Square footage calculator

How to calculate square footage

For a rectangle, square footage is length × width, both in feet. A 12 ft × 15 ft bedroom is 12 × 15 = 180 sq ft.

1. Measure wall to wall at floor level.Measure the longest straight run in each direction. If the room isn't square, the two opposite walls won't match; measure both and note the difference.
2. Write the inches down, don't round them.The calculator takes 15' 4" exactly, so there's no reason to round to the nearest foot.
3. Multiply.Or type both numbers above and let the drawing show you whether the shape looks like your room.

Feet and inches without mistakes

The most common slip is reading 10' 6" as 10.6 feet. Six inches is half a foot, so 10' 6" is 10.5 ft. 10.6 ft would be 10 ft 7.2 in. To turn inches into feet, divide by 12:

In the calculator each length has a feet box and an inches box: for 10 ft 3 in, type 10 and 3. The ▲▼ buttons (or the arrow keys) step one foot or one inch at a time, and inches roll over into feet at 12. Hold a button down to keep counting.

The boxes also read all the usual ways of writing a length: 10.5, 10 1/2, 10-1/2, 10'6", 10ft 6in and even 12'6 1/2". You can also type another unit right in the box, such as 350 cm or 6". It stores every length in inches, so switching to meters and back never drifts.

L-shaped and irregular rooms

There's no special formula for an L. Break it into rectangles. You can do it two ways, and doing both is a free check on your tape measure:

• Add two rectangles. A 12 × 10 ft section plus an 8 × 6 ft section: 120 + 48 = 168 sq ft.
• Big rectangle minus the notch. The 12 × 16 ft rectangle around the whole L, minus the missing 4 × 6 ft corner: 192 − 24 = 168 sq ft.

Pick L-shape in the calculator and switch between the two methods; the drawing shows where the split is. For rooms with more jogs, add one row per rectangle. Bay windows and angled walls are usually a rectangle plus a triangle or trapezoid.

Circles, triangles and trapezoids

Two things trip people up. For a circle, measure straight across through the center (the diameter); the calculator halves it for you. For triangles and trapezoids, the height is the straight-line distance at a right angle to the base, not the length of a slanted side. Measuring the slanted side makes the area too big.

Subtracting closets, islands and stairs

Switch a row to Subtract (or use − Subtract an area) for anything you won't cover. Subtracted rows are hatched in red in the drawing and print with "(subtract)" next to their name.

What to subtract depends on the job. For flooring, keep closets you'll floor and subtract a kitchen island or built-in cabinets if the floor won't run under them. For a stair opening in a loft, subtract it. For paint, floor area is the wrong number entirely: you need wall area (perimeter × ceiling height, minus doors and windows), which the paint calculator works out from the room itself.

If the subtractions add up to more than the total, the calculator stops and asks you to check them instead of showing a negative area.

Square feet to square meters and square yards

• Square meters = square feet × 0.09290304. That number is exact: a foot is defined as exactly 0.3048 m, and 0.3048² = 0.09290304. So 180 sq ft = 16.72 m².
• Square feet = square meters × 10.764 (approximately).
• Square yards = square feet ÷ 9, since a square yard is 3 ft × 3 ft. Carpet is often priced this way.

The result box always shows the other units too, so you can match whatever the store label uses.

Cubic feet and cubic yards

Mulch, soil, gravel and concrete are sold by volume, so you need a depth as well. Tick Add depth and each area is multiplied by its depth. In US units there's a handy shortcut:

cubic yards = square feet × depth in inches ÷ 324

Where 324 comes from: 12 inches in a foot × 27 cubic feet in a cubic yard. A 10 × 8 ft bed at 3 inches deep is 80 × 3 ÷ 324 = 0.74 yd³, which is 20 ft³ (ten 2 ft³ bags, before any allowance for settling). Each area can have its own depth if, say, one bed needs to be deeper.

Buying material: waste and whole boxes

The area is only the first number. To buy material you also need a waste allowance (cuts, breakage, pattern matching) and you can only buy whole boxes, rolls or bags. Open Buying material? in the calculator, type the waste percentage you plan for and how much one package covers, and you get three lines: the exact amount, the amount with waste, and the number of packages to buy.

The rule here is to always round up. Rounding 47.2 bags down to 47 (or to the nearest number) guarantees a second trip to the store. The page also tells you how much extra the rounding added. That's the difference between what you bought and what you needed, not a promise of usable leftovers.

The right waste percentage depends on the material and the layout: diagonal and herringbone patterns waste more than straight runs. Our upcoming calculators for flooring, tile, paint and mulch carry product-specific numbers with sources. This page leaves the number to you.

Scope and sources

This calculator does geometry only: areas, volumes, unit conversions and rounding to whole packages using the numbers you type. It contains no product coverage rates, waste factors or prices. Lengths are stored internally in inches; metric conversions use the exact definitions 1 in = 25.4 mm and 1 ft = 0.3048 m. Rows are drawn individually at a common scale; they aren't combined into a floor plan, and the page doesn't calculate the perimeter of combined shapes.

• NIST: the international foot (0.3048 m) and the retired U.S. survey foot
• NIST Special Publication 811: Guide for the Use of the International System of Units

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