△ Triangle Calculator
Give this calculator any three side lengths and it returns the area using Heron's formula — the elegant method that needs no height measurement — plus the perimeter and whether the triangle is equilateral, isosceles or scalene.
It also checks the triangle inequality first: if the sides cannot physically form a triangle, it tells you instead of producing nonsense.
How to use this calculator
Type the lengths of the three sides and it checks they can form a triangle, then returns the area, perimeter and triangle type.
- Enter the length of Side a.
- Enter the length of Side b.
- Enter the length of Side c.
- Press Calculate to see the area, the perimeter, the triangle type (equilateral, isosceles or scalene) and whether it is right-angled.
Frequently asked questions
What does this triangle calculator compute?
It computes the area, perimeter and classification of a triangle from its three side lengths: whether it is equilateral, isosceles or scalene, plus a check for a right triangle. All three sides must be positive lengths.
How is the area calculated without the height?
It uses Heron's formula: with semi-perimeter s = (a + b + c) / 2, the area equals the square root of s times (s - a) times (s - b) times (s - c). The right-triangle check tests whether the squares of the two shorter sides sum to the square of the longest.
Can you show a worked example?
Enter sides 3, 4 and 5. The perimeter is 12, the semi-perimeter is 6, and the area is the square root of (6 * 3 * 2 * 1) = 6 square units, classified as scalene and right-angled.
What if my sides cannot form a triangle?
The calculator checks the triangle inequality first: the sum of any two sides must exceed the third. Impossible combinations, like 1, 2 and 10, produce an error instead of a meaningless answer.