Paint Calculator
Enter the room size, the doors and windows, and the number of coats, and get the paint to buy in gallons or litres — plus the can combination that covers it with the least left over. Coverage defaults to the conservative end of the 350–400 ft² per gallon most labels quote, and it is yours to change.
Paint to buy
2.29 gallons
Buy 2 gallons + 2 quarts · 365 ft² × 2 coats
| Wall area (gross) | 416 ft² |
| Doors and windows deducted | 51 ft² |
| Ceiling area | — |
| Paintable area | 365 ft² |
| Paint per coat | 1.04 gal |
| Total incl. 10% extra | 2.29 gal |
| Cans to buy | 2 gallons + 2 quarts (2.50 gal) |
| Paint cost | — |
Can plan uses 5-gallon buckets, gallons and quarts; three quarts round up to a gallon, which is usually cheaper. Primer covers less than finish paint — size it separately.
Why paint estimates come up short
Paint runs out for the same three reasons on almost every job. The coverage figure on the can is a best case — a smooth, sealed, primed wall rolled by someone who knows what they are doing. The second coat gets forgotten in the arithmetic. And the room is measured as floor area, when what gets painted is the perimeter times the height.
This calculator works from walls, not floors: perimeter × height, less the doors and windows, plus the ceiling if you are doing it. It multiplies by the coats you actually intend, divides by a coverage figure you can see and change, and then turns the answer into cans you can buy — because nobody buys 2.29 gallons.
How to use the paint calculator
- Measure the room. Length and width of the floor, and the wall height from floor to ceiling. For an irregular room, measure each wall and add the lengths — the perimeter is what matters for walls.
- Choose the surfaces. Walls only, walls and ceiling, or ceiling only. Ceilings usually take a different, flatter paint, so many people run the calculator twice.
- Count doors and windows. Each standard door is deducted at 21 ft² and each window at 15 ft². Leave large openings such as patio doors out of the count and subtract their area by reducing the wall instead.
- Set coats and coverage. Two coats is the normal finish, and three for a strong colour change. Take coverage from the paint label; 350 ft² per gallon is a safe default for smooth, primed walls.
- Keep some extra. Ten per cent covers rollers, trays and the inevitable touch-up. The leftover from the same batch is worth more than any later can, because it will match.
- Buy the can plan. The result lists the cans that cover the job with the least left over. Mix ("box") them together in a bucket before you start, so every wall is the same shade.
Key formulas
- Wall area: 2 × (length + width) × height − doors × 21 ft² − windows × 15 ft²
- Ceiling area: length × width
- Paint: area × coats ÷ coverage × (1 + extra%)
- Metric coverage: 1 ft² per gallon = 0.0245 m² per litre, so 350 ft²/gal ≈ 8.6 m²/L
Worked example
A 14 × 12 ft bedroom with 8 ft walls, one door and two windows, walls only, two coats at 350 ft² per gallon with 10% extra. Gross wall area = 2 × 26 × 8 = 416 ft²; less 21 + 30 for openings = 365 ft². Paint = 365 × 2 ÷ 350 × 1.10 = 2.29 gallons, so the can plan is 2 gallons + 2 quarts. In metric, the same room comes to about 8.4 litres — a 5 L, a 2.5 L and a 1 L tin.
Coverage by surface and finish
| Surface or finish | Coverage per gallon | Per litre |
|---|---|---|
| Flat or eggshell on primed drywall | ≈ 400 ft² | ≈ 9.8 m² |
| Satin or semi-gloss | 350–400 ft² | 8.6–9.8 m² |
| High gloss | 300–350 ft² | 7.4–8.6 m² |
| Textured, porous or bare plaster | 250–300 ft² | 6.1–7.4 m² |
| Primer | 200–300 ft² | 4.9–7.4 m² |
Typical one-coat figures as quoted on product labels; the label on your own can always wins. Dark colours over light, and light over dark, often need an extra coat rather than a different coverage figure.
Things to keep in mind
- Box your paint. Pour every can into one large bucket and mix before starting, so a slight batch difference cannot show up halfway along a wall.
- Trim is a separate job. Skirting, door casings and window frames take a different finish and far less paint — a quart covers the trim in most rooms.
- Ceilings eat paint. Rolling overhead loads less paint per pass and spatters more; if the ceiling is textured, use the low end of the coverage range.
- Keep the leftover. Label it with the room and date. A matched touch-up next year is worth far more than the shelf space.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and two windows, a 12 × 12 room has about 333 ft² of paintable wall. Two coats at 350 ft² per gallon is about 1.9 gallons — 2.1 with a 10% allowance — so the can plan is two gallons and a quart. Including the 144 ft² ceiling adds about another 0.8 gallon for two coats, taking the room to roughly three gallons.
How many square feet does a gallon of paint cover?
Most interior wall paints quote 350 to 400 ft² per gallon for one coat on a smooth, primed surface. Gloss finishes cover a little less, and porous or textured surfaces — new drywall, bare plaster, popcorn ceilings — can drop to 250 to 300. The label on the can is the number to use; this calculator defaults to 350 because estimates that assume the best case are the ones that run short.
Do I need two coats of paint?
Usually, yes. A second coat evens out the sheen, hides roller marks and gives the colour its intended depth. One coat can be enough when repainting in the same colour over a sound finish, and a dramatic change — dark to light especially — can need three, or a tinted primer first.
Should I subtract doors and windows from the wall area?
Yes for estimating, and that is what this tool does: a standard door is around 21 ft² and a window around 15 ft². The deduction is small in a typical room, but in a room with a wall of glass it is the difference between buying the right amount and a can too many. Trim and door faces are painted separately and usually in a different finish.
How much primer do I need?
Primer covers less than finish paint — often 200 to 300 ft² per gallon — because it is formulated to soak into and seal the surface. Run the calculator for one coat with the primer’s own coverage figure. New drywall, patched areas, stains and big colour changes are the cases where primer is worth it.
Is it cheaper to buy a 5-gallon bucket?
Per gallon, almost always. Once a job needs four gallons or more, a 5-gallon bucket is usually cheaper than four single cans and guarantees a single batch. The can plan here switches to a bucket as soon as five gallons are needed, and rounds three quarts up to a gallon because the gallon generally costs less than three quarts.
Last updated: 25 October 2026