Math

Right Triangle Side Calculator

Find Any Missing Side of a Right Triangle

Select which side you need to find -- the hypotenuse or either leg -- then enter the two known sides. The calculator shows the step-by-step solution using the Pythagorean theorem.

Pythagorean Theorem Tips

Click to show tips

Try an Example

Pick a scenario to see how the calculator works, then adjust the values

Ladder Against Wall

A 10-foot ladder leans against a wall with the base 6 feet away. How high does it reach?

Key values: Leg a = 6 ft · Hypotenuse c = 10 ft · Solve for leg b

TV Screen Diagonal

Find the diagonal of a TV with a 16:9 aspect ratio (width 40", height 22.5").

Key values: Width = 40 in · Height = 22.5 in · Find diagonal

Classic 3-4-5 Triangle

Verify that sides 3, 4, and 5 form a right triangle (the most famous Pythagorean triple).

Key values: Sides: 3, 4, 5 · Right triangle · Primitive triple

Documentation

This calculator is also known as Right Triangle Side Calculator.

Read the complete guide

Which Formula to Use

The formula depends on which side is missing:

Category	Value
Missing hypotenuse	c = √(a² + b²)
Missing leg a	a = √(c² - b²)
Missing leg b	b = √(c² - a²)

Examples

Finding a Leg

A right triangle has hypotenuse 13 and one leg 5. Find the other leg.

b = √(13² - 5²) = √(169 - 25) = √144 = 12

Key takeaway: (5, 12, 13) is a well-known Pythagorean triple.

Tips for Finding Missing Sides

Avoid common mistakes:

The hypotenuse is always longer than either leg
When solving for a leg, subtract (not add) the squares
Check your answer by verifying a² + b² = c²

Frequently Asked Questions about Right Triangle Side Calculator

How do I find the missing side of a right triangle?

Use the Pythagorean theorem: if the hypotenuse is unknown, c = √(a² + b²). If a leg is unknown, a = √(c² - b²). Choose the correct mode in the calculator and enter the known sides.

Which side is the hypotenuse?

The hypotenuse is always the longest side of a right triangle and is located directly opposite the 90-degree angle.

What if I only know the hypotenuse and one leg?

Use the rearranged formula: missing leg = √(c² - known leg²). Select the appropriate solve mode in the calculator and enter the hypotenuse and the known leg.