Paint Coverage Calculator
Planning an interior or exterior paint job? Enter your room dimensions, minus doors and windows, to calculate exact gallons of paint and primer needed.
Results
Paint is bought by wall area, not floor area
The number that matters is the perimeter of the room multiplied by its height, which has almost nothing to do with square footage of floor. A long narrow 6 × 24 ft room and a square 12 × 12 ft room both have 144 sq ft of floor, but the narrow one has 480 sq ft of wall against the square one's 384. That is 25% more paint for the same floor.
Formula: net area = 2 × (length + width) × height − (doors × 20) − (windows × 15)
Then gallons = net area × coats ÷ 350, rounded up. A standard interior door is about 20 sq ft and an average window about 15, which is what those deductions represent. Skip the deductions on rooms with only one or two openings — the rounding up to a whole gallon absorbs them anyway.
Why 350 square feet per gallon
Manufacturers print 350 to 400 sq ft on the can. That figure assumes a smooth, previously painted, similar-colored wall and a roller in unhurried hands. This calculator uses 350, the low end, because almost every real condition takes coverage down rather than up.
| Surface | Coverage per gallon |
|---|---|
| Smooth, previously painted drywall | 350 to 400 sq ft |
| Light orange-peel texture | 300 to 350 sq ft |
| Knockdown or heavy texture | 250 to 300 sq ft |
| Bare drywall (prime first) | 200 to 250 sq ft |
| Rough masonry or stucco | 150 to 200 sq ft |
Coats, and when primer replaces one
Two coats is the working default and it is what produces an even sheen — a single coat almost always dries with visible roller variation, even when the color looks solid while wet. One coat is enough only for a touch-up in the identical color. Three is for light over dark, or for a saturated red or yellow, where the pigments are translucent and simply need more film.
Primer is a separate calculation at one coat, and it is the cheaper way to solve a coverage problem. A tinted primer under a dark color routinely saves a full finish coat, and primer costs less per gallon than paint does. Prime bare drywall, patched areas, bare wood, anything glossy, and any surface with stains or water marks that would bleed through.
Buy the whole job at once
Order all the paint for one color in a single purchase, and if it is more than a couple of gallons ask the store to box it — combine the cans into one container and remix. Tinted paint is mixed per can, and small machine variations between batches are invisible on a stir stick but obvious as a seam down a long wall.
The ceiling is a separate line item: it is the floor area, not the wall area, and it usually takes flat white rather than the wall color. Trim, doors and baseboards are separate again, normally in a different sheen, and a quart covers far more trim than people expect.
Frequently asked questions
How many square feet does 1 gallon of paint cover?
Manufacturers quote 350 to 400 square feet, which assumes a smooth, previously painted wall. This calculator uses 350 because most real surfaces take coverage down: light texture gives 300 to 350, knockdown texture 250 to 300, and bare drywall only 200 to 250 until it is primed.
How many gallons of paint do I need for a 12x12 room?
A 12 × 12 room with 8 ft ceilings has 384 sq ft of gross wall area, or about 349 after deducting one door and one window. At two coats that is 698 sq ft of coverage, so you need 2 gallons. Add a third if you are going light over dark.
Do I need primer, or just an extra coat of paint?
Primer is the cheaper fix. It costs less per gallon than finish paint, and a tinted primer under a dark color often saves a whole finish coat. Prime bare drywall, patched areas, bare wood, glossy surfaces, and anything with stains or water marks that would otherwise bleed through.
Why buy all the paint at once?
Tinted paint is mixed one can at a time, and small batch variations that are invisible on a stir stick show up as a seam down a long wall. Buy the full quantity in one purchase, and for more than two gallons ask the store to box it — combining the cans and remixing guarantees a single color.