Area 1
Advanced options
Flooring 10%, diagonal or herringbone 15–20%, paint 0%.
Leave at 0 if you only want the area.
This square footage calculator finds the area of a room, floor, wall, yard or lot — and lets you add several shapes together or subtract cut-outs like islands and closets. It returns square feet, square metres, square yards, acres, perimeter and material cost.
The short answer
Square footage is length × width, with both measurements in feet. A room 20 ft by 14 ft is 20 × 14 = 280 square feet.
If your measurements include inches, convert them first: 14 ft 6 in is 14.5 ft, because 6 ÷ 12 = 0.5.
On this page
How to use this square footage calculator
Pick a shape, enter the dimensions, and the square footage calculator returns the area along with conversions, perimeter and cost. Four shapes cover almost everything:
- Rectangle — rooms, floors, walls, decks, lots, most of what you’ll measure
- Circle — round patios, tree rings, above-ground pools, rotundas
- Triangle — gable walls, corner beds, wedge-shaped sections
- Trapezoid — pie-shaped lots and rooms where opposite walls aren’t parallel
The feature that makes this genuinely useful for real rooms is add versus subtract. Every area you add carries a toggle. Set it to subtract and it comes off the total — for a kitchen island you’re not flooring, a closet you’re excluding, a pool inside a patio, or a stairwell opening in an upper floor. Most square footage calculators only add, which forces you to do the subtraction on paper.
The square footage formula
Square feet = Length (ft) × Width (ft)
20 ft × 14 ft = 280 ft²
Both measurements must be in feet before you multiply.
That’s the whole thing for a rectangle. Everything else on this page is about handling measurements that aren’t clean feet, shapes that aren’t rectangles, and rooms that aren’t one simple box.
Worked example — 20 × 14 ft living room
Measurements that include inches
This is where most manual calculations go wrong. Rooms are rarely whole feet, and you cannot multiply feet by inches directly. Convert the inches to a decimal fraction of a foot first by dividing by 12.
| 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 |
Worked example — a room measuring 12 ft 7 in by 10 ft 4 in
Doing it wrong — multiplying 12.7 × 10.4 — gives 132 ft². Small here, but the error compounds across a whole house.
Circles, triangles and trapezoids
Square feet = π × (Diameter ÷ 2)²
12 ft diameter = 3.1416 × 6² = 113 ft²
Square feet = ½ × Base × Height
16 ft base × 10 ft height = 80 ft²
Square feet = ½ × (Side A + Side B) × Height
20 ft and 14 ft parallel sides, 12 ft apart
= 0.5 × 34 × 12 = 204 ft²
For a triangle, “height” means the perpendicular distance from the base to the opposite point — not the length of a sloping side. Using a slope length instead of the perpendicular height is the most common error on gable walls and corner lots.
Measuring an irregular room
Almost no real room is a perfect rectangle. There are bays, alcoves, chimney breasts, islands and closets. The method is always the same: break the space into rectangles, circles and triangles, then add the parts and subtract the holes.
- Sketch the room roughly to scale, on paper or your phone.
- Draw straight lines dividing it into simple shapes. Make the cuts so shapes butt against each other rather than overlapping.
- Measure each shape and label the sketch as you go, rather than trusting memory.
- Enter each as a separate area in the calculator, set to add.
- Add cut-outs as subtract areas — islands, closets you’re not covering, stairwell openings.
Rooms are rarely truly square. Measure the width at both ends and the length at both sides. If they differ by more than an inch or two, use the larger figure for material ordering and be generous with the waste allowance — the difference has to be absorbed in cuts somewhere.
Whole-house square footage
House square footage is a specific term with rules, and it’s not simply the total of every room you can walk into.
The widely used standard in US residential real estate is ANSI Z765, which measures finished living area from the exterior face of exterior walls. Under that standard, the figure includes finished, heated space with adequate ceiling height, and excludes garages, unfinished basements and attics, and open porches. Below-grade finished space — a finished basement — is typically reported separately rather than folded into the above-grade total.
