Estimate paint quantity and optional cost with area units, coats, coverage, and wastage controls.
How this calculator works
Paint required scales with surface area, number of coats, and the manufacturer coverage rate.
gallons = (area × (1 + wastage%) × coats) ÷ coverage_per_gallonrecommended_purchase = ceil( gallons )
- Coverage varies by paint type: latex / oil ≈ 350, primer ≈ 250, elastomeric ≈ 150 sq ft per gallon.
- Porous surfaces, deep colour shifts, and rough textures all reduce effective coverage — bump wastage to 15–20%.
Common questions
How much paint do I need for two coats?
Set "Coats" to 2 and enter the same area. The calculator multiplies your surface area by the number of coats before dividing by coverage, so the gallon estimate is for the full job.
Why is my actual coverage less than the can says?
Manufacturer numbers assume smooth, sealed, primed surfaces. Texture, porosity, drastic colour shifts, and inexperienced applicators all reduce yield. Add 10–20% wastage when any of those apply.
Should I include the ceiling area?
Only if you're painting it. Add ceiling area to total area when ceilings get the same paint. If they're a different paint or colour, run the calculator twice — once per surface — so each gets its own coverage rate.
Run short on paint mid-room and the second can off the same shelf will be a different batch — slightly different sheen, slightly different undertone, and now your wall has a vertical line where the lap sits. Run long and you’ve got a half-empty gallon drying out in the basement that you’ll discover, hardened, in three years. This calculator returns the gallons that cover your walls at the coverage rate the can claims and the gallons you actually buy, rounded up because paint comes in whole containers.
Inputs and outputs
Enter wall length and ceiling height, subtract openings (windows and doors), choose a coat count and coverage rate. The calculator returns paintable square footage, gallons needed at the coverage rate, and the round-up to the next whole gallon — plus an estimated cost if you supply a per-gallon price.
A sample room
Say you’re painting a 12 ft × 12 ft bedroom with 8 ft ceilings, two coats, with one window (15 ft²) and one door (21 ft²). Wall area is the perimeter times the height; subtract the openings:
Most flat or eggshell wall paint claims 350 ft² per gallon on smooth drywall. Two coats:
Round up to 2 gallons exact, or buy a 3-gallon can if your supplier sells the pail size cheaper than three singles. The leftover from the 3-can solves the touch-up problem for the life of the room — that’s the can to label and store.
On coverage rates: the can vs. the wall
350 ft² per gallon is the manufacturer claim on smooth, primed drywall. Rough or textured surfaces drop coverage to 250-300 ft²; unprimed drywall or unprimed wood drinks the first coat at half the rated coverage. The “primed-vs-unprimed” question matters more than people admit — tinted primer that gets you to colour in one coat is cheaper than buying an extra gallon of finish to cover bare drywall.
Where this falls short
This is a four-walls-and-trim room calculator. It handles bedrooms, hallways, basements, and most renovation rooms cleanly. It is not the right tool for ceilings (different paint, often different colour), exterior siding (square footage doesn’t account for lap and crevice), or anywhere a colour change requires a tinted primer to block the previous colour. For those, calculate the surfaces separately and watch the coverage rate on each can — the same brand can vary by 50 ft² between a flat ceiling paint and an eggshell wall paint.
Brushes, rollers, sleeves, and tape are not in the calculator — figure one good 9 inch roller sleeve per 600 ft² of finish, plus 3 inch and 1.5 inch brushes for cutting in. Drop cloths and the painter’s life-saver: a wet rag in your back pocket.
If you’re scoping a whole-house repaint, a colour change that needs primer, or any commercial space where uniformity matters more than the can claim, a sit-down with a painting contractor who can spec coverage from a sample area is usually cheaper than the second supply run.
Other calculators on this site
If you’re scoping a whole project, you’ll usually need more than one of these. They share the same approach — plain math, honest about edge cases, with the practical notes someone who has done the work would actually need.
- Asphalt Calculator. Tonnage for a driveway, parking patch, or walkway pour, including the depth-to-tonnage curve a supplier will quote against.
- Concrete Pour Calculator. Cubic yards of concrete for a slab, footing, or post hole, plus the waste factor most pours actually need.
- Drywall Calculator. Sheets of drywall and bundles of mud for a renovation room, plus the cut and waste allowance.
- Excavation Calculator. Cubic yards of dirt to remove for a basement, pool, or footing — including the bulking factor that surprises first-timers.
- Fence Calculator. Lineal feet of fence, post count, rails, and gates for a property line that is rarely perfectly straight.
- Fill Calculator. Cubic yards of fill for a regrade, a backyard level, or a low spot, plus a sensible compaction factor.
- Lumber Calculator. Board feet for a deck, frame, or addition, in the rough framing pattern a supplier will price against.
- Tile Calculator. Square footage of tile for a floor or wall, plus the breakage and cut waste a real installation needs.
