This calculator works only for regular polygons - those polygons which have ALL sides equal & ALL interior angles equal.
The figure below shows a regular octagon.
The outer circle surrounding it is called a circumscribed circle (or circumcircle) and the inner circle which is surrounded by the octagon is called the inscribed circle (or incircle).
Angle EAB is one of the 8 interior angles.
Angle EAF is one of the 8 exterior angles and equals central angle ACB.
Line AC (which also equals Line EC) is called the circumradius (the radius of the outer, circumscribed circle).
Line DC is called the inradius or the apothem (the radius of the inscribed circle).
To see a list of the 22 polygons that have integer angles, click here.

Sum of Interior Angles = (Number of Sides -2) • 180 degrees

Number of Sides = (Sum of Interior Angles ÷ 180) + 2

Each Exterior Angle = Each Central Angle = (360 degrees) ÷ (Number of Sides)

Polygon Names

Sides	Name	Sides	Name	Sides	Name
3	triangle	9	nonagon	15	pentadecagon
4	quadrilateral	10	decagon	16	hexadecagon
5	pentagon	11	hendecagon	17	heptadecagon
6	hexagon	12	dodecagon	18	octadecagon
7	heptagon	13	tridecagon	19	enneadecagon
8	octagon	14	tetradecagon	20	icosagon

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Significant Figures >>>

The default setting is for 5 significant figures but you can change that by inputting another number in the box above.

Answers are displayed in scientific notation and for easier readability, numbers between .001 and 1,000 will be displayed in standard format (with the same number of significant figures.)
The answers should display properly but there are a few browsers that will show no output whatsoever. If so, enter a zero in the box above. This eliminates all formatting but it is better than seeing no output at all.

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