Enter a side length, perimeter, area, or height to calculate every other measurement of an equilateral triangle.

How to Use the Equilateral Triangle Calculator

Pick which value is known — Side, Perimeter, Area, or Height — enter it, and the calculator works backward to the side length before deriving everything else from there.

Every equilateral triangle has three equal 60° angles and three equal sides, so a single measurement is always enough to reconstruct the whole shape:

perimeter = 3 × side
area = (√3 ÷ 4) × side²
height = (√3 ÷ 2) × side

Working backward from area or height means inverting those formulas — side = √(4 × area ÷ √3) from area, or side = 2 × height ÷ √3 from height — which is why this calculator exists instead of leaving the algebra to be rearranged by hand.

Circumradius and Inradius

Two circles are baked into every equilateral triangle: one passing through all three corners (circumradius), and one touching the midpoint of all three sides (inradius).

circumradius = side ÷ √3
inradius = side ÷ (2√3)

The circumradius is always exactly twice the inradius in an equilateral triangle — a relationship that doesn't hold for any other triangle shape, where the two circles don't even share a center.