Triangle Calculator
Area, perimeter, angles and type of a triangle from its three sides.
Your result will appear here as soon as you add some input.
How to use it
- 1Enter your values in the fields above.
- 2The answer and the working appear immediately below.
- 3Check the working to see the formula with your numbers substituted.
Example
Input
Sides 3, 4, 5
Output
Area 6, right-angled, angles 36.87, 53.13, 90
Formulas used
- Heron's formula: A = sqrt(s(s-a)(s-b)(s-c)) where s = (a+b+c)/2
- cos A = (b^2 + c^2 - a^2) / (2bc)
About the Triangle Calculator
Heron's formula gives the area from the three sides alone, with no need for a height or an angle. The semi-perimeter s is half the perimeter.
The three lengths must satisfy the triangle inequality - the two shorter sides together must exceed the longest - or no triangle exists. That is checked first and reported plainly.
Angles come from the law of cosines, and the type is classified from the sides: equilateral, isosceles, right-angled or scalene.
Questions people ask
Is your figures uploaded to a server?
No. This tool runs entirely in your browser using standard web APIs, so your figures never leaves your device. You can confirm it by opening your browser's network panel while the tool runs, or by disconnecting from the internet after the page has loaded.
Is it free, and do I need an account?
Yes, it is free, and there is no account, no sign-up and no watermark on the result. The project is funded by optional donations.