Triangle Calculator
Calculate the area, perimeter, and other properties of a triangle given three side lengths.
Published by:
isCalculator Editorial Team
Reviewed by:
Review Board
Last Updated:
Was this calculator helpful?
How to use this tool?
How to Use the Triangle Calculator
- Enter side lengths – Input positive numbers for Side A, Side B, and Side C (e.g., 3, 4, 5). Any unit works (cm, inches, etc.), but all sides must use the same unit.
- Click "Calculate" – The calculator validates that each side is a positive number and that the three sides satisfy the triangle inequality (each side must be less than the sum of the other two).
- View results – After a valid calculation, you'll see:
- Area – Computed using Heron's formula: √(s(s-a)(s-b)(s-c)) where s = (a+b+c)/2.
- Perimeter – Sum of all three sides.
- Triangle Type – Equilateral (all sides equal), Isosceles (two sides equal), or Scalene (no equal sides).
- Fix errors – If a side is invalid (empty, zero, or negative), the input field highlights red. If the sides don't form a triangle, an alert appears. Correct the values and recalculate.
Tip: Use any consistent unit (metric or imperial). The area and perimeter will be in the same squared/linear units.
Previous Results
| Result | Inputs | Date |
|---|
Frequently Asked Questions
It calculates the area, perimeter, and type of a triangle given three side lengths using Heron's formula.
Enter the lengths of the three sides (positive numbers) and click Calculate.
The calculator checks the triangle inequality theorem. If the sum of any two sides is not greater than the third, it will show an error.
