Enter your room dimensions to calculate the exact number of gallons needed for walls, ceiling, and touch-ups. Adjustable for number of coats, coverage rate, and waste factor.
Quick Presets
Room Dimensions
Doors & Windows (deduct area)
Number of Coats
2Coverage (sq ft / gallon)
350Price per Gallon
Waste / Overage Factor
10%Results
Enter room dimensions to calculate
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Buy in 5-gallon buckets when you need 3 or more gallons
A 5-gallon bucket is cheaper per gallon than five individual gallons and ensures color consistency — no risk of slight batch-to-batch color variation between cans.
Calculate wall area before going to the store
Don't eyeball it. Measure the actual perimeter and ceiling height, subtract large openings (standard door = 20 sq ft, standard window = 15 sq ft), then divide by the coverage rate on the can label. Being off by half a gallon mid-job costs you a trip back to the store.
Order 10–15% extra for touch-ups
Even on finished jobs, you'll need touch-up paint later. Store the leftover in a tightly sealed container, label it with the room name and paint color code, and keep it — properly sealed latex paint lasts 2–5 years in storage.
A standard gallon of paint covers about 400 square feet of smooth wall with one coat. To find how much you need, calculate your wall area (perimeter × ceiling height) and divide by 400, then multiply by the number of coats. Always add 10% for waste and touch-ups.
Most contractors use a separate flat white ceiling paint. Ceiling paint is formulated to hide roller marks and has a flat sheen that reduces glare. Wall paint on a ceiling tends to drip and shows lap marks more easily. Budget for a separate gallon of ceiling paint per 350–400 sq ft of ceiling area.
Two coats is the standard for most interior paint jobs — one coat for coverage, a second for evenness and durability. If you're going from a dark color to a light one, or painting raw drywall, you may need a primer coat plus two finish coats. For new drywall specifically, use a PVA drywall primer — it seals the surface and prevents uneven absorption. One coat is rarely sufficient for a professional result.
Paint coverage (400 sq ft per gallon) is the manufacturer's spread rate on smooth, primed surfaces. On textured walls, rough surfaces, or unprimed drywall, actual coverage drops to 300–350 sq ft per gallon. Always check the paint can label, and use 400 sq ft as your planning baseline rather than an optimistic ceiling.
Primer is required when painting new drywall, covering a dark color, or painting over a glossy surface. For repainting with a similar color on a sound existing finish, a self-priming paint (paint + primer in one) may be enough. Skipping primer on bare drywall leads to uneven absorption and more coats needed.