How to Calculate Paint Coverage
Learn how to calculate paint coverage for interior and exterior surfaces by measuring square footage and accounting for doors, windows, and porosity. Get the right number of gallons on the first trip to the store.
Standard Paint Coverage Rates
Most interior latex paints cover approximately 350–400 square feet per gallon on a smooth, previously painted surface. Exterior paints and primers on porous or rough surfaces cover less, typically 250–300 sq ft per gallon. Always check the manufacturer's data sheet for the specific coverage rate of the product you are using.
Measuring Wall Area
For each wall, multiply its width by its height to get the surface area in square feet. Sum all four wall areas in a room, then subtract the area of doors (approximately 20 sq ft each) and windows (approximately 15 sq ft each). This net paintable area is what you divide by the paint's coverage rate to find gallons needed per coat.
Calculating Ceiling Area
Ceiling paint is calculated separately because it uses a different sheen (usually flat) and may be a different color. Multiply the room's length by its width to get the ceiling area. A 12 ft × 15 ft room has a 180 sq ft ceiling, requiring 180 ÷ 400 = 0.45 gallons, so you would buy one quart (0.25 gal) plus one additional quart for a second coat.
Number of Coats Required
One coat is generally sufficient over a similar existing color with a high-quality paint, but two coats are standard for new drywall, color changes, and dark-to-light transitions. Multiply your single-coat gallon requirement by the number of coats to get the total gallons needed. New drywall should always receive a dedicated primer coat before the finish paint.
Accounting for Surface Porosity
Unpainted drywall, masonry, bare wood, and stucco absorb significantly more paint than previously painted surfaces. Reduce the effective coverage rate by 20–30 % for these porous substrates. Priming first with a purpose-made sealer or primer normalizes absorption and can actually reduce the total amount of finish paint needed over two coats.
Exterior Paint Calculations
Measure the perimeter of the building and multiply by the wall height, then subtract large openings like garage doors and windows. Siding with deep grooves (such as channel rustic) increases effective surface area by 10–15 % — apply a siding factor multiplier accordingly. Rough masonry like brick or textured stucco may cut coverage by up to 50 % compared to smooth surfaces.
Buying the Right Amount
Always round up to the nearest whole gallon or standard container size (quart, gallon, or 5-gallon pail). Keeping a quart of leftover paint for touch-ups is worthwhile; tint-matched paint rarely matches perfectly once dry. Note the paint's lot number on the can label before discarding it, as future purchases from a different lot can have slight color variations.
Try These Calculators
Put what you learned into practice with these free calculators.
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