To estimate paint for a room, multiply the wall perimeter by the wall height, subtract openings you will not paint, multiply by the number of coats, and divide by your chosen paint’s coverage per gallon. Then round the result up to container sizes actually available. Calculate ceilings, trim and primer separately, especially when they use different products.
A room’s floor area cannot tell you its wall-paint quantity. Two rooms with the same floor area can have different perimeters, ceiling heights and window sizes. A few measurements give you a much more useful shopping estimate than a room-size label such as “small bedroom.”
Measure the surfaces that will receive this paint
For a rectangular room, perimeter equals twice the length plus twice the width. Multiply that perimeter by the wall height. A room measuring 12 feet by 10 feet with 8-foot walls has a perimeter of 44 feet and a gross wall area of 352 square feet.
For an irregular room, measure each wall section separately, multiply each length by its height, then add the areas. Include alcove walls and other recessed surfaces you intend to coat. Where a wall rises into a triangular section, calculate the rectangle below it and add half the triangle’s base times its vertical height.
Keep units consistent. Write 8 feet 6 inches as 8.5 feet, not 8.6 feet. If measuring in meters, calculate square meters and use the product’s stated square-meters-per-liter coverage. Do not divide square feet by a metric coverage figure.
Subtract relevant doors and windows
Subtract the measured area of openings that will not receive the wall paint. Measure the opening or the outer trim boundary that you are excluding, rather than subtracting glass alone while silently counting the surrounding trim as wall. Include any window returns that will receive wall paint as separate small surfaces.
In the example room, suppose the excluded doorway measures 3 by 7 feet and the excluded window area measures 4 by 4 feet. Their areas are 21 and 16 square feet. The net wall area is therefore 352 − 21 − 16 = 315 square feet.
Do not subtract furniture that will be moved or surfaces you still intend to paint behind. Likewise, do not subtract the entire window opening and then subtract the glass again. Small switches rarely warrant a separate deduction, but large doors and windows can materially change the purchase.
Use the actual product’s coverage and coat count
Look up the exact product, sheen and market version on its container or current technical sheet. Coverage is an area per unit of paint, usually for one coat. There is no reliable universal number for every interior paint and surface.
For a concrete reference, BEHR PREMIUM PLUS Interior Eggshell Enamel lists 250–400 square feet per gallon. That range belongs to this product, not every paint. For the worked estimate below, use the lower listed figure, 250 square feet per gallon, as a cautious planning assumption. Check the actual can before buying.
The calculation is: net wall area × coats ÷ coverage per gallon = gallons required. Assuming two coats in our example, 315 × 2 ÷ 250 = 2.52 gallons. Two coats are an explicit assumption here; use the number required for your product, surface and color change.
If the appropriate coverage were instead 400 square feet per gallon, the same calculation would give 1.575 gallons. This difference shows why selecting the most optimistic coverage without checking conditions can leave you short. Rough or absorbent surfaces and application losses can increase consumption; follow the manufacturer’s advice for the actual surface rather than treating the printed maximum as a guarantee.
Round up to a purchase you can actually make
At 2.52 gallons, two gallons are insufficient. If only whole gallons are available, buy three. If the identical product, sheen and color are available in U.S. quarts as well, compare three gallons with two gallons plus three quarts. The latter totals 2.75 gallons because four U.S. quarts equal one U.S. gallon.
Two gallons plus two quarts totals 2.5 gallons, just below the calculated requirement. Do not round 2.52 down to 2.5 simply because the difference looks small. Compare total prices too: several small cans can cost more than another gallon.
Decide whether you want a separate touch-up reserve before rounding. For example, adding a chosen 0.25-gallon reserve to 2.52 gives 2.77 gallons, so the 2.75-gallon combination would no longer meet your plan. Three gallons would. This reserve is an example of a personal allowance, not a manufacturer-required percentage.
Keep ceiling, trim and primer on separate lines
A flat rectangular ceiling in the example room has an area of 12 × 10 = 120 square feet. Multiply by its required coats and divide by the ceiling paint’s own coverage. A sloping ceiling needs its actual sloping surface area, not just the floor footprint.
For trim, add the lengths being painted and multiply by their painted width in matching units. Forty feet of trim with a simple 6-inch-wide face has 40 × 0.5 = 20 square feet before counting other exposed edges. Doors need their own measurements, including whichever faces and edges will receive that product. Detailed profiles need a more careful surface allowance.
Primer is a separate quantity even when the topcoat description includes “paint and primer.” Estimate only the area that requires the specified primer, multiply by its required coats, and divide by that primer’s coverage. Do not subtract primer volume from the topcoat requirement.
Check the calculator before checking out
An online paint calculator is useful if its assumptions match your job. Benjamin Moore’s calculator, for example, states that its rough estimate assumes two coats and excludes ceilings. Do not multiply its result by two again or assume it includes every painted surface.
Keep one shopping line for each product, sheen and color. Combine quantities across rooms only when those specifications match, then round the combined requirement rather than each wall individually. Record your measurements, coverage assumption, coat count and chosen reserve so the estimate is easy to revise. For the work itself, follow our room-painting preparation and application guide.
For measuring supplies, browse tape measures on Amazon. Choose a readable scale and enough reach for your room measurements.
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