Triangle
Three straight sides enclose the simplest polygon. A triangle’s angles total 180°, and its area depends on a base and its perpendicular height.
Give your shape a size.
Use the same unit for every dimension. Changing the unit relabels the measurements; it does not convert them.
From dimensions to an answer
Know its properties
| Sides / vertices | 3 / 3 |
| Interior-angle sum | 180° |
| Angle types | Acute, right, or obtuse |
| Side types | Equilateral, isosceles, or scalene |
Useful formulas
Area: A = bh / 2
Perimeter: P = a + b + c
Angle sum: A + B + C = 180°
Triangle: common questions
Wondering about the why? Start here.
Can I use a sloping side as the height of a triangle?
Only if that side is perpendicular to your chosen base. In A = bh/2, height means the perpendicular distance from the opposite vertex to the line containing the base. In an obtuse triangle, that height may lie outside the triangle.
Understand the area formulaHow can I find area if I only know the three side lengths?
Use Heron’s formula: A = √[s(s − a)(s − b)(s − c)], where s = (a + b + c)/2. The side lengths must form a valid triangle. Choose SSS in the triangle solver to calculate it with worked steps.
Calculate from three sides