Formulas, without the guesswork.
Choose a relationship from the measurements you know. The variable names below belong to each calculator; a letter can have a different role in another method. π is evaluated using the browser’s full numerical constant, rather than replacing it with 3.14.
From a side
Inputs: Side a (length units).
From circumradius
Inputs: Circumradius R (length units).
From the apothem
Inputs: Apothem r (length units).
From area
Inputs: Area A (square units).
Reading the notation
√ means a square root, ∛ means a cube root, and a superscript ² or ³ means a square or cube. Sine, cosine and tangent use angles converted from entered degrees. Where an inverse trigonometric function is used, its displayed angle is converted back to degrees. The ellipse perimeter is approximate; other formula labels describe the ideal mathematical model.
A general eight-sided polygon is not determined by one side. These tools apply only to regular octagons.
Reference reading: Regular octagon at Wolfram MathWorld.