What is the Triangle Calculator?
Solve a triangle from its three sides: area by Heron's formula, perimeter and all three angles.
Enter side a, side b and side c and the Triangle Calculator returns area, perimeter, angle A, angle B, angle C and type. Results update as you type, and nothing you enter leaves your device.
How it is calculated
Heron: Area = √(s(s − a)(s − b)(s − c)), s = (a + b + c) ÷ 2
Law of cosines: cos A = (b² + c² − a²) ÷ 2bc
How to use it
- Enter side a.
- Enter side b.
- Enter side c.
- Read area in the results panel. It updates as you type.
- Change any value to compare scenarios, or use Share to send the result as a link.
Worked example
Common questions
What formula does the Triangle Calculator use?
It uses: Heron: Area = √(s(s − a)(s − b)(s − c)), s = (a + b + c) ÷ 2; Law of cosines: cos A = (b² + c² − a²) ÷ 2bc.
What do I need to use the Triangle Calculator?
You need side a, side b and side c. Sensible starting values are filled in so you can see how it works straight away.
Is the Triangle Calculator free, and is my data stored?
It is free with no sign-up. The calculation runs in your browser, so the values you enter are not sent to or stored on our server.