Right Triangle Calculator

How it is calculated

You enter the two legs, a and b, the sides that meet at the 90-degree corner. The tool computes the hypotenuse c = √(a² + b²), the area A = a × b ÷ 2 and the perimeter P = a + b + c. Each result is rounded to two decimals. Both legs must be greater than zero. Use any length unit, as long as both legs share it; lengths come out in that unit and the area in its square. The calculator cannot start from the hypotenuse and one leg, so if you know c, you must work out the missing leg yourself first (b = √(c² − a²)). Angles are not shown.

Common mistakes

• Entering the hypotenuse as one of the legs. The longest side must not be typed in; it is calculated, and putting it in a or b gives a larger triangle than the one you have.
• Using a side that does not meet the right angle. Both inputs must be the two sides forming the 90-degree corner.
• Mixing units between the two legs, for example feet and inches, which makes the hypotenuse meaningless.
• Expecting an answer when the right angle is not exactly 90 degrees. For any other angle use a law-of-cosines tool instead.
• Reading the area as being in the same unit as the legs. If legs are in meters, the area is in square meters.