Use this free online Pythagorean theorem calculator to find any missing side of a right triangle - the hypotenuse or either leg. Just choose what you want to solve for, enter the known side lengths, and get an instant, accurate result.
How to Use the Pythagorean Theorem Calculator
Select what you want to "Solve for" - the hypotenuse (c), leg a, or leg b.
Enter the known side lengths that appear (Side a, Side b, and/or Hypotenuse (c)) based on your selection.
Click "Calculate" to compute the missing side.
Review the result shown below the button.
Frequently Asked Questions
The calculator uses a² + b² = c². To solve for the hypotenuse it computes c = √(a² + b²); to solve for a leg it rearranges the formula, e.g. a = √(c² - b²) or b = √(c² - a²).
When solving for a leg, the hypotenuse (c) must be strictly greater than the known leg. If it isn't, the calculator shows "The hypotenuse (c) must be greater than side b" (or side a) instead of a result, since a right triangle would be impossible otherwise.
For side a = 3 and side b = 4, solving for the hypotenuse gives c = √(3² + 4²) = √25 = 5 - the classic 3-4-5 right triangle.
No, all side lengths must be valid positive numbers greater than zero. Entering zero, a negative number, or leaving a field blank triggers an error message instead of a calculation.