Paint Calculator

Take the wall area, subtract doors and windows, multiply by the number of coats, and divide by the coverage printed on the can: gallons = (area − openings) × coats ÷ coverage. A 400 sq ft wall minus 40 sq ft of openings, at 2 coats and 350 sq ft per gallon, needs 360 × 2 ÷ 350 ≈ 2.06 gallons — so buy 3, or 2 gallons plus a quart. The step people get wrong is the first one, because wall area is not floor area. Walls come from perimeter × ceiling height. A 12 by 14 ft bedroom has a 52 ft perimeter, and with an 8 ft ceiling the walls add up to 52 × 8 = 416 sq ft — more than twice the 168 sq ft of floor underneath. Then subtract the openings. A standard interior door of 3 ft by 6 ft 8 in removes about 20 sq ft; a window removes whatever its own measurements give. The deduction is worth making when the openings are large or numerous. On a wall with a single door it changes little and quietly becomes your safety margin. If you are painting the ceiling, add the floor area, because that is the ceiling's footprint.

The square-feet-per-gallon figure on the label is the most misread number in painting, and three qualifications explain why the paint runs out before the wall does. First, it is per coat. If the can claims 350 sq ft and you are applying two coats, it paints 175 sq ft of wall, not 350. The one-coat number is what gets the large type; the qualification lives in the fine print. Second, it is measured on an ideal surface — smooth, sealed, uniform, and already close to the finished color. Real walls are rarely in that condition. Third, many labels give a range rather than a single number, and the top of that range is the best case. Calculate with the bottom of it. Coverage varies enough between a builder-grade line and a premium line from the same manufacturer that the only figure worth trusting is the one on the can you actually bought.

A single coat rarely lands uniform, and the reason is physical: the applied film is thin, and wherever it goes slightly thinner the old surface shows through. The second coat evens out those variations rather than adding a whole new layer of color. Some jobs need three. Covering a dark color with a light one is the obvious case. Saturated yellows and reds also hide poorly, because the pigments in those families have less hiding power than whites and earth tones — a property of the pigment, not a defect in the paint. A tinted primer in a shade near the finish color often does more than a third coat of finish paint, and usually costs less per gallon. Execution matters too. Every product specifies a recoat time on the label. Rolling the second coat too early drags the first one and forces a third; waiting the stated interval saves paint.

New drywall, fresh joint compound and bare wood are porous and drink the first coat like a sponge: it disappears into the wall and real coverage falls well below the printed figure. The fix is not more finish paint but a primer or sealer first, which closes the surface and costs less per gallon than burning a full coat of topcoat. A previously painted wall in good condition is the opposite case, and it is where coverage comes closest to the label. Textured surfaces — knockdown, orange peel, popcorn ceilings, stucco, masonry — consume noticeably more, because the true surface area is larger than the flat rectangle you measured with a tape. One more surface consideration. Homes built before 1978 may have lead-based paint, and sanding or scraping it releases lead dust. The EPA's Renovation, Repair and Painting rule governs how that work is done and who may be paid to do it, so check before you start prepping an older wall.

Sheen changes both the look and the arithmetic. Flat hides wall imperfections but scuffs easily; eggshell and satin clean up better and are the usual choice for living spaces; semi-gloss and gloss go on trim, doors and bathrooms. Different sheens of the same color are different products with their own coverage figures, so trim and walls are separate calculations, not one number split two ways. Buy the whole job in one trip. Custom colors are mixed by machine from a base plus a tint formula, and two batches mixed weeks apart can land slightly different. When the order comes as several gallons of one color, boxing them — pouring them all into one larger bucket and stirring — removes any batch variation before it reaches the wall. Running out mid-wall and returning for one more gallon is exactly how a visible seam gets created. Keep the leftovers, and keep them labeled with the color code, the product line and the sheen. A machine-mixed color is nearly impossible to reproduce from memory, and a quart on a garage shelf is the difference between a touch-up and repainting the whole room.