Measuring each room from the inside and adding them will come out lower than the ANSI figure, because interior measurements exclude wall thickness, hallways and stair openings. For a typical house the gap is often 5–10%. This calculator gives you true measured area, which is what you want for buying materials — not the number for a listing. For anything that affects a sale, appraisal or tax assessment, use a licensed appraiser working to the local standard.
How much waste should you add?
Waste isn’t guesswork — it depends on the material and the pattern you’re installing.
| Material or pattern | Waste | Why |
|---|---|---|
| Straight-lay plank flooring | 10% | End cuts and starter boards |
| Straight-lay tile | 10% | Perimeter cuts and breakage |
| Diagonal tile or plank | 15% | Every perimeter piece is a cut |
| Herringbone or chevron | 15–20% | High cut ratio throughout |
| Carpet | 10–20% | Roll width forces seam placement |
| Large-format tile (24 in+) | 15% | A cut wastes more material |
| Sod or turf | 5–10% | Trimming to curved edges |
| Paint | 0% | Round up to the next can instead |
| Drywall | 0–10% | Depends on how sheets lay out |
Two practical notes. Buy flooring from a single dye lot in one order — matching a later top-up is often impossible. And keep the leftovers: a box of spare planks or tiles is what lets you repair damage in five years without re-doing the floor.
Using square footage to estimate materials
| Material | Typical coverage | For 280 ft² |
|---|---|---|
| Interior paint, one coat | 350–400 ft² per gallon | ~1 gallon |
| Drywall, 4 × 8 ft sheet | 32 ft² per sheet | 9 sheets |
| Drywall, 4 × 12 ft sheet | 48 ft² per sheet | 6 sheets |
| Plywood or OSB sheet | 32 ft² per sheet | 9 sheets |
| Sod, standard slab | ~2.7 ft² per piece | ~104 pieces |
| Roofing, one square | 100 ft² | 2.8 squares |
| Concrete at 4 in thick | 81 ft² per cubic yard | 3.5 yd³ |
| Mulch at 3 in deep | 108 ft² per cubic yard | 2.6 yd³ |
Flooring is sold by the carton, not by the square foot, and carton coverage varies by product — commonly 18 to 24 ft² per box for planks. Divide your total including waste by the box coverage and round up to whole boxes.
Square foot conversions
| Conversion | Factor |
|---|---|
| Square feet to square inches | × 144 |
| Square feet to square yards | ÷ 9 |
| Square feet to square metres | × 0.09290304 |
| Square metres to square feet | × 10.7639 |
| Square feet to acres | ÷ 43,560 |
| Acres to square feet | × 43,560 |
| Hectares to square feet | × 107,639 |
| Square feet to roofing squares | ÷ 100 |
The acre figure has a historical origin worth knowing: an acre was one chain by one furlong, or 66 ft × 660 ft, which is exactly 43,560 square feet. It is not a square shape, which is why an acre doesn’t convert to a tidy number of feet per side.
Square footage mistakes to avoid
- Treating 12 ft 7 in as 12.7 ft. Seven inches is 0.583 ft, not 0.7. Divide inches by 12.
- Confusing square yards with square feet. A square yard is 9 square feet, not 3.
- Using a sloping side as triangle height. Height is the perpendicular distance to the base.
- Measuring only one wall of each pair. Rooms are rarely square; check both.
- Forgetting to subtract cut-outs like islands and closets you aren’t covering.
- Adding room-by-room totals and calling it house square footage. That’s not how listed figures are measured.
- Ordering exact area with no waste. Every material with cuts needs an allowance.
Frequently asked questions
How do I calculate square footage?
Multiply length by width, both in feet. A 20 ft by 14 ft room is 280 square feet. If your measurements include inches, divide the inches by 12 and add them as a decimal first — 14 ft 6 in becomes 14.5 ft.
