Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Calculate paint gallons needed for walls or ceilings, accounting for door and window openings and coats.
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Painting a room isn't just total wall area divided by a coverage rate -- doors and windows don't get painted, and most rooms need more than one coat. This calculator subtracts your openings first, then applies your coat count, so the gallon figure reflects what you'll actually paint.
Enter your total wall (or ceiling) area, the combined area of doors and windows to subtract, the number of coats, and your paint's coverage rate (350 sqft per gallon is a common average, though it varies by paint type and surface porosity).
Net Area = Wall Area − Openings Area
Gallons = (Net Area × Coats) ÷ Coverage per Gallon
For a surface without door/window subtraction logic -- like a fence, deck, or cabinets -- use the more general Paint Coverage Calculator instead. Once you know your gallons, the Paint Cost Calculator turns that into a budget.
Measure the height of the room and the length of each wall, multiply height by the total perimeter, and you have the gross wall area. A 12 by 14 foot room with 8 foot ceilings has a perimeter of 52 feet, so its gross wall area is 416 square feet. For ceilings, multiply length by width. Then measure each door and window opening and add them up for the openings field. A standard interior door is often around 20 square feet and a typical window may be about 15, but measure yours rather than guessing, since those sizes vary.
Suppose the walls of that 12 by 14 room are 416 square feet, two windows and one door total 55 square feet, you plan 2 coats, and the paint covers 375 square feet per gallon (hypothetical values). Net area is 361 square feet. Gallons are 361 times 2, divided by 375, which is 1.93. You would buy 2 gallons in this case, though if your can is exactly a gallon and the estimate is close to a whole number, a small margin for touch-ups or a cut-in pass makes a second can worthwhile. Note that this leaves almost nothing spare, so a smaller extra quart is a practical way to add safety.
Trim, baseboards, and crown molding are often painted with a different product and finish, so they are usually calculated separately. Use the Paint Coverage Calculator with their total area, or estimate a quart for a typical room's trim. The ceiling is another separate surface, and it is usually painted with a flat ceiling paint.
The gallons figure is a decimal because paint cannot be bought in fractions of an exact amount. Round up to the next available can size. Half-gallon and quart sizes let you match the result more closely and avoid buying a whole extra gallon. Also consider that tinted paint is usually not returnable, so order enough for the whole project in one batch to keep the color consistent.
The calculator does not model primer, repairs, or the difference between roller and sprayer. Coverage numbers on cans are ideal averages, and your real result depends on the surface and technique.
350 is a common average, but actual coverage varies by paint brand, sheen, and the surface being painted -- always check your specific product's label for its stated coverage.
2 coats is standard for most color changes and new construction. A single coat may suffice for touch-ups on an already-similar color, while dramatic color changes (especially light over dark) sometimes need a third coat or a tinted primer first.
Yes -- paint is sold in fixed gallon/quart sizes, and running short mid-job risks a subtle color/batch mismatch if you have to buy more later.
A small margin is sensible for touch-ups and for uneven absorption. Tinted paint from a single batch keeps color consistent, so buying it all at once is better than returning later for more.
Bare drywall, patched areas, stains, and big color changes often benefit from primer. The calculator does not include it, so size it separately based on your surface.
Run the calculation again with the ceiling area and a ceiling paint's coverage, since the ceiling has no openings to subtract in most rooms.
A textured surface has more actual area than the flat measurement suggests, and it absorbs more paint, so use a lower coverage rate than the can states.