How to use the triangle calculator
- Choose a method that matches the information you have.
- Enter positive side lengths or valid angles in degrees.
- Click Calculate triangle to see the missing measurements, area and perimeter.
Triangle formulas
P = a + b + c
A = √[s(s − a)(s − b)(s − c)], where s = P/2
c² = a² + b² − 2ab cos(C)
A + B + C = 180°
Types of triangles
A triangle can be classified by its sides as equilateral, isosceles or scalene, and by its angles as acute, right or obtuse. The calculator reports a combined description based on the calculated values.
Frequently asked questions
What measurements do I need?
Most triangles can be solved from three suitable measurements, including three sides (SSS), two sides and their included angle (SAS), or two angles and a side (ASA/AAS). Some combinations can be ambiguous or impossible.
How is triangle area calculated?
When all three sides are known, the calculator uses Heron's formula. For base and height, area is one-half times base times height.
Can any three side lengths form a triangle?
No. The sum of any two side lengths must be greater than the third side. The calculator checks this triangle inequality before displaying a result.
Are angles measured in degrees?
Yes. Enter angles in degrees. Trigonometric calculations internally convert degrees to radians where required.
Are results sent to a server?
No. The calculation runs in your browser, and the measurement inputs are not submitted to the calculator page's server.
Results are mathematical estimates based on the measurements entered. Check units before using measurements for construction or other precision-critical work.