To calculate square footage, measure the length and width of the space in feet and multiply them. A room that is 12 feet by 14 feet has 12 × 14 = 168 square feet. A square foot is the area of a square that measures one foot on each side, so the answer counts how many of those squares fit in the space.
What is the formula for square footage?
For a rectangle or square:
area (sq ft) = length (ft) × width (ft)
OpenStax states that the area of a rectangle is its length times its width and that area is measured in square units such as square inches or square feet. The two sides must use the same unit. Multiplying feet by feet gives square feet; multiplying feet by inches does not.
Step-by-step example
A bedroom measures 12 ft 6 in by 10 ft 8 in.
- Convert inches to feet: 6 in ÷ 12 = 0.5 ft, and 8 in ÷ 12 ≈ 0.6667 ft.
- Length = 12.5 ft; width = 10.6667 ft.
- Area = 12.5 × 10.6667 ≈ 133.3 square feet.
If the measurements are in inches, use the inches to feet converter first, or multiply the inch measurements and divide by 144 (a square foot is 12 × 12 = 144 square inches).
How do you calculate square footage of an L-shaped room?
Split an irregular floor plan into rectangles, calculate each one and add the results. An L-shaped room is two rectangles.
Worked example: the main part measures 20 ft × 15 ft and the extension measures 10 ft × 8 ft.
- Main part = 20 × 15 = 300 sq ft
- Extension = 10 × 8 = 80 sq ft
- Total = 300 + 80 = 380 sq ft
There is usually more than one way to cut the shape, and every valid cut gives the same total, so cut it a second way as a check. When part of the room is missing, such as a stairwell or a built-in closet, calculate the full rectangle and subtract the missing piece.
How do you find the area of a triangle or a trapezoid?
Some spaces are not rectangles: an attic with a sloped end, a gable wall, a corner lot. OpenStax gives these formulas:
| Shape | Formula | Example |
|---|---|---|
| Rectangle | A = L × W | 12 × 14 = 168 sq ft |
| Triangle | A = ½ × base × height | ½ × 24 × 8 = 96 sq ft |
| Trapezoid | A = ½ × height × (b + B) | ½ × 8 × (10 + 16) = 104 sq ft |
In the trapezoid formula, b and B are the two parallel sides and the height is the distance between them. For the triangle, the height is measured at a right angle to the base, not along the slanted side.
A gable end that is 24 feet wide and 8 feet tall at the peak is one triangle: ½ × 24 × 8 = 96 square feet.
How do you convert square feet to other units?
Square units multiply the conversion factor of the length unit by itself. A yard is 3 feet, so a square yard is 3 × 3 = 9 square feet. NIST defines the international foot as exactly 0.3048 meters, so a square foot is 0.3048² = 0.09290304 square meters.
| From | To | Rule | 168 sq ft room |
|---|---|---|---|
| Square feet | Square yards | ÷ 9 | 18.67 sq yd |
| Square feet | Square meters | × 0.09290304 | 15.61 sq m |
| Square feet | Square inches | × 144 | 24,192 sq in |
| Square feet | Acres | ÷ 43,560 | 0.00386 acre |
Some flooring is priced per square yard, in which case the 168 square foot room is 168 ÷ 9 ≈ 18.67 square yards before any allowance for waste. For other conversions, see the unit converters and the length, distance and area guide.
How many acres is a lot?
The acre is defined through the square foot: NIST lists one acre as exactly 43,560 square feet when the international foot is used. A lot that measures 100 ft × 150 ft has 15,000 square feet, which is 15,000 ÷ 43,560 ≈ 0.344 acre. See the acre entry for the survey-foot nuance.
How much flooring, paint or material do you need?
Square footage is the starting point for most estimates: divide the area by the coverage printed on the product to find how many units to buy. The coverage figure varies by product, so use the manufacturer's number rather than a general rule. Most manufacturers also recommend ordering extra for cuts and waste, and the right amount depends on the product and the layout.
For mulch, soil or concrete, the quantity is a volume, so multiply the area by the depth. The cubic yard calculator does that conversion, and the percentage calculator is handy for adding a waste allowance.
What counts as a home's square footage?
Measuring your own floor area and a home's listed square footage are different things. Room measurements are taken between walls. For single-family homes, the ANSI Z765 standard covers how to calculate and report above-grade and below-grade square footage; it does not cover room dimensions, and Fannie Mae requires appraisers to use the 2021 edition when determining gross living area. A listing's number may therefore differ from the sum of the room areas you measured, and it is reasonable to ask which method the number follows. The exact rules, such as which spaces are counted, are in the standard itself.
Common mistakes when calculating square footage
- Mixing units, such as feet by inches. Convert first.
- Multiplying the perimeter, or adding length and width, instead of multiplying them.
- Forgetting to subtract closets, stairwells or built-ins when the room is cut out of a larger rectangle.
- Using the slanted side as the height of a triangle.
- Rounding early. Keep the decimals (such as 10.6667 ft) until the last step.
- Confusing square feet with feet. A room that is 10 feet on each side is 100 square feet, not 40.