How to use this calculator
- Enter each room’s floor dimensions. Not the wall lengths — the length and width of the room. Wall area comes from the perimeter, so a 12 × 12 room and a 16 × 9 room have the same floor area but different amounts of wall.
- Set the ceiling height. Eight feet is standard in most US homes; older houses and newer builds often run nine or ten.
- Count the doors and windows. They come off the total. One or two barely matter; six make a real difference.
- Choose the surface and the number of coats. These two settings move the answer more than anything else on the page.
Painting several rooms in the same colour? Add a section for each and buy once — a five-gallon bucket is usually cheaper than five gallons in separate cans, and it guarantees the colour matches across the whole job.
The formula
The step people skip is that wall area comes from the perimeter, not the floor area.
wall area = perimeter (ft) × ceiling height (ft)
= 2 × (length + width) × height
openings = doors × 20 sq ft + windows × 15 sq ft
paint area = (wall area − openings) × number of coats
gallons = paint area ÷ coverage (sq ft per gallon)
Worked example — a 12 ft × 12 ft bedroom with 8 ft ceilings, one door, two windows, two coats:
- Perimeter: 2 × (12 + 12) = 48 ft
- Wall area: 48 × 8 = 384 sq ft
- Openings: 20 + (2 × 15) = 50 sq ft
- Net wall area: 384 − 50 = 334 sq ft
- Two coats: 334 × 2 = 668 sq ft of coverage needed
- At 350 sq ft per gallon: 668 ÷ 350 = 1.91 gallons
So: buy two gallons. Adding the ceiling costs another 144 sq ft per coat, or 288 sq ft for two — about eight tenths of a gallon, so a third gallon covers it with enough left for touch-ups.
Coverage: how far a gallon actually goes
| Surface | Square feet per gallon, per coat |
|---|---|
| Smooth, previously painted, primed drywall | 400 |
| Average interior wall | 350 |
| Lightly textured (orange peel, knockdown) | 300 |
| Rough, masonry, or unprimed new drywall | 250 |
These match what the manufacturers publish. Behr states a gallon covers 250 to 400 sq ft with one coat; Glidden puts it at “up to 400” on a primed, smooth, non-porous surface. That upper figure assumes a wall that is already sealed and someone rolling it who is not in a hurry. Real walls are rarely that.
Three things quietly eat coverage:
- Texture. Knockdown or orange-peel texture increases the actual surface area you are covering, sometimes by 15 to 20 percent.
- Porosity. New drywall, bare wood, joint compound and masonry all drink the first coat. This is what primer is for — it is cheaper than finish paint and it stops the expensive can disappearing into the wall. Note that primer itself does not go as far: budget 200 to 300 sq ft per gallon rather than 350.
- Application. Spraying uses noticeably more paint than rolling; a roller with too thick a nap holds more paint than it puts on the wall.
How many coats do you actually need?
One coat works only when you are repainting the same colour, with a quality paint, over a sound and previously painted surface. It is the exception, not the plan.
Two coats is the default and what almost every manufacturer specifies for their stated durability and washability. The first coat is doing the covering; the second is what gives you an even sheen. A single coat often looks fine until you see it in raking afternoon light.
Three coats — or two coats over a tinted primer — is for dramatic colour changes: dark to white, or moving to a saturated red or yellow, which are the hardest pigments to build up. Tinted primer is usually cheaper than a third finish coat and gets there faster.
Doors, windows and when to stop deducting
This calculator uses 20 sq ft per door and 15 sq ft per window, which are the same figures the manufacturer calculators use. A standard US interior door is 3 ft × 6 ft 8 in, which works out to exactly 20 sq ft; a common double-hung window is around 3 ft × 5 ft.
These are rules of thumb, and they are good enough for a normal room. Two situations where they are not:
- A wall that is mostly glass. Sliding doors, picture windows and window walls should be measured properly, or you will buy a third more paint than you need.
- Trim, doors and window frames. These are a separate job with a separate product — usually a semi-gloss or satin enamel in a quart rather than a gallon. This calculator covers walls and ceilings only.
Buy the extra gallon now
If the number lands just over a whole gallon, buy the next one up rather than planning a second trip.
Paint is tinted to order from a base, and colour varies very slightly between batches. A can mixed two weeks later, from a different base can, may not blend invisibly on a wall you have already painted. And an unopened can stores for years — it is the simplest insurance against the scuff that appears next spring.
This is an estimate. The coverage figure printed on your specific can is the authoritative number. Use this to know whether you are buying two gallons or five; use the can to fine-tune.