How to use this paint calculator
Measure the length and width of the room and the height of the walls from floor to ceiling. Count the doors and windows; the calculator subtracts a typical 20 ft² for each door and 15 ft² for each window. Choose how many coats you need and, if you're painting the ceiling with the same paint, include it.
The calculator shows the exact amount of paint and then rounds it into what you'd actually buy: whole gallons plus quarts in US units, or whole liters in metric. If you'd need three quarts, it rounds up to another gallon, because a gallon usually costs about the same as three quarts.
The paint formula
Paint = (wall area − openings + ceiling) × coats ÷ coverage
Wall area is the room's perimeter times its height: 2 × (length + width) × height. Coverage is printed on the can, usually as square feet per gallon or square meters per liter.
Worked example: a 14 × 12 ft bedroom with 8 ft walls, one door and two windows. The perimeter is 2 × (14 + 12) = 52 ft, so the walls cover 52 × 8 = 416 ft². Subtract 20 ft² for the door and 30 ft² for the windows to get 366 ft². Two coats is 732 ft², and at 350 ft² per gallon that's 2.09 gallons, so you'd buy 2 gallons and 1 quart.
How many coats do you need?
| Situation | Coats |
|---|---|
| Refreshing the same color in good condition | 1 |
| Most repaints, or a similar color | 2 |
| Dark to light, or a bold red, yellow or orange | 2 + primer, sometimes 3 |
| New drywall or bare plaster | Primer + 2 |
Primer usually covers less area than paint, often 200–300 ft² per gallon, so calculate it separately by lowering the coverage figure and setting coats to 1.
What affects how far paint goes
- Surface texture. Textured walls, popcorn ceilings and rough plaster can need 10–25% more paint than smooth drywall.
- Porous surfaces. Fresh drywall, patched areas and bare wood soak up the first coat. Prime them first.
- Application method. Spraying uses noticeably more paint than rolling, because some of it drifts away as overspray.
- Color change. Covering a dark color with a light one takes more coats. A tinted primer can save a coat of the more expensive finish paint.
Tips before you buy
- Buy all the paint at once and mix the cans together in a bucket. Tints can vary slightly from batch to batch.
- Keep a little left over for touch-ups, labeled with the room and the date.
- Trim needs its own paint. Doors, baseboards and window frames usually use a semi-gloss, so estimate them separately.
Frequently asked questions
How much paint do I need for a 12×12 room?
With 8 ft walls, a 12 × 12 ft room has 384 ft² of wall. Subtracting one door and one window leaves about 349 ft². Two coats at 350 ft² per gallon need just under 2 gallons, so 2 gallons is usually enough for the walls.
How many square feet does a gallon of paint cover?
Most interior wall paint covers about 350–400 ft² per gallon per coat on smooth, primed walls. That's roughly 8.5–10 m² per liter. Rough or porous surfaces get less.
Should I subtract doors and windows?
Yes, for an accurate estimate. In a room with several large windows or a sliding door, they can account for 10% or more of the wall area. This calculator subtracts typical sizes automatically.
Do I need primer?
Prime new drywall, bare wood, patched areas, glossy surfaces and stains, and use primer when making a big color change. Over clean paint in good condition, a quality paint-and-primer product is often enough.
Results are estimates. Actual coverage depends on the product, surface and application method; always check the manufacturer's label.