The formula for each shape
Every area formula answers the same question — how much surface fits inside a boundary — just with different inputs depending on the shape.
| Shape | Formula |
|---|---|
| Rectangle | A = length × width |
| Square | A = side² |
| Circle | A = π × r² |
| Triangle | A = ½ × base × height |
| Trapezoid | A = ½ × (base1 + base2) × height |
| Ellipse | A = π × a × b |
A 20 × 15 ft rectangle comes to 300 ft², while a circular patio with a 10 ft radius works out to π × 10² ≈ 314.16 ft² — close in size, despite looking very different laid out on the ground.
Measuring a room that isn't a clean rectangle
Most real rooms aren't perfect rectangles — there's a bump-out for a bay window, an L-shaped combined living/dining area, or a closet cut into a corner. The standard approach is to break the irregular shape into simple pieces (rectangles, triangles), calculate each piece's area separately with this calculator, then add them together for the total. Subtract any area you want to exclude, like a closet footprint, if you're after usable floor space rather than the full enclosed area.
What "square footage" means on a real estate listing
Listed square footage isn't always every enclosed square foot of the building. Most measurement standards count finished, heated, above-grade living space, and exclude things like garages, unfinished basements, and open (unenclosed) porches. That's why two houses with a similar footprint can show noticeably different listed square footage — one may be counting a finished basement that the other, built the same size, leaves out because its basement is unfinished.
Acres and hectares for larger areas
Once an area gets larger than a building lot, square feet stops being a convenient unit. An acre — 43,560 square feet — and a hectare — 10,000 square meters, about 107,639 square feet — exist for exactly that reason, and this calculator converts straight to either from the same shape inputs used for a small room.
Turning an area into a materials list
Once you know a space's square footage, the tile calculator turns it into a tile count and box quantity, and for a roof rather than a floor, the roofing calculator converts area into roofing squares and shingle bundles.