About Missing Leg Calculator
This missing leg calculator finds leg a of a right triangle from leg b and the hypotenuse c. It subtracts the square of the known leg from the square of the hypotenuse and takes the square root. It is free, needs no sign-up and runs in your browser, so nothing you enter is uploaded.
The hypotenuse must be longer than the known leg, otherwise no right triangle exists and the calculator says so. Use the find leg b option when it is leg a that you know.
How to use Missing Leg Calculator
- Choose what to find: the hypotenuse c, leg a, leg b, or check three sides.
- Type the known sides. Only the boxes you need are shown.
- Pick a length unit if you want it printed in the answer.
- Read the answer, the area, the perimeter and the two acute angles.
- Open the step-by-step working to see the squares, the subtraction or addition and the square root.
- In check mode, read the verdict and the triple note, for example whether the sides are a primitive triple.
Features
- Find the hypotenuse, either leg, or test three sides.
- Step-by-step working with KaTeX formulas.
- Pythagorean triple detection, with the primitive triple and Euclid’s m and n.
- Right, acute or obtuse verdict from the converse of the theorem.
- A diagram with a labelled square on each side.
- Area, perimeter and both acute angles of the triangle.
- Clear errors, for example when the hypotenuse is not the longest side.
Missing leg formula
The difference of squares under a root.
a = √(c² − b²)Worked examples
b = 12, c = 13 gives a = 5
169 − 144 = 25, so a = 5.
Privacy: how it works
- All the arithmetic runs in your browser tab. The sides you enter are not sent to Ulkaa or to anyone else, and there is nothing to sign in to.
Tips
- The hypotenuse is always the longest side. If you enter a leg longer than c, the calculator refuses and says so.
- To test a right angle on site, measure 3 units along one wall and 4 along the other. The diagonal should be 5.
- Keep all sides in the same unit. The calculator does not convert between units for you.
- A triple needs whole numbers. 1.5, 2, 2.5 satisfy the theorem but are not counted as an integer triple.
Frequently asked questions
Is it really free?
Yes. There is no price, no trial and no credit card. Ulkaa Tool does the work with your own device, so there is no server bill to pass on to you.
Do I need an account?
No. There is no sign-up and no login. Open the page and use it. An Ulkaa account is not needed for anything on this tool.
Does it run in my browser?
Yes. Your values are processed by code running in your browser tab, on your own device.
Is my data uploaded?
No. Your values are never sent to Ulkaa or to anyone else. You can check this yourself by watching the Network tab in your browser developer tools while you use the tool.
How do I find a missing leg?
Square the hypotenuse, subtract the square of the known leg and take the square root.
Why does it say the hypotenuse must be longer?
Because the hypotenuse is the longest side. A leg as long as it or longer cannot belong to a right triangle.
More Pythagorean Theorem Calculator pages
- Pythagorean Theorem CalculatorSolve for any side of a right triangle, check whether three sides are a Pythagorean triple, and see the proof in a picture.
- Hypotenuse CalculatorEnter the two legs and get the hypotenuse.
- Find Leg b CalculatorEnter leg a and the hypotenuse to solve for leg b.
- 3-4-5 Triangle CalculatorSee why 3, 4 and 5 make a right angle and how to scale it.
- Pythagorean Triples CalculatorEnter three whole numbers to test them and see the primitive triple and Euclid’s m and n.
- Right Triangle CheckerEnter three sides in any order and see whether the triangle is right, acute or obtuse.
- Converse of the Pythagorean Theorem CalculatorEnter three sides and apply the converse to classify the triangle.