Square Feet of a Triangle Calculator

The formula

A = ½ × base × height

The height must be perpendicular

The height is the straight-line distance from the base to the opposite vertex, measured at right angles to the base — not the length of a slanted side. For an obtuse triangle that perpendicular can fall outside the triangle entirely, and the formula still holds.

What always holds

• The three angles add to 180°, without exception.
• Any two sides must add to more than the third — the triangle inequality. Sides of 4, 5 and 12 cannot close.
• The longest side always faces the largest angle.
• Area = ½ × base × height for any triangle, with the height measured perpendicular to that base.

Which formula to reach for

With three sides, use Heron’s formula. With a base and its height, use ½bh. With two sides and the angle between them, use ½ab·sin C. With a right angle, the two legs are already base and height, so the area is just half their product.