Square footage is length times width for a rectangle: a room 15 ft by 12 ft 6 in is 15 × 12.5 = 187.5 sq ft. Add a 12 × 11 ft bedroom and the two rooms total 319.5 sq ft, which is 35.5 square yards or 29.68 square meters. Other shapes have their own formula, and an odd-shaped area is split into pieces that each have one.
How square footage is calculated
Inches become feet first (divide by 12: 12 ft 6 in is 12.5 ft), then each shape's formula applies. The calculator adds every area on your list and subtracts the ones marked as cut-outs.
Rectangle: length × width · Triangle: base × height ÷ 2 · Circle: π × radius² · Trapezoid: (side a + side b) ÷ 2 × height
Square yards are square feet ÷ 9, square meters are square feet × 0.092903 and an acre is 43,560 sq ft, all exact definitions in NIST Handbook 44.
Worked example: two rooms in feet and inches
The calculator opens with a living room 15 ft by 12 ft 6 in and a bedroom 12 ft by 11 ft.
- Living room: 15 × 12.5 = 187.5 sq ft
- Bedroom: 12 × 11 = 132 sq ft
- Total: 187.5 + 132 = 319.5 sq ft
- Square yards: 319.5 ÷ 9 = 35.5 sq yd
- Acres: 319.5 ÷ 43,560 = 0.0073 acres
Each shape is drawn to scale from the numbers you type, so a typo shows up as a room with the wrong proportions.
Square footage of a triangle
Pick "Triangle: base and height" when you can measure square off the base to the far corner: a 12 ft base with a 10 ft height is 12 × 10 ÷ 2 = 60 sq ft. When all you have are the three sides, as on a corner lot or an angled deck, pick "three sides". The calculator uses Heron's formula: for sides of 10, 12 and 14 ft, half the perimeter is 18, and √(18 × 8 × 6 × 4) = 58.79 sq ft.
Square footage of a circle
Measure straight across through the middle. A round patio 12 ft across has a 6 ft radius: π × 6² = 113.1 sq ft.
Irregular shapes
Four sides and a diagonal. Four sides alone can make a square or a flattened diamond; one diagonal locks the shape into two triangles. A lot with sides of 40, 55, 42 and 50 ft and a 68 ft diagonal is 2,146.1 sq ft, or 0.0493 acres.
Corner coordinates. Pick a starting corner, lay a tape along one wall and measure each corner's distance across and up from there. The shoelace formula turns the list into an area. The default example, a 30 × 20 ft block with a 12 ft gable-shaped bump on one side, gives 780 sq ft.
An L-shaped room is its overall rectangle minus the missing corner: 20 × 15 ft minus an 8 × 6 ft notch is 300 − 48 = 252 sq ft. A trapezoid with parallel sides of 20 and 14 ft, 10 ft apart, is (20 + 14) ÷ 2 × 10 = 170 sq ft.
Square footage of multiple rooms
Add one line per room and use "How many like this" for repeats, such as three identical bedrooms. Mark islands, columns and built-ins as subtracted: a 14 × 12 ft kitchen with a 6 × 3 ft island leaves 168 − 18 = 150 sq ft of floor to cover. For one room with closets and bay windows, and its walls and ceiling, the room square footage calculator handles the pieces directly.
Measuring from a map
"Draw on a map" puts your address on satellite imagery; tap each corner of a lot, driveway or roof and the area appears as you go. Aerial photos lean and roofs overhang the walls, so treat it as a planning number and use a tape for anything you buy material for.
Square footage for flooring, tile and carpet
Flooring is sold by the square foot or by the carton, so start from the floor area and add cutting waste in the flooring calculator or the tile calculator. Carpet is priced by the square yard: the 319.5 sq ft example is 35.5 sq yd before seams, which the carpet calculator works out from roll width.
Square footage for HVAC
Heating and cooling loads start from each room's floor area. Measure rooms separately here, then take the numbers to the BTU calculator.
Square footage for concrete
A slab needs area times thickness. A 10 × 10 ft patio is 100 sq ft; at 4 in thick that is 100 × 4 ÷ 12 = 33.3 cu ft, or 1.23 cubic yards. The concrete slab calculator adds waste and bag counts, and the cubic yard calculator converts any area and depth.
Square footage for wallpaper and paint
Wall coverings use wall area, not floor area: perimeter times height, minus doors and windows. The room calculator gives that number, and the wallpaper calculator and paint calculator turn it into rolls and gallons.
Countertop square footage
Measure each run along the wall and its depth to the front edge, including any overhang. A 9 ft 6 in run that is 25½ in deep is 9.5 × 2.125 = 20.19 sq ft; an L-shaped counter is two runs with the corner counted once. The countertop calculator turns that into slabs and seams.
Conversion table
| Square feet | Square yards | Square meters | Acres |
|---|---|---|---|
| 100 | 11.11 | 9.29 | 0.0023 |
| 500 | 55.56 | 46.45 | 0.0115 |
| 1,000 | 111.11 | 92.90 | 0.0230 |
| 2,000 | 222.22 | 185.81 | 0.0459 |
| 43,560 | 4,840 | 4,046.86 | 1 |
More tools for measuring a project, from volumes to dumpster sizes, are in the measuring and planning calculators.
Measuring a lot or roof outline? Read the side lengths from the measure-distance tool in Google Maps, then enter them here for the square footage. A quick check on the sq ft of a 12x12 room: 12 ft × 12 ft = 144 sq ft.