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Perimeter of a Right Triangle Calculator.

Calculate the hypotenuse and perimeter from two legs.

On-device calculationNo signup
01

Set your values

Results update as you type.

Hypotenuse: 5

Hypotenuse

5.000000
Perimeter: 12

Perimeter

12.000000

Results update automatically as you type.

Use Cases

Construction and DIY Projects

Determine the total length of material needed to frame a right-angled corner, such as a deck, fence, or roof truss.

Example: If leg a = 3 ft and leg b = 4 ft, the perimeter is 12 ft.

Geometry Homework

Quickly verify your manual calculations for right triangle perimeters in math assignments.

Example: Check that a right triangle with legs 5 and 12 has a perimeter of 30.

Frequently Asked Questions

How do I calculate the perimeter of a right triangle?
Enter the lengths of the two legs (leg a and leg b) into the calculator. It computes the hypotenuse using the Pythagorean theorem (c = √(a² + b²)) and then adds all three sides: perimeter = a + b + c.
What if I only know the hypotenuse and one leg?
This calculator requires both leg lengths. If you have the hypotenuse and one leg, you can find the other leg using the Pythagorean theorem (leg = √(hypotenuse² - known leg²)) and then use this calculator.
Can I use this calculator for any triangle?
No, this calculator is specifically for right triangles. For other triangles, you need the lengths of all three sides or other measurements like angles.

Tips & Common Mistakes

Tips

  • Ensure both leg lengths are in the same unit before entering them to get an accurate perimeter.
  • The calculator uses the Pythagorean theorem, so it works for any right triangle, even with decimal or fractional leg lengths.
  • Double-check your inputs: swapping leg a and leg b does not change the perimeter, but entering a negative or zero value will produce an error.
  • Use the result to quickly estimate material needs or verify your own math.

Common Mistakes to Avoid

  • Entering the hypotenuse as one of the legs – this calculator only accepts the two perpendicular sides.
  • Forgetting to convert units (e.g., mixing feet and inches) leads to incorrect perimeter values.
  • Assuming the perimeter is just the sum of the two legs – you must include the hypotenuse, which the calculator computes for you.

Last updated: August 13, 2026