How does the octal number relate to permissions?

Each of the three digits represents one group — owner, group, others — and is the sum of read (4), write (2) and execute (1). So 7 is read+write+execute, 5 is read+execute, 4 is read only. 755 therefore means the owner has full access and everyone else can read and execute.

What do the letters in rwxr-xr-x mean?

The symbolic form shows the same permissions as letters: r for read, w for write, x for execute, and a dash where a permission is absent. It is read in three groups of three — owner, group, others — so rwxr-xr-x is full access for the owner and read+execute for the other two.

What is the difference between read, write and execute?

Read lets you view a file's contents (or list a folder), write lets you modify it (or add and remove files in a folder), and execute lets you run a file as a program or enter a folder. On directories, execute means the ability to access what is inside.

Why is 777 considered risky?

777 grants read, write and execute to everyone — owner, group and all other users. That means any user can modify or run the file, which is usually far more access than needed and a security risk. Most files only need 644 and most scripts 755, so 777 should be used sparingly.

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