How much finish paint and primer does the job require?
The amount of paint to buy depends on the area that actually receives a coating, the number of coats, and the spread rate stated for the selected product. Measure the combined linear length of walls being painted, multiply by wall height, and subtract any door or window surfaces that will remain unpainted. Add the ceiling area only if the ceiling will receive the same finish. For connected rooms with different ceiling heights, calculate each group separately to avoid concealing important differences.
Primer and finish paint should be estimated independently. A porous new drywall surface may require a primer with a different spread rate from the finish coating, and a high-contrast color change may need an additional topcoat. Enter the coverage on the actual product label instead of assuming every gallon will cover the same area. Purchasing allowance and whole-container rounding are then applied separately, preventing primer from being incorrectly counted as finish paint.
Surface, coverage and container formulas
Finish gallons = paintable area × finish coats ÷ finish label coverage × (1 + allowance ÷ 100)
Primer gallons = paintable area × primer coats ÷ primer label coverage × (1 + allowance ÷ 100)
Containers to buy = round up(required gallons ÷ gallons per container)
Worked example: walls and a ceiling with separate primer
Suppose 50 linear feet of walls are 9 feet high, and 50 square feet of openings will not be painted. Including a 180-square-foot ceiling gives 50 × 9 − 50 + 180 = 580 square feet for each coat. Two finish coats at a stated coverage of 350 square feet per gallon, with a 10% allowance, require approximately 3.65 gallons. Four 1-gallon finish cans would cover that calculation.
One primer coat at 300 square feet per gallon with the same allowance requires about 2.13 gallons, rounded up to three 1-gallon primer cans. Selecting 5-gallon finish containers instead changes the purchasing count to one container but also increases the unused paint left after the project.
Coverage assumptions that can change the order
Rough masonry, textured ceilings and fresh unsealed surfaces can absorb more coating than a smooth, properly prepared wall. Spraying may have different transfer losses from rolling, and application rate, temperature and product formulation influence actual coverage. If the chosen product recommends a different number of coats for particular substrates, calculate those surfaces separately. Paint used on doors, cabinet fronts or trim should not be added to the wall area unless the same product and finish are appropriate.
Buying a larger container may improve the purchase price per gallon, but container size does not improve paint coverage. This calculator reports the resulting whole-can purchase and uses only the optional prices entered for those cans. Brushes, masking supplies, surface repair and labor are outside the paint-quantity calculation.
Paint quantity questions
Does a second coat double the number of gallons?
Two uniform finish coats consume approximately twice the amount required for one coat on the same surface, before can-size rounding. Actual second-coat consumption may vary because the surface is already sealed and the spread rate may improve. Enter the coat count recommended for the finish and condition of your walls rather than automatically doubling a single-container purchase.
Should I deduct windows and doors?
Subtract openings that will not receive paint when they are large enough to make a meaningful difference. Small cutouts and complex trim edges can create additional cutting and handling losses, so an allowance may still be appropriate. If the door itself will be painted in the same finish, calculate its surface separately instead of subtracting it permanently.
Can primer and finish paint use the same coverage number?
Only if the applicable product labels support that assumption. Primer and finish formulations can have different spread rates, and highly porous substrates often behave differently. The calculator keeps the two values separate so a change in primer coverage does not alter the calculated quantity of finish paint.