How this instrument works
Paint coverage is usually printed right on the can, and this calculator turns that number into a shopping list: multiply area by number of coats, divide by coverage per gallon, then round up, since paint is sold by the gallon and a store won't sell you a fraction of one.
Coverage per gallon is not a fixed number — it's a manufacturer-stated figure that commonly falls somewhere in the 250-400 sq ft per gallon range per coat, and it depends on the paint's formulation and the surface being painted. Smooth drywall lets a gallon go further, while textured walls, wood siding, brick, or stucco all soak up more paint per square foot, dragging real coverage down. The 350 sq ft/gal default used here is a reasonable mid-range placeholder — always check your specific can's label for the actual rated coverage before buying.
Number of coats matters just as much as area: most paint jobs use two coats for even color and durability, especially over a different existing color or a bare surface, though a single coat of a high-hiding paint over a similar existing color can sometimes suffice. Each additional coat multiplies the gallons needed proportionally, so getting this input right matters as much as the area measurement itself.
- Enter Area to paint (sq ft) — the total wall, ceiling, or surface area you're covering.
- Enter Coverage per gallon, per coat (sq ft) — check your specific paint can's label; the typical range is 250-400 sq ft/gal.
- Enter Number of coats — 2 is the common default for full coverage and even color.
- Read Gallons to buy (rounded up) — the number of full gallons to purchase.
Worked example — a 400 sq ft wall, 2 coats
Set Area to paint to 400 sq ft, Coverage per gallon to 350 sq ft/gal, a mid-range paint-can-label figure, and Number of coats to 2. Gallons needed = (400 × 2) ÷ 350 = 2.2857; rounded up to buy = 3 gallons.
A larger 1,000 sq ft area using a single coat of a higher-coverage paint (400 sq ft/gal) needs (1,000 × 1) ÷ 400 = 2.5, rounding up to 3 gallons as well — the same '3 gallons to buy' result from very different area and coat combinations, showing why checking your actual can's coverage rating matters more than eyeballing a rough area-to-gallons ratio.
Questions
How many square feet does a gallon of paint cover?
Most interior latex paints cover somewhere in the 250-400 sq ft per gallon range per coat, with smoother surfaces like drywall toward the higher end and rougher or more porous surfaces like brick, stucco, or wood siding toward the lower end. The exact figure is printed on your specific paint can's label — that's the number to use for an accurate estimate, since coverage varies by manufacturer, paint line, and sheen, not just surface type.
Do I really need two coats of paint?
For most projects, yes. Two coats give more even color, better hide any underlying color or stain, and generally hold up better over time than a single coat, especially when painting over a significantly different existing color or a bare, unpainted surface. A single coat can sometimes suffice with a high-hiding paint applied over a similar existing color, but check your specific paint's product information, since one-coat coverage claims vary in how reliable they are in practice.
Why does the calculator round up to a whole gallon?
Because paint is sold in whole containers — gallons, quarts, or five-gallon buckets — not arbitrary fractional amounts, so a calculated need of 2.29 gallons still means buying 3 individual gallon cans, or an equivalent larger container. Rounding up also builds in a small natural buffer for touch-ups, uneven application, or minor measurement error, which is generally preferable to running short mid-project.
Does this calculator account for primer separately?
No — it's built around finish or topcoat paint coverage. Primer typically has different, often lower, coverage than finish paint, commonly in the 200-300 sq ft per gallon range, so if your project needs a primer coat, calculate that separately using primer's own coverage rate rather than assuming it matches your topcoat paint's coverage figure.
Should I subtract doors and windows from my area measurement?
Many painters skip subtracting small openings like standard doors and windows, since the paint saved by not painting them is a small fraction of the total wall area and the built-in buffer from rounding up to a whole gallon usually absorbs the difference. For a room with unusually large windows, sliding doors, or other big openings, subtracting their area from your total gives a more accurate estimate and can meaningfully change the gallon count.