Equilateral Triangle Calculator

How it is calculated

In an equilateral triangle all three sides equal s, so one number is enough. The tool computes area = (√3 / 4) × s², height = (√3 / 2) × s and perimeter = 3 × s. √3 is about 1.732, so the area is roughly 0.433 × s² and the height roughly 0.866 × s. The side may be in any one length unit (cm, m, inches); the tool labels results as plain units, so the height and perimeter are in that same unit and the area in its square. It must be greater than zero. Results are rounded to 2 decimals. If you know the area or height and need the side, reverse the formula yourself.

Common mistakes

• Using the slanted side as the height. In an equilateral triangle the height is shorter than the side, about 87% of it, so 8 as a side gives 6.93 as a height, not 8.
• Using ½ × base × side for the area. The second length must be the height, otherwise the area comes out too large (32 instead of 27.71 here).
• Mixing units. A side in centimetres gives an area in square centimetres, so convert before comparing it with a figure in square metres.
• Applying this to a triangle that only looks regular. The formulas need all three sides to be equal, and an isosceles triangle with two equal sides gives wrong answers.