Quick answer
A 12 x 12 foot room with 8 foot ceilings needs about 3 gallons of paint for two coats. The estimate is wall area, minus doors and windows, times the number of coats, divided by the coverage printed on the can, usually 350 to 400 square feet per gallon.
How many gallons of paint for common room sizes
Wall paint for rooms with 8 foot ceilings, one door, and two windows, using 350 square feet per gallon and a 10 percent buffer.
| Room size | Wall area | One coat | Two coats |
|---|---|---|---|
| 10 x 10 ft | 270 sq ft | 1 gal | 2 gal |
| 10 x 12 ft | 302 sq ft | 1 gal | 2 gal |
| 12 x 12 ft | 334 sq ft | 2 gal | 3 gal |
| 12 x 14 ft | 366 sq ft | 2 gal | 3 gal |
| 14 x 16 ft | 430 sq ft | 2 gal | 3 gal |
| 16 x 20 ft | 526 sq ft | 2 gal | 4 gal |
| 20 x 20 ft | 590 sq ft | 2 gal | 4 gal |
Results are rounded up to whole gallons. Taller ceilings, extra doors, or lower coverage change the answer, so enter your own numbers above.
How do you calculate how much paint you need?
Paint is sold by coverage, so the job is really an area problem. The calculator adds the four walls together, removes the space taken by doors and windows, multiplies by the number of coats, and then adds a waste buffer for roller loss and touch-ups.
It rounds up to whole gallons because that is how most people buy paint. If the result is only a little over a whole gallon, a quart can cover the gap for less money. Ceilings and trim are not included in the wall number, so add them separately if you are painting them.
The formula
- Wall area = 2 x (length + width) x ceiling height
- Paintable area = wall area - (doors x 20 sq ft) - (windows x 15 sq ft)
- Coverage needed = paintable area x coats x (1 + waste %)
- Gallons = coverage needed / coverage per gallon, rounded up
Worked example
With the starting values in the calculator (Room length: 12 ft, Room width: 12 ft, Ceiling height: 8 ft, Doors: 1, Windows: 2, Paint coats: 2, Paint coverage: 350 sq ft/gal, Waste buffer: 10%, Paint price: $42/gal), the result is 3 gallons at an estimated $126.
- Wall area = 2 x (12 + 12) x 8 - openings = 334 sq ft
- Coverage needed = 334 x 2 coats x 1.1 waste = 734.8 sq ft
- Gallons = 734.8 / 350, rounded up = 3
What this calculator assumes
- Each door removes 20 square feet and each window removes 15 square feet.
- Coverage defaults to 350 square feet per gallon, the low end of most interior paint labels.
- A 10 percent buffer is added before rounding up to whole gallons.
- Walls only. Ceilings, trim, and primer are separate purchases.
Tips for a more accurate estimate
- Use the coverage number on your can. Rough, porous, or textured walls can drop coverage well below the label.
- Add ceiling paint separately: ceiling area is simply room length times room width.
- Buy all the finish paint for one room in a single trip so the color comes from the same batch.
- Going from a dark color to a light one usually needs primer or a third coat, so raise the coat count.
Paint Calculator FAQ
How much paint do I need for a 12 x 12 room?
About 3 gallons for two coats on the walls of a 12 x 12 foot room with 8 foot ceilings, one door, and two windows. That is 334 square feet of wall, 668 square feet for two coats, and about 735 square feet after a 10 percent buffer.
How do I calculate paint for a ceiling?
Multiply room length by room width to get ceiling area, multiply by the number of coats, and divide by the coverage on the can. A 12 x 12 ceiling is 144 square feet, so one gallon covers two coats.
Is one gallon of paint enough for a bedroom?
One gallon covers roughly 350 to 400 square feet, which is one coat on a small bedroom. Most bedrooms need two coats, so plan on two to three gallons.
How many square feet does a gallon of paint cover?
Most interior paint covers about 300 to 400 square feet per gallon per coat. Check the can label and lower the coverage number for rough or dark surfaces.
Should I buy extra paint?
A 10 percent buffer is usually enough for touch-ups, roller loss, and small measuring mistakes.
Does this include primer?
The main gallons are for paint. Add primer if you are covering stains, bare drywall, or a major color change.
Use this estimate as a planning starting point. Product coverage, waste, local prices, code rules, and site conditions can change the final amount.