Room Paint Calculator
Enter the room size; doors, windows, ceiling, and primer are below.
Walls & Paint
Paint Calculator
Quick answer: gallons = (wall area − doors and windows + ceiling) × coats ÷ coverage per gallon. A 12 × 14 ft room with 8 ft walls has 416 ft² of wall; minus one door and two windows (50 ft²) leaves 366 ft². Two coats at 350 ft²/gal need 2.09 gallons — 2 gallons plus 1 quart.
Work out how much interior paint a room needs: wall area from the room size and height, deductions for doors and windows, optional ceiling and primer, and the cans to buy at your prices.
How to calculate how much paint you need
Paint is sold by coverage: the label tells you how many square feet one gallon covers in one coat. Find the area you are painting, multiply by the number of coats, and divide by that coverage figure.
(Walls − Openings + Ceiling) × Coats ÷ Coverage
= gallons of paint; walls = 2 × (L + W) × H
- Find the perimeter: 2 × (length + width). A 10 × 12 ft room has a 44 ft perimeter.
- Multiply by wall height for gross wall area: 44 × 8 = 352 ft².
- Subtract openings: about 20 ft² for a standard interior door and 15 ft² for an average window. Skip deductions for small windows if you want a margin.
- Add the ceiling (length × width) only if it gets the same paint.
- Multiply by coats and divide by the coverage on the can — 350 ft² per gallon is a common figure for smooth, primed drywall.
- Round to cans you can buy: whole gallons, plus quarts when that is cheaper than another gallon.
Worked example: a 10 × 12 ft bedroom
8 ft walls, one door, two windows, two coats, walls only, 350 ft² per gallon.
- Gross walls: 2 × (10 + 12) × 8 = 352 ft²
- Openings: 20 + 2 × 15 = 50 ft², so net walls = 302 ft²
- Two coats: 604 ft² ÷ 350 = 1.73 gallons
- That is 1 gallon plus 3 quarts. Three quarts at $17 cost more than a second gallon at $40, so buy 2 gallons ($80).
Painting the 120 ft² ceiling with two coats as well would add 240 ÷ 350 = 0.69 gallon.
Paint needed by room size
8 ft walls, one door (20 ft²), two windows (15 ft² each), two coats, 350 ft² per gallon. Gallons are before rounding up to cans.
| Room size | Net wall area | Walls, 2 coats | Ceiling, 2 coats |
|---|---|---|---|
| 10 × 10 ft | 270 ft² | 1.54 gal | 0.57 gal |
| 10 × 12 ft | 302 ft² | 1.73 gal | 0.69 gal |
| 12 × 12 ft | 334 ft² | 1.91 gal | 0.82 gal |
| 12 × 14 ft | 366 ft² | 2.09 gal | 0.96 gal |
| 14 × 16 ft | 430 ft² | 2.46 gal | 1.28 gal |
| 16 × 20 ft | 526 ft² | 3.01 gal | 1.83 gal |
| 20 × 20 ft | 590 ft² | 3.37 gal | 2.29 gal |
What changes paint coverage
Most interior wall paints list 250 to 400 ft² per gallon. The number you actually get depends on the surface more than the brand. Smooth, previously painted drywall in a similar color reaches the top of the label range. Bare drywall and joint compound absorb the first coat, so prime them first or expect the first coat to cover noticeably less. Heavy texture such as knockdown or popcorn adds surface area and can drop coverage toward 250 ft². Rough wood, masonry, and stucco absorb even more — use the coverage stated for that surface on the product label.
Color change matters too. Going from a dark color to a light one, or painting deep reds and yellows that have less hiding power, often needs a tinted primer plus two coats, or a third coat. Rollers with a thicker nap hold more paint and lay it on heavier; that improves hide on rough surfaces but uses more per square foot.
Paint estimating tips
- Count every coat. The can's coverage is for one coat. Two coats doubles the paint — the most common reason people run out.
- Use one batch for one room. Buy enough to finish a wall without opening a new can mid-way, or box (intermix) all cans in a bucket so slight tint differences do not show.
- Trim and doors are separate. Baseboard, casing, and doors usually get a semi-gloss or enamel; a quart covers trim in a typical bedroom.
- Ceilings need their own estimate. Flat ceiling paint is a different product; switch on the ceiling option only if the walls and ceiling share a paint.
- Keep a little extra. A half-full quart of the same batch is the easiest way to touch up scuffs later. Note the color formula on the lid.
Paint calculator FAQ
How much paint do I need for a 12x12 room?
A 12 × 12 ft room with 8 ft walls has 384 ft² of wall. Subtracting one door and two windows (50 ft²) leaves 334 ft², and two coats at 350 ft² per gallon need 1.91 gallons — buy 2 gallons. Add about 0.8 gallon if the 144 ft² ceiling gets two coats of the same paint.
How many square feet does a gallon of paint cover?
Most interior paints cover 250 to 400 square feet per gallon per coat, and 350 ft² is a common planning figure for smooth, primed drywall. Textured, porous, or unprimed surfaces fall toward the low end. Always check the coverage printed on the can.
Do I need two coats of paint?
Two coats is the standard for even color and durability, especially over new drywall, patches, or a different color. One coat can be enough when repainting the same or a similar color over a clean, sound finish. Plan on a third coat or a tinted primer when covering a dark color with a light one.
How much paint do I need for a ceiling?
Ceiling area is the room length times width. A 12 × 14 ft ceiling is 168 ft², so two coats at 350 ft² per gallon need 336 ÷ 350 = 0.96 gallon — one gallon. Ceiling paint is usually a separate flat product, so estimate it apart from the walls.
Do I need to prime walls before painting?
Prime bare drywall, large patches of joint compound, stains, glossy surfaces, and dramatic color changes. Repainting a clean wall in a similar color usually does not need primer. Primers commonly list 200 to 400 ft² per gallon; this calculator defaults to 300 ft² and one coat.
How we built this calculator: results use the standard volume, area, and coverage formulas shown on this page, with typical U.S. material densities, bag yields, and waste factors that you can override. Written and checked by the YardsCalculator editorial team. Last reviewed: October 2026.