Triangle Square Footage Calculator

When to use the triangle calculator

Triangles turn up wherever a space stops following the grid: an attic corner under a sloping roof, a gable end you are siding, a wedge of garden between two fences, a plot on a street corner, or the offcut left when a diagonal wall crosses a rectangular room. The formula is base × height ÷ 2 — a triangle is always exactly half the rectangle that would enclose it.

The full method, covering right triangles and ones with three unequal sides, is in the guide to calculating the square footage of a triangle.

Getting the height right

Any of the three sides can be the base, but the height must be measured perpendicular to whichever side you chose. This is the part that goes wrong: the sloping side of a triangle is always longer than the perpendicular height, and using it in place of the height overstates the area, sometimes badly.

On the ground, run the tape from the far corner until it meets the base at a right angle. If the opposite corner sits outside the span of the base — a very slanted triangle — extend the base line with a chalk line or a string and measure to that instead.

When to use three sides instead

On a real site, a perpendicular height is often impossible to take, while three side lengths are easy. If that is your situation, use the three-sides triangle calculator, which applies Heron’s formula and gives exactly the same answer.

Base and height is quicker when the triangle is a corner cut off a rectangular room, because two of the sides are already square to each other — measure along the two walls and you have your base and height without any extra work. To combine a triangle with other shapes in one figure, add each piece as a row in the multi-room calculator.