Right Triangle Side Calculator
Missing leg or hypotenuse, plus both acute angles.
Enter your values
ⓘ Live results
Results
- Hypotenuse
- 5
- Legs
- 3 and 4
- Hypotenuse
- 5
- Angle opposite leg a
- 36.87°
- Other acute angle
- 53.13°
- Area
- 6
- Perimeter
- 12
- Altitude to the hypotenuse
- 2.4
Quick answer
In a right triangle a² + b² = c², so the hypotenuse is √(a² + b²) and a missing leg is √(c² − a²).
a² + b² = c²
How to use the Right Triangle Side Calculator
- 1Choose whether you are finding the hypotenuse or a leg.
- 2Enter the two lengths you know.
- 3Read the missing side plus both angles for layout or cutting.
Worked example
Using the values the calculator loads with:
Inputs
- What are you solving for?Hypotenuse (both legs known)
- Leg a3
- Leg b or hypotenuse c4
Results
- Hypotenuse5
- Legs3 and 4
- Hypotenuse5
- Angle opposite leg a36.87°
- Other acute angle53.13°
- Area6
- Perimeter12
- Altitude to the hypotenuse2.4
Frequently asked questions
What is the 3-4-5 rule?
A 3-4-5 triangle is exactly right-angled, which is why builders use it to square up corners on site.
Does this work in metres and inches?
Any unit, as long as both inputs use the same one — the output matches.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.