Quick start
- Enter side a, side b, and side c using the same unit.
- Add a unit label if you want the result to show cm, m, in, or another unit.
- Press Calculate triangle.
- Review area, perimeter, semiperimeter, angles, side type, and angle type.
When to use three-side triangle mode
Use this calculator when you know all three side lengths but do not know the height. The calculator first checks that the sides can form a real triangle.
After the sides pass the triangle inequality check, it uses Heron's formula for area and the law of cosines for angle estimates.
Reading the answer
Area is shown in square units. Perimeter and semiperimeter are shown in the same length unit you entered.
The classification helps you spot whether the triangle is scalene, isosceles, equilateral, acute, right, or obtuse.
Common mistakes to avoid
Do not mix units in the same calculation. Convert all sides first if one side is in inches and another is in centimeters.
If the calculator says the triangle inequality fails, the three side lengths cannot close into a triangle.
Examples from the calculator
Area = 84
Area = 6
Area = 31.2249899919
History, privacy, and copying
Recent answers stay visible in the page while you work. The history is kept only in the current browser tab and is not sent to a server.
Copy answer copies the expression and result so you can paste it into notes, homework, a message, or another document.