How to use this calculator
- Choose Room size and enter the length, width and wall height, or choose Wall area if you’ve already added up the walls.
- Enter the number of doors and windows to leave out. Their sizes are editable under Door and window sizes.
- Decide whether you’re painting the ceiling too.
- Set the number of coats and the coverage per gallon from your paint can or the manufacturer’s data sheet.
- Turn on primer if you need it.
- Optionally edit the example prices for a gallon, a quart and a gallon of primer.
The result gives the total gallons of paint, exactly what to buy in gallons and quarts, and an estimated cost. With a ceiling or primer, each product is listed on its own line, because you’ll usually buy ceiling paint and primer separately from the wall color.
The formula
For a rectangular room, the wall area is the perimeter times the height: 2 × (length + width) × height. The paintable area is that, minus the doors and windows. The ceiling is length × width.
To turn gallons into something you can buy, the calculator rounds up to whole quarts (a quarter gallon each), then swaps quarts for another gallon when the quarts would cost as much or more. Without prices, it treats three quarts as a gallon.
Worked example
The default is a 14 × 12 foot bedroom with 8 foot walls, one door and two windows, painted with two coats at 350 square feet per gallon:
- Wall area: 2 × (14 + 12) × 8 = 416 sq ft.
- Openings: 1 door × 21 + 2 windows × 15 = 51 sq ft.
- Paintable wall: 416 − 51 = 365 sq ft.
- Paint: 365 × 2 ÷ 350 = 2.09 gallons.
- Buy: 2.09 gallons is 8.4 quarts, so 9 quarts: 2 gallons + 1 quart.
At the example prices of $45 a gallon and $18 a quart, that’s $108. Add the ceiling (14 × 12 = 168 sq ft, two coats) and you need 0.96 gallons more, so 1 gallon of ceiling paint. A coat of primer over walls and ceiling, 533 square feet at 300 square feet per gallon, takes 1.78 gallons, so 2 gallons. All together at the example prices: $213.
Paint for common room sizes
Two coats at 350 square feet per gallon, 8 foot walls, one door and two windows subtracted:
| Room | Paintable wall | Wall paint | Buy for walls | Ceiling paint | Buy for ceiling |
|---|---|---|---|---|---|
| 10 × 10 ft | 269 sq ft | 1.54 gal | 2 gallons | 0.57 gal | 1 gallon |
| 12 × 12 ft | 333 sq ft | 1.90 gal | 2 gallons | 0.82 gal | 1 gallon |
| 14 × 12 ft | 365 sq ft | 2.09 gal | 2 gallons + 1 quart | 0.96 gal | 1 gallon |
| 16 × 14 ft | 429 sq ft | 2.45 gal | 2 gallons + 2 quarts | 1.28 gal | 1 gallon + 2 quarts |
| 20 × 16 ft | 525 sq ft | 3.00 gal | 3 gallons | 1.83 gal | 2 gallons |
The buying column uses the example prices of $45 a gallon and $18 a quart. With one coat instead of two, the 14 × 12 room needs 1.04 gallons: 1 gallon + 1 quart.
Coverage by brand
Coverage is listed per gallon for one coat on a smooth, sealed surface. These are the manufacturers’ own figures:
| Product | Listed coverage per gallon |
|---|---|
| Sherwin-Williams SuperPaint Interior Latex Flat | 350 – 400 sq ft (at 4 mils wet) |
| Behr Premium Plus Interior | 250 – 400 sq ft |
| Benjamin Moore Regal Select Interior | 400 – 450 sq ft |
| KILZ 2 All-Purpose primer | 300 – 400 sq ft |
The calculator defaults to 350 for paint and 300 for primer, toward the low end, because real walls are rarely perfect and rollers leave some paint behind. New drywall, textured walls, rough plaster and bare wood soak up more paint, so use a lower figure for those. If your can lists a range, the low number is the safer one to plan with.
Tips for buying
- Buy one batch. Tinted paint can vary slightly between batches. Buying all the wall color at once keeps it consistent; for four gallons or more, a 5-gallon bucket is often cheaper per gallon.
- Keep a little extra. A labeled quart of leftover paint makes touch-ups invisible later.
- Count the trim separately. Baseboards, door frames and window casings are usually painted in a different sheen, often semi-gloss, so buy that paint on its own; trim takes far less than the walls.
- Measure the real height. Rooms with 9 or 10 foot ceilings need 12% to 25% more wall paint than the standard 8 feet.
- Planning a bigger project? For floors, use the square footage calculator; for the outside of the house, the roofing calculator estimates squares and bundles.
Assumptions and limitations
- Rooms are rectangles with walls of one height. For vaulted or angled walls, add up each wall’s area and use the Wall area option.
- Door and window sizes are averages: a 3 × 7 foot door (21 sq ft) and a 3 × 5 foot window (15 sq ft). Change them under Door and window sizes if yours differ, and enter 0 for openings you’ll paint over, like a closet door.
- Coverage figures are the manufacturers’ for smooth surfaces. Spraying usually uses more paint than rolling.
- Primer is bought in whole gallons. Prices are editable examples, not quotes.
Sources
- Sherwin-Williams SuperPaint Interior Latex Flat, product data sheet
- Behr Premium Plus Interior Flat, product page
- Benjamin Moore Regal Select Interior Matte, product page
- KILZ 2 All-Purpose Primer, product page
Frequently asked questions
How much area does a gallon of paint cover?
Most interior wall paints list 250 to 450 square feet per gallon for one coat on a smooth, primed surface. Sherwin-Williams SuperPaint Interior Flat lists 350 to 400, Behr Premium Plus Interior 250 to 400, and Benjamin Moore Regal Select Interior 400 to 450. Rough, porous or textured walls take more.
How much paint do I need for a 12 by 12 room?
A 12 by 12 foot room with 8 foot walls has 384 square feet of wall, or 333 after subtracting one door and two windows. Two coats at 350 square feet per gallon take 1.9 gallons, so buy 2 gallons. Painting the ceiling as well adds about 0.82 gallons, so 1 more gallon.
Do I need two coats of paint?
Two coats is the standard recommendation for an even color and a durable finish, especially when changing colors. One coat can be enough when you repaint with the same color over sound paint. Plan for two and keep any leftover for touch-ups.
Should I buy quarts or another gallon?
Quarts cost more per ounce, so three quarts usually cost more than a full gallon. The calculator buys extra quarts only while they cost less than another gallon at the prices you enter. Leftover paint is useful for touch-ups.
Do I need primer?
Primer is worth it on new drywall, bare wood, patched or stained areas, glossy surfaces, and when you're making a big color change such as light over dark. Many paints include primer, but a separate primer coat often saves a coat of more expensive paint. KILZ 2 lists 300 to 400 square feet per gallon.
How do I measure walls for paint?
Measure the length of each wall and the height from floor to ceiling, multiply, and add the walls up. For a rectangular room that's simply the perimeter times the height. Then subtract doors and windows, about 21 square feet for a standard door and 15 for an average window.
Updated November 2026. Results are estimates for educational purposes and depend on the inputs you provide.