Key takeaways
- Paint needed = surface area × coats ÷ coverage per gallon.
- Published coverage runs from about 250 to 450 sq ft per gallon, so one small room can swing by more than a gallon.
- In a home built before 1978, read about lead before any sanding or scraping.
Enter your room's size, the door and window area you measured, your coats, and the coverage on your paint's label. The tool shows what to buy and how the answer changes across published coverage figures.
How the calculator works
- Walls: 2 × (length + width) × height − openings
- Surface: walls, plus length × width if you include the ceiling
- Gallons: surface × coats ÷ coverage
- To buy: whole gallons, then the remainder rounded up to quarts
Worked example
A 12 × 12 foot room with 8-foot walls and one 3 × 7 foot door:
- Walls: 2 × 24 × 8 = 384 square feet
- Less the door: 384 − 21 = 363 square feet
- Two coats: 726 square feet of coverage
| Coverage | Gallons |
|---|---|
| 450 sq ft per gallon | 1.61 |
| 400 sq ft per gallon | 1.82 |
| 350 sq ft per gallon | 2.07 |
| 300 sq ft per gallon | 2.42 |
| 250 sq ft per gallon | 2.90 |
So the same room can be one trip to the store or two, which is why we ask for your product's figure rather than assuming one.
Choosing a coverage figure
Published figures vary by product:
- Benjamin Moore Regal Select matte lists 400 to 450 square feet per gallon, and says bases and colors can differ.
- Behr's guide gives 250 to 400 for its Ultra and Premium Plus interior paints, and says texture can raise what you need.
Use the lower end for rough, textured or unprimed surfaces. To see where these figures come from, read paint coverage and wet film thickness.
Measuring openings
We don't subtract a standard door or window, because we found no manufacturer figure for one. Measure each opening and add the areas. If you skip this step, you'll just overestimate a little, which is often fine for small rooms.
Trim, ceilings and primer
- Trim: Behr's method treats trim as total length × 0.5 feet, assuming trim about 6 inches wide. Work it out separately if you're using a different paint.
- Ceilings: these are often a different paint, so run them separately unless you're using the same product.
- Primer: run the tool again with one coat and the primer's coverage. Sherwin-Williams' ProMar 200 Zero VOC primer sheet, for example, lists 400 square feet per gallon and specifies one primer coat and two topcoats over drywall.
Older homes
If your home was built before 1978, or you don't know, read about lead paint before any sanding, scraping or removing.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8-foot walls, one door and two coats, about 1.6 to 2.9 gallons, depending on the paint. At 400 square feet per gallon, it's about 1.8 gallons.
Does a gallon cover 400 square feet?
Some products list that, but it isn't universal. Check the label, and allow more on rough or unprimed surfaces.
Should I buy extra?
Rounding up gives you a little left for touch-ups. Whether to buy more depends on your surface and paint, and a store can tell you its return policy for tinted paint.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- Benjamin Moore: Regal Select matte product data
- Behr: How to determine how much interior paint to buy
- Sherwin-Williams: ProMar 200 Zero VOC primer data sheet