Length times width
Square footage is the area of a rectangle: length in feet times width in feet. A room 12 feet long and 15 feet wide is 12 × 15 = 180 square feet. Divide by 9 for square yards, because a square yard is 3 feet by 3 feet: 180 ÷ 9 = 20 square yards. The international foot is exactly 0.3048 meters, so one square foot is 0.3048 × 0.3048 = 0.09290304 square meters. Those 180 square feet are 16.72 square meters.
Measure each wall in feet and inches, then convert inches to a fraction of a foot before you multiply. Six inches is half a foot. A length of 12 feet 6 inches is 12.5 feet. A width of 10 feet 0 inches stays 10. The area is 12.5 × 10 = 125 square feet. Forgetting to convert the inches, and multiplying 12 × 10 while ignoring the extra 6 inches, drops 5 square feet on this room.
Worked example
An L-shaped room is two rectangles. Measure each rectangle, compute each area, and add them. Do not multiply the longest length by the longest width. That rectangle contains floor you do not have. A triangle, such as a gable end, is base times height divided by 2, with both measurements in feet. A circle is π times the radius squared. The shape menu uses those formulas in feet and reports square feet.
- Length: 12 feet 6 inches = 12 + 6/12 = 12.5 feet.
- Width: 10 feet = 10 feet.
- Area: 12.5 × 10 = 125 square feet.
- In square meters: 125 × 0.09290304 = 11.61 m².
- The other way: a 20 m² room is 215.28 square feet, because 1 ÷ 0.09290304 is about 10.7639 square feet per square meter.
Flooring, paint, and the extra you actually buy
Square footage of the floor is the starting number for tile, carpet, or hardwood, not the number of boxes to buy. Installers add waste for cuts and pattern match, often 10% for a straight lay and more for a diagonal. This calculator does not add waste, because the right allowance depends on the material and the room. Take the square footage it gives you and apply the percentage your supplier asks for with the percentage calculator.
Walls are a different rectangle: width of the wall times height, minus doors and windows if you are buying paint. A 12-foot wall that is 8 feet high is 96 square feet before openings. Ceilings use the same footprint as the floor in a room with a flat ceiling. Keep the feet consistent. Mixing inches on one side and feet on the other, without converting, is the error that makes a room look ten times too small or too large.