Octal Addition Calculator

About this calculator

Octal addition works in base 8, using only the digits 0 through 7 — there is no digit 8 or 9, so any column that sums to 8 or more carries into the next column exactly the way a column reaching 10 carries in ordinary decimal addition. For example, 7 + 3 in octal is not “10” in the decimal sense; it rolls over to octal 12 (one group of eight, plus two).

Octal saw heavy use in early computing because it groups binary digits neatly in threes (three bits per octal digit), which made it a convenient shorthand on machines like the PDP-8 before hexadecimal became the standard grouping for most later architectures. It survives today mainly in Unix and Linux file permission notation, where modes like 755 or 644 are octal numbers representing read, write and execute bits for owner, group and others.

Enter two octal numbers and this calculator adds them, showing the sum in both octal and its decimal equivalent so you can verify the carries by hand.