Direct answer
Octal is base-8, using digits 0–7. Each octal digit packs 3 bits. Common use: Unix permissions (chmod 755 = rwxr-xr-x).
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Decimal input
Octal output
About Decimal → Octal
Octal's most common modern use is Unix file permissions. chmod 644, 755, 777 are octal. Each digit represents one of (owner, group, world) and bundles 3 permission bits (read=4, write=2, exec=1). 7 = read+write+exec, 6 = read+write, 5 = read+exec.
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Frequently asked questions
Why does chmod 755 mean "owner full, others read+exec"?
7 = 111 binary = read(4)+write(2)+exec(1). 5 = 101 binary = read(4)+exec(1). The three digits cover owner, group, world.
When is octal used outside Unix?
Rarely now. C-language number literals starting with 0 are octal (012 = 10), which trips up newcomers. Modern languages avoid the convention.