Octagon Calculator

Calculate the properties of a regular octagon from one side length.

Last updated: June 2026 | By Patchworkr Team

Regular Octagon

Side Length

Math check: area = 2(1 + sqrt(2))s^2

Result

Enter a side length to compute octagon properties.

Formulas

A = 2(1 + sqrt(2))s^2

P = 8s

apothem = (1 + sqrt(2))s / 2

If the side length is 5, the perimeter is 40 and the area is about 120.7107.

The circumradius and inradius follow directly from the side length.

A regular octagon has several diagonal lengths; this calculator reports the three common chord lengths by vertex separation.

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