Paint calculator guide
How much paint do I need for a 12×12 room?
A 12×12 ft room with 8 ft walls, one door and two windows needs 2 gallons of wall paint for two coats — and one more gallon if you are painting the ceiling too. In the UK that is 7.1 litres for the walls (buy a 5 L tin plus a 2.5 L tin). Here is the maths, so you can check it against your own room instead of trusting a round number.
The quick answer
- Walls, 2 coats (US): 1.90 gallons → buy 2 gallons
- Walls, 2 coats (UK): 7.1 litres → buy 5 L + 2.5 L
- Ceiling, 2 coats: 0.82 gallons → buy 1 gallon of ceiling paint (UK: 3.0 litres → 2.5 L + 1 L)
- Primer (bare plaster or new drywall, 1 coat): 0.95 gallons → buy 1 gallon (UK: 3.5 litres → one 5 L tin)
Run your exact room through the paint calculator — it deducts your doors and windows and works out the exact tins to buy, in gallons or litres.
The maths, step by step
Paint is sold by area, not by room. The wall area is the room's perimeter times its height. A 12×12 ft room has a perimeter of 2 × (12 + 12) = 48 ft, so 8 ft walls give 384 sq ft of wall. Doors and windows are not painted, so subtract them: a standard door is 21 sq ft and a typical window is 15 sq ft, so one door and two windows remove 51 sq ft, leaving 333 sq ft to paint.
Two coats double the area to 666 sq ft. Divide by coverage — this calculator's default is 350 sq ft per gallon, the cautious end of the 350–400 sq ft per gallon most interior paints list (Sherwin-Williams guidance) — and you get 666 ÷ 350 = 1.90 gallons. You cannot buy 0.90 of a gallon, so you buy 2.
The same room in metric: 3.6 × 3.6 m with 2.4 m walls has 34.6 m² of wall. Minus a door (1.9 m²) and two windows (2.4 m²) that is 30.3 m². Two coats = 60.6 m². At the calculator's default of 8.6 m² per litre (the metric equivalent of 350 sq ft per gallon) that is 7.05 litres — buy 5 L + 2.5 L. If your can claims 11 m² per litre, the same room takes 5.5 litres, so 5 L + 1 L is enough. Always check the figure printed on your own can: it beats any article's default.
How much paint for common room sizes
Walls only, 8 ft walls, one door and two windows, at 350 sq ft per gallon. Ceiling paint is a separate column because it is usually a different (flat white) paint.
| Room size | Net wall area | 1 coat (walls) | 2 coats (walls) | Ceiling, 2 coats |
|---|---|---|---|---|
| 10 × 10 ft | 269 sq ft | 1 gallon | 2 gallons | 1 gallon |
| 10 × 12 ft | 301 sq ft | 1 gallon | 2 gallons | 1 gallon |
| 12 × 12 ft | 333 sq ft | 1 gallon | 2 gallons | 1 gallon |
| 12 × 14 ft | 365 sq ft | 1 gal + 1 qt | 2 gal + 1 qt | 1 gallon |
| 14 × 16 ft | 429 sq ft | 1 gal + 1 qt | 2 gal + 2 qt | 1 gal + 1 qt |
| 15 × 20 ft | 509 sq ft | 1 gal + 2 qt | 3 gallons | 2 gallons |
Taller walls change the answer: the same 12×12 room with 9 ft walls has 381 sq ft of net wall, so two coats take 2.18 gallons — buy 2 gallons plus a quart. If your room is not in the table, the paint calculator does the same maths with your exact dimensions.
When one coat is enough — and when it is not
- One coat can work for a same-colour refresh over clean, previously painted walls with a good-quality paint. The table above shows a 12×12 room needs just 1 gallon for one coat.
- Two coats is the default for a colour change, over patches or filler, over a stain, or on bare plaster. One coat over any of those almost always shows through in daylight — and a second trip to the shop costs more than the extra gallon.
- Three coats is rare for walls, but normal when going from very dark to very white, or over unprimed new plaster that drinks the first coat. Prime first instead: one coat of primer plus two coats of paint beats three coats of paint on bare plaster.
Do not forget the primer and the waste allowance
Primer is a separate purchase, and it follows the same maths for one coat: a 12×12 room takes about 1 gallon (3.5 litres in the UK). You only need it on bare plaster, new drywall, patched areas or a big colour change — skip it on sound, previously painted walls.
Add roughly 10% for what stays in the roller tray and on the roller, plus spills and touch-ups. For the 12×12 room that lifts 1.90 gallons to 2.09 — still 2 gallons plus a quart if you want the safety margin, or just 2 gallons if you are happy to touch up from the same tin later. Keep the leftover: a scuff repaired with the exact same paint is a five-minute job, while a colour-matched repaint is a weekend.
Painting a rental between tenants? The order of work matters as much as the quantity — see repainting a rental between tenancies for the full void-period checklist, including why ceilings and trim belong in the calculation from the start.
Frequently asked questions
How many litres of paint do I need for a 12×12 room?
For a 3.6 × 3.6 m room with 2.4 m walls, two coats of wall paint take about 7.1 litres at the calculator's default coverage of 8.6 m² per litre — buy a 5 L tin plus a 2.5 L tin. If your can claims 11 m² per litre, 5.5 litres is enough, so 5 L + 1 L will do. The ceiling adds about 3.0 litres for two coats.
Is 2 gallons enough for a 12×12 room?
Yes, for the walls in two coats — the maths comes to 1.90 gallons with one door and two windows deducted. It is not enough if you also want the ceiling painted (add 1 gallon of ceiling paint) or if the walls are bare plaster (add 1 gallon of primer first).
How much does one gallon of paint cover?
Most interior wall paints list 350–400 sq ft per gallon per coat on smooth, primed walls — about 8.6–9.8 m² per litre. This calculator uses the cautious 350 sq ft (8.6 m²/L) figure by default. Rough, textured or unprimed surfaces cover less; always check the coverage printed on your can.
Should I buy extra paint for touch-ups?
Yes — a quart (or a litre in the UK) over the calculated amount is cheap insurance. Paint from a different batch can differ slightly in shade, so touch-ups from the original tin always match. Write the brand, colour name and code on the tin before you store it.
Coverage figures are estimates based on the 350 sq ft per gallon default — always confirm coverage on the can. Read how the calculator's default assumptions were chosen on the About page.