This free online triangle calculator finds the perimeter, area, and all three interior angles of a triangle from its three side lengths. Enter sides a, b, and c to get an instant solution using Heron's formula and the law of cosines.
How to Use the Triangle Calculator
Enter the length of "Side a".
Enter the length of "Side b".
Enter the length of "Side c".
Click "Calculate" to see the perimeter, area, and all three angles.
Frequently Asked Questions
It uses Heron's formula: first the semi-perimeter s = (a + b + c) / 2 is found, then the area is the square root of s(s - a)(s - b)(s - c).
Each angle is found with the law of cosines, for example Angle A = arccos((b² + c² - a²) / (2bc)), then converted from radians to degrees.
The calculator checks the triangle inequality theorem (the sum of any two sides must exceed the third). If your sides fail this check, or any value is zero, negative, or missing, it shows an error instead of a result.
Yes, it's completely free with no sign-up required.