Triangle Area Calculator
Area from base and height, or all three sides.
Enter your values
Choose one of: Base and height, Three sides (Heron).
Enter the base or side a you are working with.
Enter the height or side b you are working with.
Enter the side 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.
- Area
- 26.832816
- Perimeter
- 24
- Semi-perimeter
- 12
- Height on side a
- 7.666519
Working
- 1s = 12
- 2A = √(12 × 5 × 4 × 3) = 26.8328
Quick answer
Triangle area is ½ × base × height, or Heron's formula √(s(s−a)(s−b)(s−c)) when you only know the three sides.
A = ½bh or √(s(s−a)(s−b)(s−c))
At a glance
| What it does | Area from base and height, or all three sides. |
|---|---|
| Category | Math |
| Inputs needed | Method, Base or side a, Height or side b, Side c |
| Main output | Area |
| Formula | A = ½bh or √(s(s−a)(s−b)(s−c)) |
| Cost | Free — no sign-up, no download |
How to use the Triangle Area Calculator
- 1Choose whether you know base and height or all three sides.
- 2Enter the measurements.
- 3Read the area and perimeter.
Inputs explained
- Method
- Choose one of: Base and height, Three sides (Heron).
- Base or side a
- Enter the base or side a you are working with.
- Height or side b
- Enter the height or side b you are working with.
- Side c
- Enter the side c you are working with.
Worked example
Using the values the calculator loads with:
Inputs
- MethodThree sides (Heron)
- Base or side a7
- Height or side b8
- Side c9
Results
- Area26.832816
- Perimeter24
- Semi-perimeter12
- Height on side a7.666519
- s = 12
- A = √(12 × 5 × 4 × 3) = 26.8328
Frequently asked questions
What is Heron's formula?
A way to get the area from side lengths alone, using the semi-perimeter s = (a+b+c)/2.
Why does my triangle fail?
Any two sides must sum to more than the third — otherwise the shape cannot close.
What do I need to enter into the Triangle Area Calculator?
Just 4 values: method, base or side a, height or side b, side 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 Triangle Area Calculator work out the answer?
Triangle area is ½ × base × height, or Heron's formula √(s(s−a)(s−b)(s−c)) when you only know the three sides. It applies the formula A = ½bh or √(s(s−a)(s−b)(s−c)) and shows the working so you can check each step by hand.
Is the Triangle Area 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.