How much paint do I need?
Short answer: gallons = wall area minus doors and windows, times coats, divided by coverage per gallon. A 12 × 12 ft room with 8 ft walls, one door and two windows has 333 sq ft to paint. Two coats at 350 sq ft per gallon need 1.9 gallons, so buy 2.
1. Your room
Measure the floor and the wall height in feet. Count only the doors and windows you will paint around.
Door 21 sq ft and window 15 sq ft are averages, not your sizes. Measure yours if they differ. 350 sq ft per gallon is the low end of Sherwin-Williams' 350-400 sq ft range. Check the label on your can, and use about 250 for rough or textured walls.
2. Paint you need
- Paintable area
- 333 sq ft
- Gallons to buy
- 2
- Or gallons + quarts
- 2 gal
333 sq ft of wall
Exact need: 1.90 gal for 2 coats
1 gallon = 4 quarts
How this is calculated
- Wall area = 2 × (length + width) × height = 384 sq ft.
- Minus openings: 1 × 21 + 2 × 15 = 51 sq ft.
- Gallons = area × coats ÷ coverage = 333 × 2 ÷ 350 = 1.90.
- The quarts option keeps the whole gallons and rounds the rest up to the next quarter gallon.
- Primer covers less: Sherwin-Williams gives 200-300 sq ft per gallon. Set coverage to 250 and coats to 1 to estimate it.
The formula
- Wall area = 2 × (length + width) × wall height.
- Subtract openings. The Old Farmer's Almanac puts the average door at 21 sq ft and the average window at 15 sq ft.
- Add the ceiling if you are painting it: length × width.
- Gallons = paintable area × coats ÷ coverage per gallon. Round up to buy.
Sherwin-Williams says a gallon typically covers about 350-400 sq ft, and a quart about 100 sq ft. The calculator starts at 350, the low end, so you are less likely to run short.
Worked example: a 12 × 12 ft room with 8 ft walls
One door and two windows, at 21 and 15 sq ft each.
| Job | Area | Coats | Coverage | Exact gallons | Buy |
|---|---|---|---|---|---|
| Walls, smooth | 333 sq ft | 2 | 350 | 1.90 | 2 gal |
| Walls, best case | 333 sq ft | 2 | 400 | 1.67 | 2 gal, or 1 gal + 3 qt |
| Walls, same color refresh | 333 sq ft | 1 | 350 | 0.95 | 1 gal |
| Walls + ceiling | 477 sq ft | 2 | 350 | 2.73 | 3 gal, or 2 gal + 3 qt |
| Walls, rough or textured | 333 sq ft | 2 | 250 | 2.66 | 3 gal, or 2 gal + 3 qt |
| Primer, one coat | 333 sq ft | 1 | 250 | 1.33 | 2 gal, or 1 gal + 2 qt |
Gross wall area is 2 × (12 + 12) × 8 = 384 sq ft. Openings take off 21 + 2 × 15 = 51 sq ft, leaving 333.
Tips before you buy
- Plan on two coats. Bob Vila says to generally count on two. One coat can work when you repaint in the same or a similar color.
- Rough, textured or porous walls take more. Bob Vila notes a gallon may cover only about 250 sq ft on rough or textured surfaces.
- Primer covers less than paint. Sherwin-Williams gives 200-300 sq ft per gallon. Primer matters most over new drywall or a dark color.
- Read your can. The coverage on the label is the best number for that product. Enter it in the calculator.
Common questions
How much does one gallon of paint cover?
About 350-400 sq ft on smooth walls, according to Sherwin-Williams. That is one coat. Two coats halve the area a gallon covers.
How many gallons for a 12 × 12 room?
About 2 gallons for two coats on the walls, with one door and two windows. Add about another gallon if you also paint the ceiling.
Should I subtract doors and windows?
Yes, if you are not painting them the wall color. Use 21 sq ft per door and 15 sq ft per window, or measure your own.
When does buying quarts make sense?
When the leftover is small. A quart covers about 100 sq ft. Compare prices, because several quarts can cost more than one gallon.
Does primer count toward my coats?
No. Primer is a separate product with lower coverage. Estimate it on its own with one coat at about 250 sq ft per gallon.