Paint Calculator: How Many Gallons of Paint for a Room?

How to use the paint calculator

1. Enter the room. Give the length, width and wall height, and the calculator works out the wall area as perimeter × height. For a single accent wall, a stairwell or an odd-shaped room, switch to “Wall area” and type in the total of width × height for each wall you will paint.
2. Count doors and windows. The calculator takes off 21 sq ft per door and 15 sq ft per window. These are the same allowances our drywall calculator uses. Patio doors and picture windows are larger, so count a 6 ft slider as two doors.
3. Set the coats and coverage. Two coats is the safe default. For coverage, use the figure on your can or the product data sheet. Our default is 350 sq ft per gallon, the low end of the 350–400 printed on several Sherwin-Williams interior latex sheets.
4. Ceiling and primer. Turn the ceiling on to estimate ceiling paint separately, at the same coats and coverage. Turn primer on for new drywall, repairs, stains or a big color change.

The result is in gallons, along with the cans to buy. It rounds up to whole quarts, and three or more quarts become another gallon.

How much paint do I need? The formula

Coverage on a data sheet is measured at a set film thickness. For Sherwin-Williams interior latex, 350–400 sq ft per gallon is quoted at 4 mils wet (about 1.5–1.6 mils dry). Benjamin Moore lists 400–450 sq ft for its Regal Select line. You only get that coverage if you apply the paint at that thickness, on a smooth, sealed wall. Rough or porous surfaces such as heavy orange peel, knockdown texture, bare plaster and brick soak up more, so drop the coverage to 300 or lower. A thin coat rolled out to stretch the paint will cover more area but hide less, and it usually means an extra coat.

Worked examples

12 × 14 ft bedroom, 8 ft ceiling, one door, two windows

The perimeter is 2 × (14 + 12) = 52 ft, so the walls are 52 × 8 = 416 sq ft. Take off 21 for the door and 2 × 15 for the windows to get 365 sq ft. Two coats at 350 sq ft per gallon is 365 × 2 ÷ 350 = 2.09 gallons, so buy 2 gallons and 1 quart, or 3 gallons if you want leftover paint for touch-ups. The ceiling is 168 sq ft, and two coats is 0.96 gallons, so 1 gallon of ceiling paint. Priming the walls and ceiling of a newly drywalled room is 533 ÷ 400 = 1.33 gallons, so 2 gallons of primer.

One accent wall, 15 ft wide and 9 ft high

135 sq ft. A deep color over a light wall usually needs two coats or a tinted primer first: 135 × 2 ÷ 350 = 0.77 gallons. That is just over 3 quarts, so buy 1 gallon.

Open living room with textured walls

For a total wall area of 900 sq ft with three windows and two doors, take off 87 sq ft to get 813 sq ft. On knockdown texture use 300 sq ft per gallon: 813 × 2 ÷ 300 = 5.42 gallons, which is 5 gallons and 2 quarts. At that size, one 5-gallon bucket plus a gallon can be cheaper and keeps the color consistent.

Do you need primer? When to prime and how much

Many wall paints are now sold as “paint and primer in one,” and on a sound, previously painted wall you usually don’t need a separate primer. Benjamin Moore’s guidance is that a previously painted surface in good condition most likely does not need priming. It recommends primer in these cases:

• bare surfaces that have never been painted, including new drywall, raw wood and spots sanded down to the substrate;
• stains and rust;
• dramatic color changes;
• hard-to-coat, glossy or non-porous surfaces such as laminate, glass, high-gloss paint and PVC.

On new drywall, the Sherwin-Williams data sheet for one interior paint offers two routes: self-prime with two coats of the paint, or use one coat of primer and then two coats of paint. The primer route usually gives a more even sheen over taped joints. Sherwin-Williams lists its Premium Wall & Wood Primer at 400 sq ft per gallon, so the calculator prices one coat of primer at that rate. Tinting the primer toward the finish color helps when you are going much darker or brighter. Hanging new board first? The drywall calculator counts the sheets, mud and tape.

Buying tips: gallons, quarts, trim and leftovers

• Trim, doors and cabinets are usually a different sheen, often semi-gloss, and aren’t included here. A quart is often enough for the trim in one room. Painting the doors themselves adds about 21 sq ft per side.
• Batch consistency. Tinted paint can vary slightly from can to can. On a large job, mix the gallons together in a bucket (called boxing) before you start.
• Keep some back. A labeled quart of leftover paint makes touch-ups invisible later.
• Coverage per coat. The calculator shows how many gallons one coat takes, so you can decide on a third coat after the second one dries.

Doing the floors too? Estimate boxes of plank with the flooring calculator, or tile and grout with the tile calculator.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Wall area = room perimeter × wall height, minus 21 sq ft per door and 15 sq ft per window (the same allowances as our drywall calculator). Measure large openings separately.
• The default coverage is 350 sq ft per gallon per coat, the low end of the 350–400 sq ft/gal printed on several Sherwin-Williams interior latex data sheets at 4 mils wet. Textured or porous surfaces cover less; your can's label wins.
• Gallons = area × coats ÷ coverage. Cans are rounded up to whole quarts; three or more quarts are rounded up to another gallon.
• Primer is one coat at 400 sq ft per gallon (Sherwin-Williams Premium Wall & Wood Primer data sheet). Trim, doors and cabinets are not included.

Sources

1. Sherwin-Williams, Duration Home Interior Latex Flat (A95 Series) product data sheet. Coverage 350–400 sq ft per gallon at 4 mils wet, 1.6 mils dry; drywall: self-prime with 2 coats, or 1 coat of primer plus 2 coats.
2. Sherwin-Williams, Premium Wall & Wood Interior Latex Primer (B28W08111) product data sheet. Coverage 400 sq ft per gallon at 4 mils wet, 1.5 mils dry.
3. Benjamin Moore, Regal Select Interior technical data sheet. Spread rate 400–450 sq ft per gallon.
4. Benjamin Moore, How to Choose the Best Paint Primer. A previously painted surface in good condition most likely does not need primer; prime bare substrates, stains and rust, dramatic color changes and hard-to-coat surfaces.

Last reviewed October 25, 2026. Product figures vary by brand; the label on what you buy wins.