The square footage of a triangle is ½ × base × height. Measure the base and the perpendicular height in feet, multiply them together, and divide by 2. For example, a triangle with an 18-foot base and a 12-foot height covers ½ × 18 × 12 = 108 square feet.
For real-world triangles, our roof square footage guide and gable wall siding guide put this formula to work.
How to Calculate the Square Footage of a Triangle
When you can measure a base and the height straight up from it, a triangle's area is half of base × height. Gable ends, triangular corners of yards and angled walls are the usual cases.
The formula
Area = ½ × base × height
The height must be perpendicular to the base. If you only have the three side lengths, use the three-sides (Heron's formula) calculator instead.
How to measure
Measure along the base, then the vertical height from the base to the opposite corner — a plumb line or level helps on a wall. Measuring along a sloped side instead of the vertical height overstates the area.
Worked example
A gable end wall is 24 ft wide at the base and 6 ft from the base to the peak.
Area = ½ × 24 × 6 = 72 square feet of siding, before windows and waste.
Where this shape is used
- Gable walls: Siding and paint for triangular wall sections.
- Yard corners: Sod, mulch or pavers for triangular areas.
- Roof sections: Triangular hip faces.
Related shape calculators
Working with a different shape? Try the triangle from three edges calculator, irregular triangle calculator, trapezoid calculator, or the parallelogram calculator. Each one shows the formula, a worked example and unit conversions.