Area Calculator

How to use Area Calculator

1. Choose the shape.
2. Enter its dimensions, all in the same unit.
3. Read the area and the formula applied — then add a waste allowance for real materials.
4. Triangle and parallelogram height is the perpendicular height, not a slanted side.
5. 1 m² = 10,000 cm², and 1 km² = 1,000,000 m² — the factor is squared.
6. Add roughly 10% for flooring and tiling waste, more for diagonal layouts.

Height means perpendicular height, and this is the usual error

For a triangle or a parallelogram, the height is the perpendicular distance from the base to the opposite vertex or side — not the length of a slanted side. On a leaning parallelogram the slanted side is longer than the perpendicular height, and using it overstates the area. For a triangle where you know all three sides but no height, Heron’s formula gives the area directly from the sides, which is often easier than constructing the height.

Squared units, and the mistake that costs money

Area scales with the square of the length ratio. A metre is 100 centimetres, so a square metre is 10,000 square centimetres, not 100. Double every dimension and the area quadruples. This is where flooring orders, paint quantities and land measurements go wrong, and the error is always in the same direction — under-ordering by a factor that looks plausible until the materials arrive. Convert all dimensions to one unit before calculating, never after.

Breaking an awkward shape into simple ones

Most real rooms and plots are not single shapes. The reliable method is to divide them into rectangles and triangles, calculate each, and add — or calculate the enclosing rectangle and subtract the missing corners, which is often fewer steps. For a genuinely irregular boundary, the surveyor’s approach is to split it into triangles from one vertex and use Heron’s formula on each.

Real-world area needs waste allowance

Calculated area is the minimum. Flooring and tiling conventionally add 10% for cuts and breakage, and more for a diagonal layout or a room with many angles, where offcuts are larger. Paint coverage figures on the tin assume a smooth primed surface; textured or porous walls absorb considerably more. Wallpaper is bought in whole rolls with pattern repeat waste. Ordering exactly the calculated area is how a job stops halfway through.