Calculators
Octagon Calculator
Calculate regular octagon area, perimeter, apothem, and diagonals from a side length locally in your browser.
Enter a side length from 0 (exclusive) to 1000000000 to see the area (2 × (1 + √2) × s²), perimeter (8 × s), apothem (s ÷ (2 × tan(22.5°))), long diagonal (s × √(4 + 2√2)), and short diagonal (s × (1 + √2)). The calculation runs locally in your browser; your value is not uploaded or stored.
Find regular octagon measurements without uploading anything.
Enter a side length from 0 (exclusive) to 1000000000 to see the area, perimeter, apothem, long diagonal, and short diagonal. Everything runs locally in your browser, so your value is not uploaded or stored.
Frequently Asked Questions
Everything you need to know about this tool, how it works, and privacy.
What does the octagon calculator show?
Enter a side length from 0 (exclusive) to 1000000000. The calculator returns the area (2 times (1 plus the square root of 2) times side squared), the perimeter (8 times the side), the apothem (side divided by (2 times tan(22.5 degrees))), the long diagonal (side times the square root of (4 plus 2 times the square root of 2)), and the short diagonal (side times (1 plus the square root of 2)). For example, a side of 5 gives area about 120.710678119, perimeter 40, apothem about 6.03553390593, long diagonal about 13.0656296488, and short diagonal about 12.0710678119.
Which formulas does the octagon calculator use?
It uses area = 2 x (1 + square root of 2) x s^2, perimeter = 8 x s, apothem = s / (2 x tan(pi / 8)), long diagonal = s x square root of (4 + 2 x square root of 2) (vertex to opposite vertex), and short diagonal = s x (1 + square root of 2) (flat to opposite flat), where s is the side length you enter. This tool covers regular octagons with eight equal sides and equal 135-degree interior angles.
Which inputs are accepted?
Enter a side length greater than zero through 1000000000. Decimal values are accepted. Zero, negative numbers, non-numeric text, and values above the limit show a clear error instead of a result.
Do my numbers stay private?
Yes. The calculation and copy action run locally in your browser. CodeASystem does not upload or store the value you enter.