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Give me two sides of a right triangle — I’ll find the third, one calm step at a time.
Choose whether to find a hypotenuse, find a leg, or check three side lengths. Enter positive lengths in the same unit and press Solve step-by-step; in Find a leg mode, put the hypotenuse in the first field and the known leg in the second.
For a right triangle, the two shorter sides a and b satisfy a² + b² = c², where c is the hypotenuse opposite the right angle. Find c with √(a² + b²), or a missing leg with √(c² − known leg²). Check mode sorts all three sides, verifies they form a triangle, and compares their squares to classify it as right, acute, or obtuse.
Choose Find the hypotenuse and enter legs 3 and 4. c = √(3² + 4²) = √25 = 5, in the same length unit.
Choose Find a leg and enter hypotenuse 13 and known leg 5. The missing leg is √(13² − 5²) = √144 = 12.
Choose Check the triangle and enter 6, 8, 10. The longest side is 10, and 6² + 8² = 36 + 64 = 100 = 10², so the triangle is right-angled.
The missing-side formulas apply only to right triangles. Check mode can classify any valid three-side triangle, but does not make a non-right triangle right-angled.
It is opposite the 90° angle and is the longest side. In Find a leg mode it must be longer than the known leg.
Yes. Positive fractional lengths are accepted. Displayed results are rounded, and classification uses floating-point tolerance rather than demanding exact equality of computer decimals.
The two shorter sides must add to more than the longest side. Lengths 1, 2, 3 form a flat line, not a triangle.
Use trigonometry when you know an acute angle and a side instead of two sides. The Pythagorean lesson develops the square relationship, and the area tool can use a height you have just calculated. Related tools: Trigonometry Calculator, Area and Perimeter Calculator. Read the Pythagorean Theorem lesson or practice pythagorean theorem to build confidence without relying on a calculator.