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How Much Paint Do You Need? Simple Formula + Examples
The formula
Wall area × coats ÷ coverage per can = litres or gallons. Measure each wall (height × width) and subtract about 20 sq ft per door and 15 per average window. Multiply the net area by coats, then divide by the coverage printed on the can. Confirm fast with our Paint Calculator.
Wall area is only the start. Ceilings, trim, doors, and closets are separate line items that quietly double a small job — and closets are the classic surprise, since nobody measures them until they are already painting one. Add them before you leave for the store, not halfway through the first coat.
Paint by room type
| Space | Net wall area | Gallons (2 coats) |
| 10×10 ft bedroom | ≈ 300 sq ft | 2 |
| 12×10 ft bedroom | 317 sq ft | 2 |
| 12×14 ft living room | ≈ 520 sq ft | 3 |
| Whole-house interior (1,500 sq ft) | ≈ 2,400 sq ft | 14 |
These totals are walls only. Add roughly one extra gallon per room for ceilings, half a gallon for trim and doors, and you get a number that survives contact with an actual paint store.
Worked example: 12×10 ft bedroom
Four walls at 8 ft ceilings total about 352 sq ft. Minus one door (20) and one window (15) = 317 sq ft net. Two coats = 634 sq ft ÷ 350 sq ft per gallon = 1.8 gallons. Buy 2 gallons and keep 10% spare for touch-ups — one bad roller sleeve wastes more paint than the spare costs.
Write the room and the date on the lid of the spare can. A labelled gallon is a five-minute repaint when a chair rail gets scuffed; an unlabelled one gets thrown out, and the shade you mixed with it is gone for good.
Coverage facts by surface
- Flat latex on sealed drywall: 350–400 sq ft per gallon (9–11 m²/L)
- Bare drywall or plaster: prime first or lose 20–30% to absorption
- Ceilings: flat white, same math, cut in with a brush before rolling
- Trim and doors: semi-gloss, roughly 1 litre per 2 doors
- Brick, stucco, rough wood: halve the label coverage
Two numbers on the can matter more than the brand: the coverage figure and the recoat time. Painting the second coat early, while the first is still tacky, is how a wall ends up with a lifted, patchy film — the label gives a window for a reason.
One coat or two?
Two coats is the honest default. One coat works only for same-color refreshes with quality paint on sealed walls. Dark-to-light color changes, reds and yellows, and cheap contractor paint always need two — sometimes three for deep reds.
Common mistakes
- Skipping primer over repairs, stains, or bare wood — flashing shows through
- Buying exact math with zero spare — tinted paint is hard to match later
- Painting in direct sun or below 50°F (10°C) — poor adhesion
- Overloading the roller — drips that dry into ridges
- Counting only walls and forgetting ceiling, trim, and closets
Limits and assumptions
Label coverage assumes smooth sealed surfaces, proper film thickness, and 50–70% humidity. Textured or porous surfaces need up to 30% more. Always verify against the technical data sheet of your exact product, and see our home improvement guides for prep checklists.
Choosing paint: what actually matters
Finish first: flat hides wall flaws but scuffs fast; eggshell and satin suit living areas; semi-gloss survives kitchens, baths, and trim. Primer-sealer on repairs and color changes saves a full coat. Cheap paint needs more coats, so price per gallon misleads — price per finished square foot is the honest metric.
How long it takes
A 12×10 ft bedroom takes a DIYer one full day: morning prep and masking, midday cutting-in, afternoon two rolled coats with 2–4 hours dry time between. Pros do it in half a day with an 18-inch roller and back-rolling technique.
FAQ
- Litre or gallon? 1 US gallon = 3.78 litres. Match your can unit to the calculator input.
- How to store leftovers? Seal, label the room and date, store upright at 50–80°F. Good for 5+ years.
- Sprayer vs roller? Sprayers are faster on exteriors but waste 15–20% to overspray; rollers win indoors.
- How much primer? One coat, roughly the same gallon count as a single paint coat, and worth it over every repair and colour change.
- Can I use ceiling paint on walls? You can, but it is formulated to be flat and drip-free overhead — it scuffs fast where hands will touch it.