Right Triangle Side Calculator
Missing leg or hypotenuse, plus both acute angles.
Enter your values
Choose one of: Hypotenuse (both legs known), A leg (one leg and the hypotenuse known).
Enter the leg a you are working with.
Enter the leg b or hypotenuse c you are working with.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Missing leg or hypotenuse, plus both acute angles. |
|---|---|
| Category | Math |
| Inputs needed | What are you solving for?, Leg a, Leg b or hypotenuse c |
| Main output | Hypotenuse |
| Formula | a² + b² = c² |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- What are you solving for?
- Choose one of: Hypotenuse (both legs known), A leg (one leg and the hypotenuse known).
- Leg a
- Enter the leg a you are working with.
- Leg b or hypotenuse c
- Enter the leg b or hypotenuse c you are working with.
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.
What do I need to enter into the Right Triangle Side Calculator?
Just 3 values: what are you solving for?, leg a, leg b or hypotenuse c. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Right Triangle Side Calculator work out the answer?
In a right triangle a² + b² = c², so the hypotenuse is √(a² + b²) and a missing leg is √(c² − a²). It applies the formula a² + b² = c² and shows the working so you can check each step by hand.
Is the Right Triangle Side Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.