In short
Multiply the base by the height and divide by 2. A triangle with a 12 ft base and a height of 10 ft is 12 × 10 ÷ 2 = 60 sq ft. The height is measured straight up from the base, at a right angle, not along the slanted side.
The formula
Square feet = base × height ÷ 2, with the base and the height in feet. A triangle is half of the rectangle around it, which is why you divide by 2.
Examples
| Base | Height | Sum | Area |
|---|---|---|---|
| 6 ft | 4 ft | 6 × 4 ÷ 2 | 12 sq ft |
| 8 ft | 5 ft | 8 × 5 ÷ 2 | 20 sq ft |
| 10 ft | 6 ft | 10 × 6 ÷ 2 | 30 sq ft |
| 12 ft | 10 ft | 12 × 10 ÷ 2 | 60 sq ft |
| 15 ft | 8 ft | 15 × 8 ÷ 2 | 60 sq ft |
| 20 ft | 12 ft | 20 × 12 ÷ 2 | 120 sq ft |
How to measure the height
- Pick one side as the base and measure it.
- Find the corner opposite the base.
- Measure from that corner straight to the base, at a right angle. In a room, a carpenter’s square or a laser helps.
- If the corner is right above one end of the base, the height is simply the other straight side.
A room with one angled wall
Split the room into a rectangle and a triangle. A room of 12 × 10 ft with a triangle on one side, 12 ft long and 4 ft deep, is 120 sq ft + 24 sq ft = 144 sq ft.
If you only know the three sides
Draw a line from the top straight down to the base and measure that, or use the three sides. A triangle with sides of 10, 10 and 12 ft has a height of 8 ft on the 12 ft side, so it is 12 × 8 ÷ 2 = 48 sq ft.
Figure 1. One floor of 25 × 17 ft. The rooms add up to 425 sq ft.
About this page · Written by the Squaremetry team. Last checked on October 1, 2026. Found a mistake? Send us a message.