Paint Calculator
Enter your room dimensions, number of coats, and price per gallon to find how much paint you need and what it will cost.
What your result means
Paint needed is the whole number of gallons to buy, after figuring your wall area, knocking off about 10% for doors and windows, and applying your coats at roughly 350 sq ft of coverage per gallon per coat. Paintable wall area is the surface you're actually covering. The cost is gallons times your price. This is walls only — ceiling and trim are separate.
How to use this calculator
- Measure the room length and width in feet.
- Measure the ceiling height (8, 9, or 10 ft for most homes).
- Pick your coats: two is standard, one can work over a similar color.
- Enter a price per gallon from your paint store.
- Read the gallons to buy and the estimated cost, and round up for touch-ups.
The formula
Perimeter times height gives the gross wall area. We trim about 10% for typical doors and windows, multiply by coats, and divide by 350 sq ft per gallon. Gallons are rounded up because you can't buy a partial can.
Worked example
A 12 by 12 ft room with 8 ft ceilings has 384 sq ft of gross wall. Take off 10% for openings and you're painting about 346 sq ft. Two coats is 692 sq ft of coverage, which needs 1.98 gallons — so buy 2 gallons. At $35 a gallon that's about $70 in paint.
Paint coverage and what affects it
| Factor | Rule of thumb |
|---|---|
| Coverage per gallon | ~350 sq ft per coat on smooth, primed wall |
| Textured or porous walls | Drops to 250-300 sq ft per gallon |
| Dark over light (or reverse) | Plan on 2-3 coats |
| Doors & windows | Deduct ~10% of wall area |
| Trim & ceiling | Estimate separately — different coverage |
Tips & gotchas
- Buy all your paint in one batch and box (mix) it so the color is uniform across cans.
- Primer first on bare drywall, patches, or big color changes — it stretches your topcoat.
- Textured walls drink more paint; drop coverage to about 300 sq ft per gallon.
- A gallon does roughly a 12x12 room in one coat; two coats needs about two gallons.
- This covers walls only. Ceilings and trim use different products and coverage.
Frequently asked questions
How much paint do I need for a 12x12 room?
About 2 gallons for two coats on the walls, assuming 8 ft ceilings. That covers roughly 346 sq ft of wall after deducting for a door and windows.
How many square feet does a gallon of paint cover?
Around 350 sq ft per coat on a smooth, primed surface. Textured or unprimed walls drop that to 250-300 sq ft.
Do I need one coat or two?
Two is standard for an even finish and true color. One coat can work when repainting a similar shade over a sound surface.
Does this include the ceiling and trim?
No. This calculator covers wall paint only. Ceilings and trim use different paints and coverage rates, so estimate them separately.
Should I buy extra paint?
Yes, a little. Round up to the next gallon and keep some for touch-ups — matching a color later is hard.
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Estimates only — see our full disclaimer.