Chmod Calculator
Unix permissions both ways — 755, rwxr-xr-x and checkboxes
✦ Nine bits that guard every Unix file
Every Unix file carries nine permission bits: read, write and execute for owner, group and others. Octal is just shorthand — each digit packs three bits (4+2+1), so 755 reads as full owner access plus read-and-run for the world. Getting one digit wrong is how sites get defaced and keys get stolen.
What this tool does
- 📁 Converts octal, symbolic and checkbox views three ways, live
- 🗣️ Explains each class in plain words (owner: read + write + execute)
- ⚠️ Rejects invalid modes instead of guessing
- ⚡ Everything updates as you type
When to use it
- Fixing a 500 error caused by wrong script permissions
- Locking down SSH keys (600, always)
- Learning what each digit actually grants
Privacy: pure bit math in your browser — nothing typed here leaves your device.
Last updated · 2026-09-01
How conversion works
Octal digits expand to bit triplets (4/2/1) and triplets fold back to digits — pure integer math with strict validators on both notations. The checkbox grid is just a third rendering of the same nine bits.
Why chmod here?
Three views, one truth
Octal digits, symbolic string and nine checkboxes stay in perfect sync — change any one and the other two follow instantly, so the representation never lies.
Plain-English verdicts
Below the numbers, each class reads out what it can actually do — owner: read + write + execute — because 6 means nothing until it says write.
Invalid input stays invalid
Bad octals and malformed symbolic strings get a clear error, never a guessed conversion that chmods your server wrong.
Frequently asked questions
What do the three digits mean?⌄
Owner, group, others — each digit sums read (4) + write (2) + execute (1). So 7 is full access, 5 is read+execute, and 755 gives the owner everything while everyone else can read and run.
Why is 777 dangerous?⌄
It grants write access to literally everyone on the system — any local user or compromised process can modify or replace the file. Web files rarely need more than 644, executables 755.
What about the fourth digit (setuid/setgid/sticky)?⌄
This calculator covers the standard nine permission bits (the common 3-digit modes). Special bits like 4755 are a separate topic with real security implications — look them up deliberately, not by accident.
Frequently asked questions
What do the three digits mean?
Owner, group, others — each digit sums read (4) + write (2) + execute (1). So 7 is full access, 5 is read+execute, and 755 gives the owner everything while everyone else can read and run.
Why is 777 dangerous?
It grants write access to literally everyone on the system — any local user or compromised process can modify or replace the file. Web files rarely need more than 644, executables 755.
What about the fourth digit (setuid/setgid/sticky)?
This calculator covers the standard nine permission bits (the common 3-digit modes). Special bits like 4755 are a separate topic with real security implications — look them up deliberately, not by accident.