Square footage is the starting point for almost every home project: flooring, tile, paint, sod, and the price of the house itself. Most rooms are rectangles, but plenty are L-shaped or have a bay, and yards come in every shape. This calculator handles the common ones, converts between units, and turns the area into quantities of the materials people most often buy by area.
How it is calculated
- Rectangle: length × width.
- L-shape: split it into two rectangles and add them. Any room made of right angles can be broken into rectangles this way.
- Circle: π × (diameter ÷ 2)².
- Triangle: ½ × base × height, where height is the perpendicular distance from the base to the opposite corner.
- Trapezoid: ½ × (top + bottom) × height.
- Conversions: 1 m² = 10.764 ft²; 1 yd² = 9 ft²; 1 acre = 43,560 ft²; 1 hectare = 10,000 m².
Odd-shaped rooms and lots
Choose "Draw my own outline" and walk the shape: the length of each wall and which way you turn at its end. Right angles are the default; corners of 45° or 135° are available for bay windows and angled lots. The shape is drawn as you type, the area comes from the coordinates of the corners (the shoelace formula), and the perimeter is shown alongside, which is what you need for baseboards, fencing or paint.
Example
A 15 × 12 ft living room is 180 ft² (16.7 m², 20 yd²). With 10% waste, buy flooring for 198 ft²: ten boxes at 20 ft² each. To paint the walls of the same room with 8 ft ceilings, the wall area is the perimeter × height: 54 × 8 = 432 ft², minus doors and windows, about two gallons for two coats.
Measuring a room
Measure at floor level along the longest walls, in feet and inches, and convert inches to decimals (6 in = 0.5 ft). For odd shapes, sketch the room, divide it into rectangles and triangles, and add the pieces; subtract fireplaces and built-ins only if the material will not go under them. For flooring, most installers measure the whole room and ignore small obstructions, since offcuts rarely fit anyway.
Square footage of a house
Listing square footage usually counts finished, heated, above-grade space measured from the exterior walls, per the ANSI Z765 standard; basements, garages and porches are listed separately. Two houses with the same number can differ a lot in usable space, so ask what was included.
Frequently asked questions
How do I calculate square footage?
Multiply length by width in feet. A 12 × 15 ft room is 180 square feet. For other shapes, use the shape options above or split the space into rectangles and add them.
How do I convert square feet to square metres?
Divide by 10.764. 180 ft² is 16.7 m². To go the other way, multiply square metres by 10.764.
How much extra flooring should I buy?
10% for straight layouts, 15% for diagonal or herringbone patterns, and keep a box for future repairs. Boxes must be whole, so round up.
Does square footage include walls and closets?
For flooring, measure the floor you will cover, including closets if they get the same material. For real-estate listings, finished space is measured from the outside of exterior walls and includes interior walls and closets.
How many square feet in an acre?
43,560. A quarter-acre lot is about 10,890 ft², roughly 100 × 109 ft.
Sources
By Calcelate Team. Formula from the sources above.
- 2026-09-14 · Formula and texts checked