Triangle calculator - area and angles
Calculate a triangle's area, angles, perimeter and height from its three sides.
Enter the three sides of the triangle
Calculations are based on Estonia's tax rates in force for 2026. Results are informational.
Last updated: 2026-06-03
How is a triangle solved?
From the three sides the area is found with Heron's formula: S = √(s(s−a)(s−b)(s−c)), where s is the semi-perimeter. The angles come from the law of cosines. A triangle exists only if the sum of any two sides is greater than the third.
How it is calculated
Formula
from three sides (SSS): semi-perimeter s = (a+b+c)÷2, area = √(s(s−a)(s−b)(s−c)) (Heron's formula). Angles from the law of cosines.
Example
Sides 3, 4, 5 → area 6, right angle C = 90°.
Frequently asked questions
How do I find a triangle's area from three sides?+
Heron's formula is used: area = √(s(s−a)(s−b)(s−c)), where s is the semi-perimeter (a+b+c)÷2.
Do any three numbers form a triangle?+
No. The sum of any two sides must exceed the third (the triangle inequality), otherwise no triangle exists.