Triangle (Three Sides) Square Footage Calculator

When to use the three-sides calculator

Use this whenever measuring a perpendicular height is impractical, which on a real site is most of the time. Three corner-to-corner tape readings are easy to take on a plot of land, a gable, an awkward garden bed or a room with no right angles, and they pin the triangle down completely — no angle or height needed.

How Heron’s formula works

Add the three sides and halve the total to get the semi-perimeter, usually written s. The area is then the square root of s × (s − a) × (s − b) × (s − c). For sides of 9, 12 and 15 ft, s is 18, and the area is √(18 × 9 × 6 × 3) = √2,916 = 54 square feet.

The calculator does that arithmetic for you, but it is worth knowing the shape of it: the formula is exact, not an approximation, and it works for any triangle at all — right-angled, obtuse or otherwise.

Both this method and the base-and-height one are walked through in the guide to calculating the square footage of a triangle.

If your three sides do not form a triangle

Any two sides added together must be longer than the third. Sides of 3, 4 and 20 ft cannot close into a shape, so there is no area to report and the calculator will say so. When that happens, the fault is nearly always a misread or a transposed digit rather than a genuinely impossible plot — re-measure the longest side first, since that is where the error usually is.

On sloping ground, measure horizontal distances rather than following the slope. Area is always the flat, plan-view figure, so a long tape held level gives the right answer where one laid on the hillside does not.