Triangle calculator.
Use this free triangle calculator to get fast estimates, compare scenarios, and understand the numbers with clear step-by-step context.
Use the Triangle Calculator.
Results update automatically as you type. No account is required.
Enter your values
About the Triangle Calculator
This triangle calculator finds the area either from a base and height or from three side lengths. With three sides it also classifies the triangle by its sides and angles and checks that the sides can actually form a triangle.
How it works
With base and height, area = ½ × base × height. With three sides it uses Heron’s formula: with s = (a + b + c) ÷ 2, area = √(s(s−a)(s−b)(s−c)). Angle type is found by comparing the squares of the sides.
Reading the result
The result is the area, plus the perimeter and the triangle’s classification (equilateral/isosceles/scalene and acute/right/obtuse) when you enter three sides.
Note: Calculations run instantly in your browser and use exact mathematical formulas. For very large numbers, results are shown to standard floating-point precision.
FAQ
What makes three lengths a valid triangle?
Each side must be shorter than the sum of the other two (the triangle inequality). If not, the lengths cannot close into a triangle.
When should I use Heron’s formula?
When you know all three side lengths but not the height. It gives the exact area without needing any angles.
How is the angle type determined?
By comparing the longest side squared to the sum of the other two squared: equal means right-angled, greater means obtuse, smaller means acute.