CalculatorEnter your measurements below and it works the answer out.
Area × coats ÷ coverage, rounded up to whole gallons. A gallon covers 350 to 400 sq ft on a smooth sealed wall and less on anything porous.
The arithmetic. Work out the area, divide by what one gallon covers, round up. Nothing more than that — the value of the page is the coverage figures, which are stated here rather than hidden inside the tool.
Why there is no waste percentage here. A gallon covers 350 to 400 square feet on a smooth sealed wall and less on anything textured, porous or previously unpainted. Two coats is the honest default: one coat only works when you are repainting the same color.
How it is sold. Paint is sold by the gallon and the quart, and there are four quarts to a gallon. Buying one gallon more than you need is cheaper than a second trip for a half-quart, and leftover paint from the same batch is what you touch up with later.
Primer, porosity and the second coat. Bare drywall, new plaster and patched repairs drink the first coat, which is what primer is for — prime those and the topcoats go back to their stated spread rate. Deduct a large picture window; do not bother deducting a door you are also painting.
Round up to how it is sold. The result below shows the raw quantity and the whole gallons to buy, so you can see exactly where the rounding happened rather than being handed one figure.
Divide your area in square feet by the coverage of one gallon, then round up. At 375 sq ft each, a 200 sq ft job needs 1.
This calculator adds no percentage, because the number of coats already carries the allowance. A gallon covers 350 to 400 square feet on a smooth sealed wall and less on anything textured, porous or previously unpainted. Two coats is the honest default: one coat only works when you are repainting the same color.
With 8 ft ceilings the walls are 384 sq ft. At 375 sq ft a gallon in two coats that is 768 sq ft of coverage, so 3 gallons — call it 2 gallons plus a quart if you are deducting a door and a window.
On a smooth, sealed, previously painted wall, yes. On new drywall, bare plaster, or anything textured, expect closer to 300. Primer first is what gets you back to the 400 figure on the second coat.
Free to use on any site, including commercial ones. Paste this where you want it to appear — it keeps working when we update the figures.