Find the missing side of a right triangle — hypotenuse or leg — with step-by-step working.
Give any two sides of a right triangle and the third is found, with area, perimeter and angles.
Missing side
—How to use this tool
Choose whether you want the hypotenuse or one of the legs.
Enter the two sides you know. When finding a leg, the hypotenuse goes in the “Hypotenuse c” box.
The missing side, area, perimeter and all three angles appear instantly, with the step-by-step working.
If all three sides are whole numbers, the result is flagged as a Pythagorean triple.
Formula & worked example
Legs a = 3 and b = 4: c² = 3² + 4² = 9 + 16 = 25, so c = 5. The area is (3 × 4) ÷ 2 = 6, the perimeter is 3 + 4 + 5 = 12, and the acute angles are about 36.87° and 53.13°. This is the famous 3-4-5 Pythagorean triple.
Limitations — read this before you rely on it
Only right triangles are supported: one angle must be exactly 90°. Sides must be positive numbers, and the hypotenuse must be longer than either leg. Results are rounded to 4 decimal places, so the a² + b² = c² check may differ in the last decimal for non-whole sides.
Frequently asked questions
Use a² + b² = c², so c = √(a² + b²). For legs 3 and 4, c = √(9 + 16) = 5.
Rearrange the theorem: a = √(c² − b²). The hypotenuse must be longer than the known leg, or no right triangle is possible.
A Pythagorean triple is a right triangle whose three sides are all whole numbers — 3-4-5 is the smallest. The calculator flags one when your sides form a triple.
No — the theorem only applies to right triangles, where one angle is exactly 90°. For other triangles you need the law of cosines.
Yes. Enter any two sides and the third is found, along with the area, perimeter and all three angles.