How to use the square footage calculator
Pick the shape that matches the floor, wall or patch you measured, then enter every dimension in the same unit. For odd layouts, split the space into simple shapes and run each one.
Gross, net and planned area stay separate in the result. Gross is the shape times the number of identical spaces. Net takes off your deductions. Planned adds the waste allowance you enter.
- Choose rectangle, square, right triangle, circle, trapezoid or L-shape.
- Choose feet, meters, yards or inches and enter the dimensions. Circles take the radius.
- Set how many identical areas you have, and enter any measured deductions such as a fireplace hearth.
- Add a material allowance if your product or installer gives one. Zero leaves the area unchanged.
- Pick Area only, Cost per area unit or Whole packages, then press Calculate square footage.
Square footage formulas
Every shape comes down to a short formula. Measure heights straight across, at a right angle to the base, not along a sloping edge.
Rectangle: A = length x widthSquare: A = side x sideRight triangle: A = base x height / 2Circle: A = pi x radius^2Trapezoid: A = (base 1 + base 2) x height / 2L-shape: A = outer length x outer width - cutout length x cutout width
Worked examples
Two identical 12 x 10 ft bedrooms make 240 sq ft gross. Take off a 20 sq ft area you won't cover and you're at 220 sq ft net. A 10% allowance brings the plan to 242 sq ft. If a box of flooring covers 20 sq ft, that's 12.1 boxes, so you buy 13 and get 260 sq ft.
| Space | Inputs | Result |
|---|---|---|
| Bedroom | 12 x 10 ft | 120 sq ft (11.15 m^2) |
| Two bedrooms with deduction and 10% | 2 x (12 x 10) - 20, +10% | 242 sq ft planned, 13 boxes of 20 sq ft |
| L-shaped room | Outer 20 x 15 ft, cutout 8 x 6 ft | 252 sq ft (28 sq yd) |
| Round patio | 6 ft radius | 113.10 sq ft (10.51 m^2) |
| Trapezoid bed | Bases 12 and 8 ft, height 10 ft | 100 sq ft |
| Triangle corner | Base 10 ft, height 8 ft, +5% | 40 sq ft, 42 sq ft planned |
| Metric room | 4 x 3 m | 12 m^2 (129.17 sq ft) |
Measuring an L-shaped room
Measure the room as if the missing corner were filled in. That's your outer length and width. Then measure the cutout. The calculator subtracts the cutout once, so 20 x 15 with an 8 x 6 notch gives 252 square feet.
You can check it by splitting the L into two rectangles that don't overlap: 12 x 15 is 180, and 8 x 9 is 72. Add them and you get 252 again. If the two methods disagree, a measurement is off.
Roof square footage
A roof is bigger than the floor under it because it slopes. Measure each roof plane along the slope: the eave length times the rafter length from eave to ridge. A simple gable roof is two equal rectangles, so enter one and set identical areas to 2.
Take a 40 ft long gable roof with rafters 16.77 ft long. One side is 670.8 sq ft and both sides are 1,341.6 sq ft. Roofers sell by the square, which is 100 sq ft, so that's about 13.4 squares before waste. The calculator doesn't take a roof pitch, so measure along the slope or work out the rafter length first.
Square feet calculator for walls and paint
The result shows the boundary length, which for a 12 x 10 room is 44 feet. Multiply that by the ceiling height and you have the wall area. With 8 ft ceilings, enter a 44 x 8 rectangle to get 352 sq ft.
Measure the doors and windows you won't paint and enter their total as the deduction. Then check the coverage printed on the paint can for the number of coats you plan.
Cost and package estimates
Cost mode multiplies the planned area by your price. At $3.50 per sq ft and a 10% allowance, a 120 sq ft room plans as 132 sq ft and costs $462 in material. Labor, underlayment, trim and tax aren't included unless your rate already covers them.
Package mode divides the planned area by the coverage on one box and always rounds up. Check the actual product label, since coverage, pattern repeats and dye lots vary.
Planned area = (gross area - deductions) x (1 + allowance % / 100)Whole packages = planned area / coverage per package, rounded up
Limits and when to double-check
The answer is only as good as the tape measure. Walls bow, corners aren't always square, and a rounded measurement adds up when you multiply. Measure twice in long rooms and use the larger figure for ordering.
For home sales, appraisals and leases, square footage follows specific measurement rules, and lenders or landlords may count space differently. Use this tool for planning, not as a legal floor area.
Sources
These references back the unit conversions and the roofing unit on this page.