What the paint calculator does
Enter the room's length, width and wall height, the number of coats, and the coverage printed on your paint can, and the calculator returns the wall area and how many gallons (or litres) you need to buy. It is a straightforward area calculation, but it is where people most often over- or under-buy: a can too few means a second trip and a visible colour-batch line; two cans too many is money on a shelf for years.
The calculation
Wall area = 2 × (length + width) × height
Paintable area = wall area − doors and windows
Paint needed = paintable area × coats ÷ coverage per can
Room 12 ft × 10 ft, 8 ft ceilings, 2 coats, 350 sq ft/gal:
wall area = 2 × (12 + 10) × 8 = 352 sq ft
minus one door (21 sq ft) and two windows (15 sq ft each) = 301 sq ft
paint = 301 × 2 ÷ 350 = 1.72 gal → buy 2 gallonsStandard deductions: an interior door is about 20 sq ft (1.9 m²), a window about 15 sq ft (1.4 m²). Skip the deduction for small windows — the leftover covers touch-ups.
Coverage figures to use
| Paint / surface | Coverage per US gallon | Per litre |
|---|---|---|
| Interior emulsion, smooth primed wall | 350–400 sq ft | 9–11 m² |
| Interior, textured or rough plaster | 250–300 sq ft | 6–8 m² |
| Primer on new drywall | 200–300 sq ft | 5–7 m² |
| Ceiling paint | 350–400 sq ft | 9–11 m² |
| Exterior masonry | 150–250 sq ft | 4–6 m² |
| Gloss / trim enamel | 400–450 sq ft | 10–12 m² |
Manufacturers print coverage on the can and it varies by brand and line; use that figure rather than the defaults when you have it. A US gallon is 3.785 litres; a UK 5-litre tin is 1.32 US gallons.
How many coats?
- Two coats is the normal assumption for a colour change on a previously painted wall, and for most paints to reach their advertised durability and washability.
- One coat can work when repainting in the same or very similar colour with a good-quality paint.
- Three coats (or primer plus two) for dark-to-light changes, strong reds and yellows, new plaster, or patched walls. A tinted primer counts as a coat and is cheaper than a third coat of finish.
- Ceilings usually need one to two coats of flat white; calculate them separately: length × width.
Buying the right amount
- Round up to whole cans and add about 10% for touch-ups — the calculator's figure is the minimum, not the purchase.
- Buy all the paint at once so it comes from the same tinting batch; box (mix) the cans together if colour consistency matters.
- Larger cans are cheaper per litre, but an unopened can is often returnable while a leftover half-can is not.
- Keep a small labelled jar for touch-ups and note the brand, line, colour code and finish on it.
Surface preparation affects coverage
Bare plaster, new drywall and previously unpainted wood absorb paint and reduce coverage by a third or more; a coat of primer or a mist coat (emulsion thinned 10–20%) seals them and restores normal coverage for the finish coats. Filler patches also absorb differently and can show through as dull spots unless spot-primed. If the walls are very rough or you are using a roller with a long nap, choose a coverage figure at the low end of the range.