Paint Calculator

How to use this calculator

1. Choose Room or House. Room paints the four walls of a rectangular room and, if you want, the ceiling. House paints the outside walls up to the eaves plus the gable triangles. Soffits, fascia and trim are not included.
2. Enter the size. For a room: length, width and wall height. For a house: length, width, wall height to the eaves, how many gable ends you will paint and the roof pitch (inches of rise per foot). The pitch sets how tall the gables are.
3. Count the doors and windows. The calculator subtracts 36 × 80 in for each door and 36 × 60 in (3 × 5 ft, 15 sq ft) for each window, the same window size our drywall calculator uses. These are placeholder sizes, not a standard, so open Door & window sizes and enter your own. Add anything else you won’t paint, like a garage door or a tiled wall, as Other areas.
4. Set the coverage and coats. Picking Smooth or Rough fills in a starting coverage. Replace it with the figure on your can. Choose Prime first if your surface needs primer (see below).
5. Answer the age question. It starts at Not sure and shows the lead-safety advice, because EPA says to assume lead-based paint when you can’t find out the age of your home. Pick 1978 or later only if you know.
6. Choose how you buy: gallons plus quarts, whole gallons, or 5-gallon pails plus gallons. Prices are examples. A blank price field counts as not entered, never as $0.

The formula

Wall area = 2 × (length + width) × wall height + gable triangles - doors - windows - other areas

Gable triangle = width × rise ÷ 2, where rise = width ÷ 2 × pitch ÷ 12

Gallons = area × coats ÷ coverage per gallon, rounded up to whole quarts, gallons or pails

The ceiling (length × width) gets its own line because ceilings usually take a different paint. Primer is one coat over everything you prime, rounded up to whole gallons.

How much does a gallon cover?

Coverage is printed on every can and data sheet, usually as a range at a set wet film thickness (how thick the wet paint goes on, measured in mils). These are the current figures from the manufacturers’ own data sheets:

Benjamin Moore says its figure depends “on surface texture and porosity”. Behr says its range depends on the application method and substrate porosity, and that it does not include paint lost to spraying. None of them publishes a multiplier for texture. So the calculator starts from the low end of a range: 350 sq ft per gallon for smooth or previously painted walls (SuperPaint) and 250 for rough, porous or textured surfaces (Behr’s lower figure). Type in the number from your own can. A thick-film paint such as Duration covers less per gallon by design.

Primer coverage starts at 400 sq ft per gallon in both modes, but from two different products. Each Sherwin-Williams primer is limited to one side of the wall (interior use only, or exterior use only), so Room uses the interior primer and House the exterior one. Neither sheet gives a separate figure for rough or porous surfaces, so the primer figure stays the same when you pick Rough. Type in the figure from your own primer.

Worked example: a 14 × 12 ft bedroom

8 ft walls, one door and two windows at the default sizes, two coats:

• Walls: 2 × (14 + 12) × 8 = 416 sq ft
• Minus one door (3 × 6.67 ft = 20 sq ft) and two windows (2 × 15 = 30 sq ft): 366 sq ft
• Per coat: 366 ÷ 350 = 1.05 gal. Two coats: 2.09 gal
• Buy: 2.09 gal = 8.4 quarts → 2 gallons + 1 quart (2.25 gal), or 3 gallons if you buy whole gallons
• At the example prices of $45 a gallon and $18 a quart: $108
• With a coat of interior primer at 400 sq ft per gallon: 366 ÷ 400 = 0.92 gal → 1 gallon of primer

For the outside of a 40 × 30 ft house with 9 ft walls and two 6/12 gables: 140 ft × 9 ft = 1,260 sq ft of walls, plus two gables of 30 × 7.5 ÷ 2 = 112.5 sq ft each, minus two doors and ten 3 × 5 ft windows (190 sq ft) = 1,295 sq ft. Two coats at 350 sq ft per gallon take 7.4 gallons: 8 gallons, or one 5-gallon pail + 3 gallons. Soffits, fascia and trim come on top.

When you need primer

The paints in the table are sold as paint-and-primer or self-priming on most properly prepared surfaces. Their data sheets still call for primer in these cases:

• Bare drywall: Sherwin-Williams lists two options for SuperPaint: two coats of the paint as its own primer, or one coat of Premium Wall & Wood Primer and then two coats. Benjamin Moore recommends two coats of Regal Select on bare surfaces.
• Big color changes: Sherwin-Williams says a coat of primer improves hiding when repainting involves a drastic color change. Behr suggests a tinted primer for deep colors marked on its color chips.
• Stains, glossy or oil-based paint: Sherwin-Williams says stains from water, smoke, ink or grease should be sealed with a primer-sealer. Behr recommends a stain-blocking primer over oil-based or glossy surfaces, and Benjamin Moore suggests special-purpose primers for “very slick” surfaces.
• Cedar, redwood and knots: these woods can bleed tannin through the paint. The data sheets name stain-blocking or oil-based primers for them.
• Masonry, stucco and metal: new masonry and stucco must cure (about 30 days), and very alkaline surfaces need a masonry primer. Bare metal needs a metal primer. Follow the data sheet for your paint.

For a sound, previously painted surface, Benjamin Moore says one or two coats will do, and Duration’s data sheet calls for one coat on repaints. The calculator defaults to two coats, so change it to suit your paint and surface.

Lead paint in homes built before 1978

If your home was built before 1978, the old paint may contain lead. Sanding, scraping or removing it can release lead dust, which is especially harmful to children and pregnant women. If you can’t find out when your home was built, EPA says it is safest to assume the paint you are working on contains lead. For any renovation, repair or painting job that disturbs paint in a pre-1978 home, EPA recommends hiring a lead-safe certified contractor.

• Contractors: under EPA’s Renovation, Repair and Painting (RRP) Rule, 40 CFR 745 Subpart E, anyone paid to disturb painted surfaces in homes, child care facilities and preschools built before 1978 must be EPA lead-safe certified. Small jobs count as minor repair and maintenance and are exempt: 6 sq ft or less per room inside, or 20 sq ft or less outside. The exemption holds only if none of the prohibited practices below are used and the work does not replace windows or demolish painted surfaces.
• Doing it yourself: EPA says the RRP Rule generally does not apply to homeowners working on their own home. It does apply if you rent out all or part of it, run a child care center in it, or renovate homes to sell. EPA still recommends lead-safe practices: have a certified lead inspector or risk assessor check the paint (or assume it contains lead), seal off the work area with plastic, turn off forced-air heating and air conditioning and cover the vents with plastic, mist surfaces before sanding or scraping, use a heat gun only below 1,100 °F, and clean up with wet wipes and mops at the end of every day.
• Protective gear: EPA suggests a disposable N-100 respirator, disposable coveralls, a painter’s hat, disposable shoe covers, eye protection and gloves. Wash your hands and face whenever you stop work, and wash work clothes separately.
• Practices banned for contractors: open-flame burning or torching, and power sanding, grinding, planing or blasting without a shroud and HEPA vacuum attachment. Avoid them yourself, too.

Tips before you buy

• Trim, doors and cabinets are not included. They usually get a different sheen, so measure them separately. Outside, the same goes for soffits, fascia and exterior trim.
• Rollers and brushes: the Sherwin-Williams data sheets call for a nylon-polyester brush and a 3/8 to 3/4 in nap synthetic roller cover for these latex paints. Check the sheet for your product.

Last reviewed Oct 9, 2026. See how we calculate. Results are planning estimates - confirm with your supplier and local code.