About 3-4-5 Triangle Calculator
This page opens on the 3-4-5 triangle, the smallest Pythagorean triple: 3² + 4² = 9 + 16 = 25 = 5². The diagram shows a square on each side with areas 9, 16 and 25. It is free, needs no sign-up and runs in your browser, so nothing you enter is uploaded.
Builders use it to square a corner: mark 3 units along one wall and 4 along the other, and the diagonal should measure 5. Any multiple works, such as 6, 8, 10 or 30, 40, 50 for a large plot.
How to use 3-4-5 Triangle 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.
Why it works
The squares of the legs add to the square of the hypotenuse.
3² + 4² = 9 + 16 = 25 = 5²Worked examples
a = 3, b = 4 gives c = 5
The angles are 36.87°, 53.13° and 90°.
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.
What is the 3-4-5 rule?
A triangle with sides in the ratio 3 : 4 : 5 has a right angle. It is used to check that a corner is square.
Do 6, 8, 10 also make a right triangle?
Yes. Doubling every side keeps the ratio, so 6, 8, 10 is a right triangle too.
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.
- Missing Leg CalculatorEnter one leg and the hypotenuse to find the other leg.
- Find Leg b CalculatorEnter leg a and the hypotenuse to solve for leg b.
- 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.