How the Estimate Is Built
Wall area = 2 × (Length + Width) × Height
Paintable area = Wall area − Doors − Windows (+ Ceiling)
Paint = Paintable area × Coats ÷ Coverage per can
The openings are subtracted at standard sizes: a door at 21 sq ft (roughly 3 × 7 ft, or 1.95 m²) and a window at 15 sq ft (3 × 5 ft, or 1.4 m²). If your openings are unusual — a picture window, a set of French doors — count them as two or three ordinary ones.
The result is rounded up to whole cans, because paint is sold that way and because running out mid-wall means a visible lap mark and a second trip.
Coverage: the Number That Matters Most
Manufacturers quote 350 to 400 sq ft per US gallon (11 to 13 m² per litre) for a second coat on a smooth, primed, previously painted wall. That figure is a best case. What you actually get depends on the surface.
| Surface | Coverage per gallon | Per litre |
|---|---|---|
| Smooth, previously painted drywall | 375–400 sq ft | 9–10 m² |
| New primed drywall | 350–375 sq ft | 8.5–9 m² |
| Light texture (orange peel) | 300–350 sq ft | 7–8.5 m² |
| Heavy texture (knockdown, popcorn) | 200–275 sq ft | 5–6.5 m² |
| Bare wood | 200–300 sq ft | 5–7 m² |
| Rough masonry, brick, block | 100–200 sq ft | 2.5–5 m² |
| Smooth ceiling | 350–400 sq ft | 8.5–10 m² |
The first coat on any porous surface always covers less than the second. Textured walls are the classic budget-breaker: knockdown texture can consume nearly twice the paint of a flat wall of identical dimensions.
How Many Coats?
- One coat — only when refreshing the same colour on a sound, clean surface. Even then the result is often patchy in raking light.
- Two coats — the standard, and what a professional quote assumes. Use two for any colour change.
- Three coats — going light over dark, or applying a saturated red, orange or yellow. Those pigments are transparent and genuinely need the extra layer.
- Primer — new drywall, bare wood, stains, patched repairs, glossy surfaces, or a drastic colour change. A coat of tinted primer is cheaper than a third coat of finish paint and blocks bleed-through that paint alone will not.
Rough Guide by Room
Assuming 8 ft walls, two coats, walls only.
| Room | Approximate size | Paint needed |
|---|---|---|
| Small bathroom | 5 × 8 ft | 1 gallon |
| Bedroom | 12 × 12 ft | 2 gallons |
| Large bedroom | 14 × 16 ft | 2–3 gallons |
| Living room | 16 × 20 ft | 3 gallons |
| Hallway and stairwell | — | 2 gallons |
| Whole 1,500 sq ft house | — | 12–16 gallons |
Trim, Doors and Ceilings
Trim is priced separately and is normally a different product — a harder, glossier enamel. A useful rule of thumb: one gallon of trim paint covers about 600 linear feet of standard baseboard or casing at two coats. A single interior door takes roughly a quarter of a quart per side, per coat.
Ceiling paint is a flat, high-hide formula sold in the same coverage range as wall paint. Tick the ceiling option above and the floor area is added to the estimate.
Buying Notes
- Buy the whole job at once and check that the cans share a batch number. Tinted colour varies slightly between mixes; two batches on one wall can show.
- Box your paint — pour all cans into a single large bucket and stir — if you have more than one can of the same colour. It removes any batch difference entirely.
- Five-gallon buckets cost roughly 15–20% less per gallon than five singles, but only if you will use it. Leftover paint has a shelf life of two to three years unopened, considerably less once opened.
- Keep a quart back for touch-ups and label the lid with the room and the date.
Frequently Asked Questions
Should I subtract the doors and windows at all?
For one or two openings the difference is small and the extra paint is useful margin. For a wall of glass it matters a great deal. This tool subtracts them, and rounds up to whole cans afterwards, which keeps a sensible cushion either way.
How much paint for a 12 x 12 room?
With 8 ft walls, one door and two windows, the paintable area is about 333 sq ft. Two coats need roughly 1.8 gallons, so buy two.
Does a dark colour need more paint?
Not more volume per coat, but often more coats. Deep bases carry less white pigment and hide poorly, so a saturated colour over a light wall typically needs three coats or a tinted primer plus two.
Is a US gallon the same as a UK gallon?
No. A US gallon is 3.785 litres; an imperial gallon is 4.546 litres. Paint sold in the UK and Europe comes in litres — switch the units above to work in metric throughout.