Square Footage Calculator

For most rectangular rooms, length × width gives the total area. Measure to the inside of walls (not including the wall thickness itself); for square footage that drives material quantities (flooring, etc.), measure the actual floor area you'll cover.

For rooms with closets, alcoves, or bumped-out areas: split the room into rectangles, calculate each, and add them. A bedroom that's 12 ft × 14 ft with a 4 ft × 6 ft closet adds: 12 × 14 = 168 sq ft + 4 × 6 = 24 sq ft → 192 sq ft total. Don't forget areas behind doors that swing into the room — the door's swept area is still floor that needs flooring.

For triangles and angled walls (like in attics or A-frame rooms): area = (1/2) × base × height. For circles and curves: area = π × r² for full circles; for partial circles, prorate by the fraction of the circle. Most rooms can be decomposed into a combination of rectangles and triangles.

Real-estate listings can use different definitions of square footage — "gross living area" (typically wall-to-wall interior, sometimes excluding stairs and certain non-conforming spaces), "usable area" (subtract walls and unusable spots), or "finished square footage" (excludes garages, unfinished basements, attics that don't meet ceiling-height minimums). Two listings of the same house can show numbers differing by 10–15% based on which definition they use.

ANSI Z765 is the standard for residential measurement in the U.S. but its adoption is voluntary by state and even by individual real-estate agent. Buyers comparing properties should ask which standard was used, and ideally measure independently. The MLS-listed 'square footage' is rarely the figure you'll use for renovations or material orders — use your own field measurements.

Flooring: most flooring is sold by the square foot, with packaging in cases or boxes. Round up to whole cases and add a waste factor (10% for straight installs, 15% for diagonals or rooms with cuts).

Paint: a gallon typically covers 350–400 sq ft on a smooth wall in one coat. Calculate wall area as wall length × ceiling height for each wall, sum, subtract doors (~21 sq ft each) and large windows. Two coats are standard, so double the gallons.

Drywall: standard 4×8 sheets cover 32 sq ft each; 4×12 sheets cover 48 sq ft. Calculate wall area, subtract major openings, round up to whole sheets. A waste factor of 10–15% accounts for cuts and breakage.

HVAC sizing rule of thumb: roughly 25–30 BTU per square foot of conditioned space, varying by climate, insulation, and ceiling height. Manual J calculation (industry standard for HVAC sizing) is more rigorous; the rule of thumb gives a starting estimate.

Forgetting closets in the floor area when calculating flooring: easy mistake that causes back-orders mid-project. Always include closets, walk-ins, and the area behind doors.

Using outside dimensions (with wall thickness) when you need usable interior area: introduces a 5–10% error that matters for materials but generally not for HVAC sizing.

Confusing wall area with floor area: wall paint calculations need wall area (length × height for each wall); floor finishes need floor area. They're computed differently and produce very different square-footage numbers from the same room.

Not subtracting major openings from paint and drywall calculations: a room with two large windows has perhaps 30 sq ft less wall area than the simple length × height calculation. Subtract doors and windows; ignore small features (outlets, switches).