How to Calculate Square Footage

Rectangles and squares

Almost every square-footage problem reduces to one formula:

area (sq ft) = length (ft) × width (ft)

12 ft × 10 ft room = 120 sq ft
16 ft × 16 ft patio = 256 sq ft

Both numbers must be in feet before you multiply. A tape reading of 12 ft 6 in becomes 12.5 ft (divide the inches by 12). If a dimension is in inches only — say a 30-inch countertop depth — that is 30 ÷ 12 = 2.5 ft.

That is the whole trick. Everything below is just ways of breaking odd shapes into rectangles (plus two formulas for circles and triangles).

L-shapes: split into rectangles

An L-shaped room is two rectangles wearing a trench coat. Draw the room, draw one line that splits it into two rectangles, calculate each, and add them.

Worked example. A living room runs 20 ft across the back and 16 ft deep, but a 6 × 8 ft corner is walled off for a closet, leaving an L. Split it:

Rectangle A: 20 ft × 8 ft  = 160 sq ft   (the back band)
Rectangle B: 14 ft × 8 ft  = 112 sq ft   (the front leg: 20 − 6 = 14)

Total: 160 + 112 = 272 sq ft

A useful cross-check: the full 20 × 16 rectangle would be 320 sq ft, and the missing corner is 6 × 8 = 48 sq ft. 320 − 48 = 272. When both methods agree, your arithmetic is right. U-shapes and T-shapes work the same way — they just split into three rectangles instead of two.

Circles

For round patios, fire-pit surrounds, and circular beds:

area = π × r²        (r = radius = half the diameter, π ≈ 3.14)

12-ft-diameter patio:
r = 6 ft  →  3.14 × 6 × 6 = 113 sq ft (rounded)

Measure the diameter (the widest distance across, through the center) because it is much easier to measure than the radius, then halve it. For a half circle, calculate the full circle and divide by 2.

Triangles

Angled bump-outs, bay ends, and corner beds are triangles:

area = ½ × base × height

Triangle with a 6 ft base and 4 ft height:
½ × 6 × 4 = 12 sq ft

The height is the straight-line distance from the base to the opposite point, measured at a right angle to the base — not along the slanted side. If you can only see slanted sides, box the triangle inside a rectangle and take half.

Closets, bays, and alcoves: add them

If flooring continues into a closet, that closet is part of the floor area. Measure each attached space as its own small rectangle and add it to the main room:

Bedroom:        12 ft × 11 ft   = 132 sq ft
Closet:         6 ft × 2.5 ft   = 15 sq ft
Doorway/nook:   3 ft × 1.5 ft   = 4.5 sq ft

Floor area: 132 + 15 + 4.5 ≈ 152 sq ft

The flooring calculator accepts multiple rooms/areas for exactly this reason — enter the closet as its own row rather than trying to mentally fold it into the room dimensions.

Islands, cabinets, and other cutouts: subtract them

Permanent objects that flooring or tile will not go under get subtracted. A kitchen island, a masonry hearth, built-in cabinetry:

Kitchen:         14 ft × 12 ft = 168 sq ft
Island cutout:   7 ft × 4 ft   = 28 sq ft

Tile area: 168 − 28 = 140 sq ft

Be conservative about subtracting. Toilets, vanities that might be replaced, and appliances usually should notbe deducted — flooring typically runs under or close enough to them that the "savings" are not real. Subtract only large, permanent footprints.

Walls: perimeter × height, minus openings

Paint and drywall need wall area, not floor area:

wall area = room perimeter × wall height − openings

12 × 10 ft room with 8 ft ceilings:
perimeter = 2 × (12 + 10) = 44 ft
gross wall area = 44 × 8 = 352 sq ft

minus one door (20 sq ft) and two windows (15 sq ft each):
352 − 20 − 30 = 302 sq ft

A common convention is to count a standard door as about 20 sq ft and a typical window as about 15 sq ft rather than measuring each one. The paint calculatorhandles these deductions and converts the result into gallons; the drywall calculator does the same for sheets. Note that many painters simply skip deductions on rooms with one door and a window or two — the "extra" conveniently covers touch-ups.

Converting square inches and square feet

Tile sizes are usually quoted in inches, so this conversion comes up constantly. One square foot is 12 in × 12 in = 144 square inches — not 12.

sq in → sq ft: divide by 144
sq ft → sq in: multiply by 144

One 18 × 18 in tile: 324 sq in ÷ 144 = 2.25 sq ft
One 3 × 6 in subway tile: 18 ÷ 144 = 0.125 sq ft (8 per sq ft)

So a 140 sq ft floor needs about 140 ÷ 2.25 ≈ 63 of the 18-inch tiles before waste. The tile calculator does this per-tile math, adds waste, and rounds up to whole boxes.

Common square-footage mistakes

• Mixing units. Multiplying 12 ft by 126 in gives a nonsense number. Convert everything to decimal feet first.
• Measuring around baseboards or furniture. Measure wall to wall in a clear straight line; move things or measure above them.
• Confusing perimeter with area. Perimeter (linear feet) is for trim, baseboard, and paver edging; area (square feet) is for the surface itself. A 12 × 10 room has 44 linear ft of perimeter and 120 sq ft of floor — unrelated numbers.
• Using 12 instead of 144 when converting square inches to square feet.
• Forgetting the closets — then coming up one box short on the last day of a flooring install.
• Buying to the exact area. Cuts and breakage mean you always need more than the measured area; see how much waste to add.