The square footage of a triangle from three sides uses Heron’s formula. Compute s = (a+b+c)/2, then Area = √[s(s−a)(s−b)(s−c)]. For example, sides 13, 14 and 15 ft give s = 21 and Area = 84 square feet.
Only know the three sides? Our triangle square footage calculator covers Heron’s formula, and the irregular acreage guide uses it to measure land.
How to Calculate the Square Footage of a Triangle from Three Sides
On a real yard corner or angled room you can usually measure the three sides but not the height. Heron's formula gives the area from the three side lengths alone, which is what this calculator uses.
The formula
s = (a + b + c) ÷ 2, then Area = √[s(s − a)(s − b)(s − c)]
s is half the perimeter. The three sides must form a real triangle: any two sides together must be longer than the third.
How to measure
Measure each side in a straight line from corner to corner. Small measuring errors matter more on long, thin triangles, so measure those twice.
Worked example
A triangular corner of a yard measures 30 ft, 40 ft and 50 ft.
s = (30 + 40 + 50) ÷ 2 = 60.
Area = √(60 × 30 × 20 × 10) = 600 square feet.
Where this shape is used
- Yard and lot corners: Sod, seed or gravel for triangular areas.
- Angled rooms: Flooring for triangular sections of a floor.
- Roof and wall sections: When only side lengths are known.
Related shape calculators
Working with a different shape? Try the triangle from base and height calculator, irregular triangle calculator, irregular quadrilateral calculator, or the rhombus calculator. Each one shows the formula, a worked example and unit conversions.