How many square feet is a 12 × 12 room?
144 square feet. That’s 13.4 square metres or 16 square yards, with a perimeter of 48 feet for baseboard. At 10% waste you’d buy flooring for about 158 square feet.
How do I calculate the square footage of an irregular room?
Break it into rectangles, triangles and circles, measure each separately, and add them. Subtract any cut-outs such as islands or closets you aren’t covering. This calculator lets you enter multiple areas and set each one to add or subtract, so the total comes out automatically.
How many square feet are in a square yard?
Nine. A square yard is 3 ft by 3 ft. This trips people up because a yard is 3 feet, so the intuitive answer is 3 — but area scales with the square, so it’s 3 × 3 = 9. Carpet is commonly priced by the square yard.
How do I convert square feet to square metres?
Multiply square feet by 0.09290304, or divide by 10.7639. A 280 square foot room is 26.0 square metres. Going the other way, multiply square metres by 10.7639.
How many square feet are in an acre?
43,560 square feet. An acre was historically one chain by one furlong — 66 ft by 660 ft. It isn’t square, which is why there’s no neat number of feet per side. An acre is roughly 4,047 square metres or 0.405 hectares.
How is house square footage measured?
US residential listings commonly follow ANSI Z765, which measures finished living area from the exterior face of exterior walls and excludes garages, unfinished basements and open porches. Finished below-grade space is usually reported separately. Adding up interior room measurements gives a lower figure, typically by 5–10%.
Does square footage include the garage?
No. Under the ANSI standard, garages are excluded from finished living area because they aren’t finished, heated space. The same applies to unfinished basements, unfinished attics and open porches. A converted garage that has been properly finished and heated may count — that’s a judgement for an appraiser.
How much extra flooring should I buy?
10% for straight-lay planks or tile, 15% for diagonal patterns and large-format tile, and 15–20% for herringbone or chevron. Buy it all in one order from a single dye lot, and keep the offcuts — matching a discontinued product later is often impossible.
How do I find the square footage of a wall?
Multiply wall height by wall length. For a whole room, add the four walls together, then subtract doors and windows — a standard door is about 21 square feet and an average window 12 to 15. For paint, most people skip the subtraction unless the openings are large, since coverage estimates are generous.
What is a roofing square?
100 square feet. Roofing materials are sold and quoted by the square, so a 2,800 square foot roof surface is 28 squares. Note that roof surface area is larger than the building footprint because of pitch — a 6/12 pitch adds about 12% to the plan area.
Can I use this calculator in metres?
Yes. Switch the toggle in the calculator header from Feet to Metres and enter all dimensions in metres. Results still show square feet, square metres, square yards and acres together, so you can work in whichever unit your supplier quotes.
Sources and method
- NIST Special Publication 811 — unit conversion factors
- ANSI Z765 — Square Footage Method for Calculating the Finished Area of Single-Family Residential Buildings
- Manufacturer installation guidance for flooring and tile waste allowances
- Published coverage rates for paint, drywall and sheet goods
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How this square footage calculator works out its numbers
Areas use standard geometric formulas: length × width for rectangles, π × radius² for circles, ½ × base × height for triangles, and ½ × (a + b) × height for trapezoids. Multiple areas are summed, with any marked as subtract removed from the total. Perimeter is calculated for added rectangles and circles only, since a subtracted cut-out doesn’t add trim length.
Unit conversions use exact NIST factors: 1 square foot = 144 square inches = 0.09290304 square metres; 1 square yard = 9 square feet; 1 acre = 43,560 square feet; 1 roofing square = 100 square feet.
This calculator measures area, not appraised living space. Figures used in property listings, appraisals and tax assessments follow standards such as ANSI Z765 and are measured from exterior walls with specific inclusion rules. For anything affecting a sale or valuation, use a licensed appraiser working to the standard that applies in your jurisdiction.