The most famous equation in geometry fits on a napkin: a² + b² = c². Give it any two sides of a right triangle and it hands you the third — no protractor, no trigonometry, just squares and a square root. Carpenters use it to check square corners (the 3-4-5 triangle: a 3-foot leg and 4-foot leg must give exactly a 5-foot diagonal), navigators use it for shortest-path distances, and it quietly powers everything from screen-size specs to GPS math. This solver works all three directions: enter the two known sides in Side A (leg) and Side B (leg), or provide one leg plus the hypotenuse, and it computes the missing side along with the Triangle Area and Perimeter as bonuses. The classic 3-4-5 triangle gives hypotenuse 5, area 6, and perimeter 12 — try it first. Units carry through whatever you choose (feet, meters, inches), and the one hard rule is that the hypotenuse, the side opposite the right angle, is always the longest.

With the pre-filled defaults — Side A (leg) 3, Side B (leg) 4 — the Pythagorean Theorem Calculator returns Hypotenuse (C) 5.0000, Triangle Area 6.0000 m², Perimeter 12.0000 m. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.