Octagon Calculator
Enter any one measurement of a regular octagon (side, area, perimeter, diagonals, circumradius or inradius) to find the rest.
Change the values and press Calculate to work out your own figures.
Enter any one measurement of the regular octagon.
Show the working
- From the side (a): side a = 5
- Area = n a² ÷ (4 tan(180° ÷ n)) = 8 × 5² ÷ (4 tan(22.5°))
- Area (A) = 120.7106781
How to use it
Enter any one measurement of the regular octagon.
Enter the measurements (decimals, fractions such as 3/4, or mixed numbers such as 1 1/2) and, if you like, their unit; the answer is then also given in other units. Show the working gives the formula.
Key facts
A = 2(1 + √2) a²
Stop signs
A stop sign is a regular octagon; each interior angle is 135°.
Questions
What is the area of a regular octagon with side 5?
50(1 + √2) ≈ 120.7106781.
Each interior angle?
135°.
Formulas
A = 2(1 + √2) a²
Sources
Limitations
- All measurements in one calculation use the same unit; the answer is also given in other units.
- Measurements up to 10¹⁵.