What Is a Right Triangle?

A right triangle is a triangle with one angle that is exactly 90°. The two sides that make the 90° angle are called the legs. The longest side is called the hypotenuse.

Remember: The hypotenuse is always opposite the 90° angle.

How to Use This Calculator

Finding the missing side of a right triangle is easy:

1. Enter any two known sides.
2. Click the Calculate button.
3. The calculator finds the missing side and other useful measurements.

Pythagorean Theorem

The Pythagorean theorem helps us find a missing side of a right triangle.

a² + b² = c²

Here, a and b are the two shorter sides, while c is the hypotenuse.

Example: A 3-4-5 Triangle

Suppose a right triangle has two sides measuring 3 units and 4 units.

c² = 3² + 4²

c² = 9 + 16

c = 5 units

So, the three sides are 3, 4, and 5 units.

Area of a Right Triangle

To find the area, multiply the two legs and divide the answer by 2.

Area = ½ × a × b

For sides of 3 and 4 units:

Area = ½ × 3 × 4 = 6 square units

Perimeter of a Right Triangle

The perimeter is the distance around the triangle. Simply add all three sides.

Perimeter = a + b + c

For a 3-4-5 triangle, the perimeter is 12 units.

Quick Facts

• A right triangle has one 90° angle.
• The hypotenuse is the longest side.
• The hypotenuse is opposite the right angle.
• The other two angles always add up to 90°.
• The Pythagorean theorem is a² + b² = c².