Triangle Sqft Calculator
Use base and height if you can measure a perpendicular, or enter three sides and let Heron’s formula do the work.
To find the square footage of a triangle, multiply the base by the height and divide by 2. A triangle with a 20 ft base and 9 ft height is (20 × 9) ÷ 2 = 90 square feet. The height must be measured perpendicular to the base, not along a sloping side.
Formula: (Base × Height) ÷ 2
Formula: Heron's formula
Measuring the height correctly
The single most common error is using a sloping side as the height. The height in the formula is the perpendicular distance from the base to the opposite corner — a right angle to the base, not a diagonal.
For a right triangle this is easy: the two sides that meet at the right angle are the base and the height, so a triangle with legs of 12 ft and 9 ft is (12 × 9) ÷ 2 = 54 sq ft.
For any other triangle, if you cannot measure a perpendicular, measure all three sides instead and use the second tab above.
Heron's formula, for when you only have three sides
When measuring a perpendicular is impractical — a triangular garden bed or an odd corner of a lot — three side lengths are enough.
First find the semi-perimeter: s = (a + b + c) ÷ 2. Then the area is √(s × (s−a) × (s−b) × (s−c)).
For sides of 12, 15 and 18 ft: s = 22.5, so the area is √(22.5 × 10.5 × 7.5 × 4.5) = √7,973 = 89.29 square feet.
Where triangular measurements come up
Triangles turn up more often than you would expect once you start splitting shapes:
- Gable ends of a house, for siding or paint
- Roof sections — hips and valleys divide into triangles
- Corner lots and wedge-shaped garden beds
- Any irregular shape, which can always be divided into triangles
That last point is the useful one: every straight-sided shape can be broken into triangles, which makes this the fallback method for genuinely awkward areas.
Triangle square footage examples
| Base | Height | Square feet |
|---|---|---|
| 8 ft | 6 ft | 24 sq ft |
| 10 ft | 8 ft | 40 sq ft |
| 12 ft | 9 ft | 54 sq ft |
| 16 ft | 10 ft | 80 sq ft |
| 20 ft | 9 ft | 90 sq ft |
| 24 ft | 12 ft | 144 sq ft |
| 30 ft | 15 ft | 225 sq ft |
Frequently asked questions
How do you calculate the sqft of a triangle?
Multiply the base by the perpendicular height and divide by two. A triangle with a 20 foot base and a 9 foot height is (20 × 9) ÷ 2 = 90 square feet.
What if I cannot measure the height?
Measure all three sides and use Heron's formula instead. Find the semi-perimeter s = (a+b+c) ÷ 2, then take the square root of s(s−a)(s−b)(s−c). The calculator above does this on the second tab.
How do I find the area of a right triangle?
The two sides meeting at the right angle are the base and height, so multiply them and divide by two. Legs of 12 ft and 9 ft give (12 × 9) ÷ 2 = 54 square feet.
Is the height the same as the sloping side?
No, and confusing the two is the most common mistake. The height is the perpendicular distance from the base to the opposite corner. A sloping side is always longer, so using it overstates the area.
How do I calculate the square footage of a gable end?
A gable is a triangle: measure the width of the house as the base and the vertical height from the ceiling line to the ridge peak, then multiply and divide by two.