Find exactly how much paint your project needs. Enter the wall area, coats, and coverage per gallon to get exact gallons, cans to buy, liters, and primer in seconds.
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Measure the wall area you plan to paint in square feet, then tell the calculator how many coats you will apply and the coverage per gallon printed on your paint can. It multiplies the area by the coats, adds your waste allowance, and divides by the coverage rate to give the exact gallons. The buy quantity is rounded up to whole gallons because paint is sold by the gallon, and the liter figure helps when cans are sold in liters. If you are starting from bare drywall or wood, the primer option adds a one-coat primer quantity at its own coverage rate.
The whole calculation rests on one simple idea: paintable area divided by coverage rate. First find the net wall area: perimeter times height, minus about 21 square feet per door and 15 square feet per window. A 12x12 foot room with 8 foot ceilings has 384 square feet of wall; minus one door and two windows leaves 333 square feet. Multiply by the number of coats, then divide by the coverage per gallon. At 350 square feet per gallon, that room needs 333 × 2 / 350 = 1.9 gallons, so you buy 2 gallons. One gallon of paint covers roughly 350 to 400 square feet per coat on smooth, previously painted walls. The result is always rounded up, because a store cannot sell you a fraction of a gallon and a mid-project store run usually ends with a color that does not quite match.
Start by entering the paintable area in square feet. If you know the room dimensions, multiply the perimeter by the ceiling height and subtract the openings, or measure each wall and add them up. Next, set the number of coats: two is the standard for most repaints. Then enter the coverage per gallon from your paint can label; 350 is a safe default for interior latex. Add a waste allowance of about 10 percent to cover rollers, cut-ins, and touch-ups. Finally, choose whether to include primer, which matters when you are painting bare drywall, bare wood, or stained areas. The calculator returns the exact gallons, the whole gallons to buy, the equivalent in liters, and the primer gallons in one pass. This estimator also works for exterior walls, fences, and decks when you enter the exterior coverage rate from your paint can.
Coverage changes with the surface, and using the wrong rate is the most common reason people buy too little paint. Smooth, previously painted drywall gets the best mileage at about 400 square feet per gallon. Standard eggshell or satin finishes cover around 350. Semi-gloss and gloss build a thicker film, so they cover closer to 300. Lightly textured walls such as orange peel or knockdown drop to about 250 to 325, and bare drywall, bare wood, plaster, or masonry can pull coverage down to 200 to 300 square feet per gallon because the porous surface soaks up the first coat. Heavy stucco or popcorn ceilings are the thirstiest of all at roughly 175. When your surface is rough or new, pick the lower end of the range rather than the printed maximum.
Primer seals porous surfaces so the topcoat covers evenly and lasts longer. You need a primer coat on bare drywall, bare wood, water-stained areas, patched repairs, glossy surfaces (sand them first), and dramatic dark-to-light color changes. You can skip it when repainting a similar color over sound, clean paint. Primer typically covers about 300 square feet per gallon, a little less than finish paint because it is thinner and soaks in. One coat is almost always enough; the primer's job is to seal and create a uniform base, not to build color. A paint-and-primer-in-one product works for moderate color changes over painted walls, but it is not a substitute for dedicated primer on bare or stained substrates.
Two coats is the residential standard for even color and full sheen, and most paint warranties assume two coats were applied. One coat can work when you are refreshing the same color with a quality paint on a sound surface. Three coats, or primer plus two coats, are needed when painting light colors over dark ones, covering bold saturated colors like deep reds and yellows, or working on highly porous surfaces. If you are unsure, enter two coats in the calculator and compare the result with one coat; the price difference of one extra gallon is small compared to the cost of a patchy wall and a second trip to the store.
The same formula works outside the house, but the coverage numbers are lower. Exterior emulsion on smooth siding covers roughly 300 to 350 square feet per gallon, while rough wood, stucco, brick, and masonry drop to 150 to 250. Measure the perimeter of the house, multiply by the wall height, and subtract doors and windows just like an interior room. For a typical two-story home with about 2,500 square feet of paintable exterior, two coats at 350 square feet per gallon means roughly 14 gallons of body paint plus trim paint. Add the waste allowance and always round up, because running short on an exterior job leaves a visible line where the new batch starts.
Most interior latex paints cover 350 to 400 square feet per gallon for one coat on smooth, sealed walls. Textured walls, bare drywall, and rough plaster absorb more paint and drop coverage to 250 to 300 square feet per gallon. The exact figure is printed on every paint can, and it is safest to use the lower end of the printed range for the first coat.
A 12x12 foot room with 8 foot ceilings has 384 square feet of wall. After subtracting one door (21 square feet) and two windows (30 square feet), you get 333 square feet net. With 2 coats at 350 square feet per gallon: 333 × 2 / 350 = 1.9 gallons, so you buy 2 gallons. Add 2 gallons of primer if the walls are bare drywall.
On smooth walls at 350 square feet per gallon, one coat needs about 1/350 of a gallon per square foot, so two coats need about 1/175 of a gallon per square foot. A 400 square foot area with two coats therefore needs about 2.3 gallons, which rounds up to 3 gallons to buy. Textured surfaces roughly double that figure.
Two coats is the standard for most interior repaints, giving even color and full sheen. Use one coat only when refreshing the same color on a sound surface in good condition. Use three coats, or primer plus two coats, for dark-to-light color changes, saturated colors like deep red or yellow, and porous bare surfaces.
Yes, use primer on bare drywall, bare wood, water-stained areas, patched repairs, glossy surfaces, and dark-to-light color changes. You can skip primer when repainting a similar color over clean paint in good condition. Primer covers about 300 square feet per gallon, and one coat is usually enough to seal the surface.
Yes. A standard interior door is about 21 square feet (3 × 7 feet) and an average window is about 15 square feet. In a typical bedroom, one door plus two windows remove roughly 50 square feet of paintable area, which is almost half a gallon at two coats. This calculator deducts them from the total so you do not overbuy.
Ceiling area is just the room's floor area: length times width. A 12x12 foot room has a 144 square foot ceiling, which needs about half a gallon per coat at 350 square feet per gallon, so one gallon covers two coats. Ceiling paint is usually flat white, and it is calculated separately from wall paint because it often uses a different product.
Yes. Textured surfaces like orange peel, knockdown, popcorn ceilings, and stucco have more actual surface area hidden inside the texture pattern, so they absorb 20 to 40 percent more paint. Drop your coverage rate to about 250 square feet per gallon for textured walls instead of the 350 used for smooth drywall.