Chmod Calculator
Convert between octal and symbolic Linux permissions, with warnings on risky ones.
Three Digits, And What They Actually Mean
Unix permissions are simpler than they look. There are three classes — the owner, the owning group and everyone else — and three actions: read, write and execute. Each action has a value: read is 4, write is 2, execute is 1. A digit is the sum, so 6 means read and write, and 755 means the owner can do everything while everybody else can read and enter.
The part that catches people is what execute means on a directory. It is not "run" — it is "may enter". A directory that can be read but not entered lists its contents and refuses to open any of them, which produces some of the more baffling permission errors.
Key features
- Both directions at once — tick the boxes or type the octal; each updates the other.
- Symbolic output — the rwxr-xr-x form, exactly as
ls -lprints it. - Special bits — setuid, setgid and the sticky bit, shown in their upper and lower case forms.
- Warns about the dangerous ones — world-writable, 777 and setuid, with the reason rather than just a flag.
- Copyable command — the full chmod line with your path in it.
This tool runs entirely inside your browser using native Web APIs. Your files and text are never uploaded to a server, never logged and never shared with third parties.
How to use: Chmod Calculator
- Tick or type
Use the checkbox grid, or type an octal value like 644 to see it broken down. - Add special bits if needed
Rarely required. The warnings explain what each one does. - Read the advice
Risky combinations are explained rather than simply flagged. - Copy the command
Set your path first so the copied line is ready to run.
Technical specifications
| Processing location | Entirely in your browser — no server round trip |
|---|---|
| Data uploaded | None. Files and text never leave your device |
| Price | Free — no account, no trial, no usage cap |
| Category | Developer |
| Works offline | Yes, once the page has loaded |
| Browser support | Chrome 90+, Edge 90+, Firefox 90+, Safari 15+ |
| Interface languages | English, 中文, हिन्दी, Español, العربية |
Frequently asked questions
What is the difference between 644 and 755?
644 gives the owner read and write, and everyone else read only. 755 adds execute for everybody. Use 644 for ordinary files and 755 for directories and for scripts meant to be run. Giving a data file execute permission does nothing useful; withholding it from a directory makes the directory unusable.
Why is 777 such a bad idea?
Because it lets any account on the system modify the file, including one an attacker has obtained through something unrelated. It is the reflex fix when a permission problem appears, and it works by removing the protection rather than by finding the cause. The cause is usually ownership: the right answer is almost always chown, not chmod 777.
What does the sticky bit actually do?
On a directory it means that only a file's owner can delete or rename it, even if the directory itself is world-writable. That is what makes /tmp workable — everyone can create files there, and nobody can delete anyone else's. It is written 1777.
Why does my permission show S instead of s?
A capital S means the special bit is set but the matching execute bit is not, which is almost always a mistake — setuid without execute does nothing. Lower case means both are set. The same applies to T for the sticky bit.
Does chmod affect existing open files?
No. Permissions are checked when a file is opened, not while it is being read. A process that already has a file open keeps its access until it closes it, which is why a permission change sometimes appears to have no effect until a service is restarted.