How much paint a room needs
Paint is sold by coverage, not by room. A gallon of interior wall paint covers roughly 350 to 400 square feet for one coat on smooth, primed drywall, which is the range printed on most Sherwin-Williams, Behr and Valspar cans (Benjamin Moore often claims 400 to 450). This calculator defaults to 350 sq ft so you round toward having enough rather than running short halfway around the room.
To get the paintable area, it multiplies the room's perimeter by the wall height, then subtracts 20 sq ft for each door (a standard 30–36 in × 80 in door) and 15 sq ft for each window (about 3 × 5 ft). That area is multiplied by the number of coats and divided by the coverage per gallon. The result is rounded up to whole gallons, because that's how you'll buy it.
The formula
total area = (wall area + ceiling area) × coats
gallons = total area ÷ coverage per gallon, rounded up
cost = gallons × price per gallon
Example: a 12 × 12 ft bedroom
A 12 × 12 ft room with 8 ft walls, one door and two windows, painted with two coats at 350 sq ft per gallon and $40 per gallon, walls only.
| Step | Result |
|---|---|
| Gross wall area (48 ft perimeter × 8 ft) | 384 sq ft |
| Minus 1 door (20) and 2 windows (30) | 334 sq ft |
| Two coats | 668 sq ft |
| Paint needed (668 ÷ 350) | 1.91 gal |
| Gallons to buy | 2 gallons |
| Estimated cost (2 × $40) | $80.00 |
Tick "Include the ceiling" and the same room adds 144 sq ft per coat, bringing the total to 956 sq ft and about 2.73 gallons, so you'd buy 3.
Getting the number right
Adjust coverage for the surface
Coverage claims assume a smooth, sealed wall. Knockdown or orange-peel texture, bare drywall, fresh plaster and masonry all absorb more. Drop the coverage to 250–300 sq ft for those. New drywall should get a primer coat first, which typically covers 200–300 sq ft per gallon.
Plan for color changes
Going from a dark color to a light one, or painting a saturated red or yellow, often takes a third coat or a tinted primer. Choosing 3 coats above is a fair way to budget for it.
Trim and doors are separate
Baseboards, casings and doors are usually painted in semi-gloss, a different product. A quart covers trim in most average rooms.
Buy it all at once
Tinted paint can vary slightly between batches. If you need several gallons, buy them together, or mix them in a larger bucket (called boxing) so the color is consistent from wall to wall. Keep any leftover sealed for touch-ups; the note under the results tells you roughly how much you'll have.
Frequently asked questions
How much does one gallon of paint cover?
Most interior wall paints list 350 to 400 square feet per gallon for one coat on smooth, primed drywall. Textured, porous or unprimed surfaces soak up more, so 250 to 300 square feet is more realistic there. Check the coverage printed on your can and enter it above.
How many gallons of paint do I need for a 12x12 room?
A 12 by 12 foot room with 8 foot ceilings has 384 square feet of wall. After subtracting one door and two windows you paint about 334 square feet. Two coats at 350 square feet per gallon need about 1.9 gallons, so buy 2 gallons. The ceiling adds another 144 square feet per coat.
Do I really need two coats of paint?
Usually, yes. Two coats give even color and sheen, hide roller marks and hold up better to cleaning. One coat can work when you repaint the same or a very similar color with a high-quality paint. Big color changes, especially light over dark, may need a primer plus two coats.
Should I subtract doors and windows when calculating paint?
Yes, for an accurate number. A standard interior door is about 20 square feet and a typical window about 15 square feet. In a room with only one or two openings the difference is small, and many painters skip the subtraction to build in a little extra.
Is it cheaper to buy quarts or a gallon?
A quart usually costs 35 to 45 percent of a gallon, so three quarts cost more than one gallon. If you only need a little beyond a whole number of gallons, one or two quarts can save money. If you need three quarts or more, buy the extra gallon.
Does ceiling paint need to be calculated separately?
Ceilings are normally painted with a flat ceiling paint in a different color from the walls, so treat them as a separate purchase. This calculator adds the ceiling to the total when you tick the box; to price it on its own, run the calculator with the wall height set to zero.
Rates and figures last checked September 2026.
This calculator gives estimates for planning. Check current rates and product labels before you buy, list or file.