How to use it
- Choose how to enter the mode: Check boxes, Octal (755) or Symbolic (rwxr-xr-x).
- With the check boxes, tick Owner: read, Owner: write and so on. Tick the special bits only when you need them.
- Type the File name you are going to change, so the commands are complete.
- Copy the chmod command and run it in a terminal on Linux, macOS or another POSIX system.
How the three forms match
A file mode has three classes of users: the owner, the group and everyone else (others). Each class has three bits: read (4), write (2) and execute (1). Adding the bits of one class gives one octal digit, so rwx is 7, r-x is 5 and --- is 0.
The mode 755 therefore means: owner 7 (rwx), group 5 (r-x) and others 5 (r-x). The symbolic form of the same mode is rwxr-xr-x.
rwx = 4 + 2 + 1 = 7, r-x = 4 + 1 = 5, --- = 0The special bits and what the calculator leaves out
Three special bits sit in front of the three digits. Set-user-ID (4) makes a program run with its owner's rights. Set-group-ID (2) gives the program its group's rights, and in a folder it makes new files take the folder's group. The sticky bit (1) stops users from deleting other users' files in a shared folder.
In the symbolic form the special bits show as s and t. An upper-case S or T means the bit is set but the execute permission is not, which usually has no effect and is often a mistake. The calculator works on one mode at a time: it does not read the permissions of an existing file and does not handle access control lists.
Worked example: a script that everyone can run
Tick owner read, write and execute, group read and execute, and others read and execute. The octal mode is 755, the symbolic mode is rwxr-xr-x and the command is chmod 755 script.sh. The symbolic command is chmod u=rwx,g=rx,o=rx script.sh.
Clear the group execute box and the octal mode becomes 745; the steps then say that the group can read only.
Frequently asked questions
What does chmod 755 mean?
It gives the owner read, write and execute (7), and the group and all other users read and execute (5 each). Anyone can run and read the file, but only the owner can change it.
What is the difference between 644 and 755?
644 gives the owner read and write, and everyone else read only, so the file cannot be run as a program. 755 adds execute permission for all users, which programs and scripts need.
When do I need four digits such as 4755?
A fourth digit appears only when a special bit is set. The first digit then holds 4 for set-user-ID, 2 for set-group-ID and 1 for the sticky bit, or the sum of them.
Why does the symbolic form show a capital S or T?
The capital letter means the special bit is set but the execute permission is not. Set-user-ID has no effect on a file that cannot be executed, so the capital letter usually points to a mistake.
Is chmod 777 safe?
No. 777 gives read, write and execute to everyone, so anyone can change the file. Use the smallest mode that works: usually 644 for a file and 755 for a program or a folder.
Does this work on Windows?
The chmod command belongs to POSIX systems such as Linux and macOS. Windows does not use these modes, so the commands on this page do not apply there; use the security settings of the file instead.
Sources
Figures and text last checked on Tuesday, November 10, 2026.