Paint & Flooring
Paint Calculator
This paint calculator tells you how much paint you need for a room or a whole house. Enter each room’s size, wall height, doors and windows, and it works out the paintable area, gallons for your number of coats, ceiling paint and primer, and the cans to buy, with an optional cost.
Result
Enter your measurements and press Calculate. What to buy, the working and the cost will appear here.
How to use the paint calculator
- Enter each room: its length, width and wall height, and the number of doors and windows. Tick Paint the ceiling too if you are doing the ceiling.
- Add rooms to estimate a whole floor or house in one go.
- Set the coats (two is standard) and the coverage printed on your paint can. Leave 350 ft² per gallon if you have not chosen a paint yet.
- Add primer for new drywall, bare wood, stains or a big color change.
- Add prices if you want a cost, then press Calculate to see gallons and the cans to buy.
How much paint do I need? The formula
Paint is worked out from wall area, not floor area. For each room:
wall area = 2 × (length + width) × wall height − doors − windows
gallons = wall area × coats ÷ coverage per gallon
Then add a small allowance and round up to the cans you can buy. By default the calculator counts a door as 20 ft² and a window as 15 ft²; change those figures if yours are larger, such as patio doors or picture windows. Ceilings are simply length × width, and are listed separately because ceiling paint is usually a different product.
Worked example: painting a bedroom
A 14 ft × 12 ft bedroom has 8 ft walls, one door and two windows. The walls and the ceiling each get two coats of paint that covers 350 ft² per gallon, at $38 a gallon, with a 10% allowance:
- Room 1 walls: 2 × (14 ft + 12 ft) × 8 ft = 416 ft² − 1 door and 2 windows (50 ft²) = 366 ft²
- Room 1 ceiling: 14 ft × 12 ft = 168 ft²
- Wall paint: 732 ft² ÷ 350 ft² per gallon × 1.1 = 2.3 gal
- Ceiling paint: 336 ft² ÷ 350 ft² per gallon × 1.1 = 1.06 gal
Wall paint: 2.3 gal (buy 2 × 1-gallon can + 2 × 1-quart can); Ceiling paint: 1.06 gal (buy 1 × 1-gallon can + 1 × 1-quart can); Estimated cost: $142.50. Open this example in the calculator.
Paint coverage per gallon
How much paint 100 ft² of wall needs at different coverage rates. Check your can: most interior paints list 250–400 ft² per gallon.
| Coverage | One coat per 100 ft² | Two coats per 100 ft² |
|---|---|---|
| 250 ft² per gal | 0.4 gal | 0.8 gal |
| 300 ft² per gal | 0.33 gal | 0.67 gal |
| 350 ft² per gal | 0.29 gal | 0.57 gal |
| 400 ft² per gal | 0.25 gal | 0.5 gal |
Coverage drops on rough, porous or textured surfaces such as stucco, brick, fresh drywall and popcorn ceilings; use a lower figure (250–300 ft²) for those.
How many coats of paint?
- Two coats is the norm for a durable, even finish, even with “paint and primer in one” products.
- One coat can work when repainting the same or a very similar color over a sound surface.
- Three coats, or a tinted primer plus two coats, for dramatic changes such as dark to light, or deep reds and yellows that cover poorly.
When do you need primer?
Use a coat of primer on new drywall or plaster, bare or knotty wood, patched areas, water or smoke stains, glossy surfaces and big color changes. Primer usually covers a little less than paint, around 200–300 ft² per gallon. Previously painted walls in good condition generally do not need it.
Buying paint: gallons, quarts and 5-gallon buckets
- Gallons are the standard size for walls. The calculator suggests quarts only for small remainders, because a gallon is usually cheaper than three quarts.
- 5-gallon buckets are cheaper per gallon for whole-house jobs; the calculator suggests one once you need about 3.5 gallons of a color.
- Keep leftovers. A little spare paint of the same batch is ideal for touch-ups later. Write the room and date on the lid.
- Trim and doors are usually painted in a different sheen (semi-gloss); a quart covers the trim of an average room.
Assumptions and limitations
- Rooms are treated as rectangles with walls of one height. For vaulted ceilings or gable walls, add the extra triangle area with the square footage calculator.
- Every door and every window in a calculation uses the same size you enter (20 ft² and 15 ft² by default); for a mix of sizes, use an average.
- Coverage is the manufacturer’s figure for a smooth surface; spraying and textured walls use more.
- Metric cans are assumed to come in 1, 2.5, 5 and 10 liter sizes.
Paint questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and one window, the walls are 2 × (12 + 12) × 8 − 20 − 15 = 349 ft². Two coats at 350 ft² per gallon need about 2 gallons, or 2.2 gallons with a 10% allowance, so buy 2 gallons and a quart. The ceiling adds about 0.8 gallons for two coats.
How many square feet does a gallon of paint cover?
Usually 250–400 ft² per coat, and about 350 ft² on smooth, previously painted walls. The can lists the manufacturer’s figure; rough surfaces cover less.
How much paint do I need for one wall?
Multiply the wall’s width by its height and subtract any doors or windows. A 12 ft × 8 ft wall is 96 ft², so two coats need 192 ft² of coverage: just over half a gallon at 350 ft² per gallon. A single gallon covers it with some to spare.
Do I need to paint the ceiling with the same paint?
Usually not. Ceilings typically use a flat, spatter-resistant ceiling paint, which is why the calculator shows ceiling paint separately.
How much paint does it take to paint a whole house interior?
Add every room to the calculator; that is far more accurate than a rule of thumb. As a rough guide, the walls of an average room need 1.5–2.5 gallons for two coats, so a whole house often runs to 15–25 gallons plus ceilings and trim.
Formulas, unit conversions and default values are explained on our methodology page. Results are estimates; check manufacturer data and local codes before you buy or build.