Square footage tells you how much flooring, paint, mulch or concrete to buy, and how to compare the size of rooms, homes and lots. The math is simple, length × width, but most errors come from measuring, mixing units or splitting shapes badly. This guide covers each step. The square footage calculator does the arithmetic for you.
What you need
- A 25 ft tape measure for rooms. A laser measure is faster for long rooms and works well alone.
- A measuring wheel, or a long tape and a helper, for yards, driveways and lots.
- Paper and pencil for a sketch. Writing every number on a drawing is the best protection against double-counting.
Step 1: Sketch the space
Draw the outline roughly to shape. Mark every wall, doorway, closet, bump-out and angled corner. You'll write each measurement on this sketch as you go.
Step 2: Measure in feet and inches, then convert
Tapes read in feet and inches. The math needs decimal feet. Divide the inches by 12 and add them to the feet.
| Inches | Decimal feet | Inches | Decimal feet |
|---|---|---|---|
| 1 in | 0.083 | 7 in | 0.583 |
| 2 in | 0.167 | 8 in | 0.667 |
| 3 in | 0.25 | 9 in | 0.75 |
| 4 in | 0.333 | 10 in | 0.833 |
| 5 in | 0.417 | 11 in | 0.917 |
| 6 in | 0.5 | 12 in | 1.0 |
So 11 ft 9 in = 11.75 ft, and 9 ft 4 in = 9.333 ft.
Alternatively, measure everything in inches, multiply, and divide the result by 144 (12 × 12) to get square feet.
Step 3: Split the shape into simple pieces
Almost any floor plan can be broken into rectangles, triangles and circles.
Rectangles
Area = length × width. A 15 × 11.75 ft living room is 176.25 ft².
L-shapes and T-shapes
Draw one line to cut the L into two rectangles that don't overlap. Measure each one separately.
Example: a 20 × 12 ft main section plus a 10 × 8 ft wing. That's 240 + 80 = 320 ft².
A common error: measuring the full 20 ft length for both pieces, which counts the corner twice.
Triangles
Area = ½ × base × height. The height must be measured at a right angle to the base, not along the sloping side.
Example: a corner garden bed with 8 ft and 6 ft sides meeting at a square corner. That's ½ × 8 × 6 = 24 ft².
Circles and semicircles
Area = π × radius², where the radius is half the diameter.
Example: a 7 ft round bay window seat area, cut in half. π × 3.5² ÷ 2 = 19.24 ft².
Subtracting areas
For a space with something you don't want to count, such as a kitchen island when you're pricing a rug, or a tree ring in a lawn you're seeding, calculate the outer area and subtract the inner one.
Example: a 30 × 20 ft lawn with a 6 ft diameter tree bed. 600 − (π × 3²) = 600 − 28.27 = 571.73 ft² of lawn.
Step 4: Add up the pieces
Add every piece from your sketch and you have the total.
Worked example, an open-plan main floor:
| Area | Size | ft² |
|---|---|---|
| Living room | 15 × 11.75 | 176.25 |
| Kitchen | 12 × 10 | 120.00 |
| Dining nook | 8 × 7 | 56.00 |
| Hall | 3.5 × 12 | 42.00 |
| Total | 394.25 |
Measuring a whole home
For a quick figure, measure the outside of the house and multiply length × width for each rectangle in the footprint. Then multiply by the number of finished floors that have the same footprint.
Example: a 40 × 30 ft two-storey house with a 12 × 10 ft single-storey extension. Each upper and lower floor is 1,200 ft², so the two storeys give 2,400 ft². Add 120 ft² for the extension: 2,520 ft² gross.
Exterior measurements include wall thickness, so the result is larger than the sum of interior room measurements. Real estate listings, appraisals and builders may follow particular standards for what counts. Garages, unfinished basements and spaces with low ceilings are often treated differently. For buying materials, always measure the rooms you are working on.
Walls and ceilings
For painting or wallpaper, you need wall area, not floor area:
Wall area = perimeter × wall height, where perimeter = 2 × (length + width).
Example: a 12 × 14 ft room with 8 ft ceilings has a perimeter of 52 ft. 52 × 8 = 416 ft² of wall.
Subtract doors and windows by measuring each one: width × height. A flat ceiling has the same area as the floor. The paint calculator does both.
Yards, lots and acres
For outdoor areas, measure with a wheel and split the space into rectangles and triangles, just like a room. Lots are usually given in square feet or acres.
- 1 acre = 43,560 ft² (NIST Handbook 44).
- Acres = ft² ÷ 43,560, so a 9,000 ft² lot is 0.207 acres.
- ft² = acres × 43,560, so a quarter acre is 10,890 ft².
A plat or survey will be more accurate than measuring a lot yourself, especially when it has angled boundaries.
Converting square feet to other units
| Convert | Multiply or divide | Example (500 ft²) |
|---|---|---|
| Square yards | ÷ 9 | 55.56 yd² |
| Square meters | × 0.09290304 | 46.45 m² |
| Acres | ÷ 43,560 | 0.0115 ac |
| Square inches | × 144 | 72,000 in² |
The square meter factor is exact: NIST defines the foot as exactly 0.3048 m, and 0.3048² = 0.09290304.
Adding a material allowance
Area is not the same as the amount of material to buy. Flooring, tile and sod are cut to fit, and the offcuts are waste. Plan the allowance on top of the area:
- Flooring: add an allowance for cutting (the calculator defaults to 10%), then divide by the square feet per box printed on the carton. Diagonal layouts and patterned tile usually need more, so follow the manufacturer's installation instructions.
- Paint: coverage per gallon is printed on the can, and textured surfaces use more.
- Mulch and gravel: multiply area by depth to get volume (see the mulch guide).
Common mistakes
- Typing 12.6 for 12 ft 6 in. It's 12.5. The decimal is a fraction of a foot, not inches.
- Overlapping rectangles. Each square foot should be counted once.
- Sloped side as height. Triangles need the perpendicular height.
- Comparing to a listing. A listing may use a different measuring standard.
- No allowance. Area plus waste is what you buy, not area alone.
Frequently asked questions
What is the formula for square footage?
Length × width in feet, for any rectangle. Other shapes have their own formulas: ½ × base × height for a triangle, and π × r² for a circle.
How do I calculate square footage with inches?
Either convert each side to decimal feet (inches ÷ 12), or multiply in inches and divide by 144.
How many square feet is a 10 × 12 room?
120 ft².
How do I measure square footage for flooring?
Measure the floor area of every room and closet getting the new floor, add the areas, add a cutting allowance, and divide by box coverage.
How do I calculate the square footage of a house?
Measure the outside footprint, split it into rectangles, multiply each one, and multiply by the number of finished floors. Interior room totals come out smaller because walls take up space.
Sources
- NIST Handbook 44 (2026), Appendix C: 1 acre = 43,560 ft², 1 yd² = 9 ft², 1 ft² = 144 in². https://www.nist.gov/document/2026-nist-handbook-44-appendix-c
- NIST, U.S. survey foot: 1 ft = 0.3048 m exactly. https://www.nist.gov/pml/us-surveyfoot
- NIST, SI units of length: 1 in = 25.4 mm exactly. https://www.nist.gov/pml/owm/si-units-length