Triangle Calculator — Area, Perimeter & Angles from 3 Sides
Enter the three side lengths:
— Area
— Perimeter
— Shape
— Angle type
— Angle A
— Angle B
— Angle C
— Height a
— Height b
— Height c
Enter the three side lengths of any triangle and instantly get the area (Heron's formula), perimeter, all three interior angles in degrees, the height from each vertex, and the triangle classification — both by side type (equilateral, isosceles, or scalene) and by angle type (right, acute, or obtuse).
How it works
- 1 Enter the three side lengths Type the length of sides a, b, and c in the input boxes. Any positive numbers work — integers, decimals, or measurements in any consistent unit.
- 2 Click Calculate Press the Calculate button (or hit Enter) to compute all triangle properties. Invalid side combinations (where the triangle inequality is violated) show a clear error message.
- 3 Read the results Results include area, perimeter, all three interior angles in degrees, the altitude from each vertex, and the triangle type by both shape and angle classification.
Your data stays private
All processing happens entirely in your browser. No files, text, or data are ever sent to our servers. You can disconnect from the internet and this tool will still work.
Frequently asked questions
- What formula is used to calculate the area?
- We use Heron's formula: area = √(s(s−a)(s−b)(s−c)), where s = (a+b+c)/2 is the semi-perimeter. This formula works for any triangle given three side lengths without needing to know the height.
- How are the angles calculated?
- We use the Law of Cosines: cos(A) = (b²+c²−a²) / (2bc), and similarly for the other angles. The third angle is derived as 180° minus the sum of the first two.
- What is the triangle inequality?
- A valid triangle requires that the sum of any two sides must be strictly greater than the third side. If your input violates this (e.g., sides 1, 2, 10), the calculator will show an error instead of a result.
- What is an equilateral triangle?
- An equilateral triangle has all three sides equal and all three angles equal to 60°. An isosceles triangle has exactly two equal sides. A scalene triangle has all sides of different lengths.
- What is a right triangle?
- A right triangle has one angle of exactly 90°. An acute triangle has all angles less than 90°. An obtuse triangle has one angle greater than 90°.
- Does it work with decimal side lengths?
- Yes. You can enter any positive decimal number for each side — for example, 3.5, 7.14, or 0.001. The calculator works in any consistent unit (centimetres, metres, inches, etc.).