How this works
Estimating paint is mostly a question of net wall area. You take the perimeter of the room, multiply by the ceiling height to get the gross wall area, then subtract the doors and windows that won't be painted. Multiply that by the number of coats (almost always two for a quality finish — one for primer-style coverage, one for the colour) and divide by the spreading rate of the paint to get litres or gallons.
The spreading rate is the number that varies most between products. A good interior emulsion covers about 10-12 m² per litre on a smooth, previously-painted wall (≈ 400 ft² per US gallon). On bare plaster, drywall, or a strong colour change, that drops to 7-8 m² per litre because the surface drinks paint and you need a heavier first coat. Textured surfaces (woodchip, masonry, brick) can be as low as 5 m² per litre. The number printed on the tin is the manufacturer's lab figure — apply a 10-15% margin in the real world for cutting in, brushwork, drips, and tray waste, which is what this calculator's default rate already accounts for.
A few practical points. (1) Always buy slightly more than the calculator says — running short halfway through means a second batch with a slightly different shade (paint mixers aren't perfect across batches), and the join will be visible in raking light. Round up to the next can. (2) Two thinner coats look much better than one thick one — the second coat is what actually gives you the even, opaque finish. Don't try to save a coat by laying it on heavy. (3) The same area takes very different amounts of paint depending on colour: going from a dark wall to white can take three or four coats; white to white is often just one. The calculator assumes a "normal" colour change and 2 coats — bump coats to 3 if you're going dark-to-light or covering strong stains.
The formula
L, W, H are the room length, width and ceiling height. Doors and windows are the total area (in m² or ft²) to subtract — the calculator above uses standard sizes (1.8 m² per door, 1.4 m² per window) and lets you adjust the count. The spreading rate defaults to 10 m²/L (≈ 400 ft²/gal) for an interior emulsion on a smooth, previously-painted wall — drop it for textured or thirsty surfaces. Can sizes default to 5 L (Europe) and 1 US gallon (3.785 L); both bag-block counts are shown.
Example calculation
- Bedroom: 4 m × 3 m × 2.5 m ceiling, 1 door and 2 windows, 2 coats.
- Wall area = 2 × (4 + 3) × 2.5 = 35 m². Subtract 1 × 1.8 + 2 × 1.4 = 4.6 m² → 30.4 m² net.
- Total to cover = 30.4 × 2 coats = 60.8 m². At 10 m²/L → 6.1 litres of paint needed.
- Round up: 2 × 5 L cans (10 L) — leaves a comfortable margin for touch-ups.
Frequently asked questions
How many coats do I really need?
Two for almost every job. The first coat seals and provides base coverage; the second is what gives you the even, opaque finish — tiny variations in roller pressure on the first coat get cancelled out by the second. You can drop to one coat only if you're refreshing wall in the same colour over a clean, undamaged previous coat (e.g. recoating a rental between tenants in identical paint). Bump to three coats if you're going from dark to light, covering strong stains, or painting over patch repairs that were spot-primed — those areas absorb paint differently and will "telegraph" through two coats. Don't try to substitute one thick coat for two thin ones; thick coats sag, dry unevenly, and the surface tension creates micro-orange-peel that looks worse close up.
Should I include the ceiling?
Only if you're painting it — and almost always with a separate tin. Ceiling paint is a different formulation (flatter, drips less, designed to be roller-applied overhead) and it's usually white, while walls are usually a colour. If you are painting both in the same paint, add the ceiling area (length × width) to the wall total before dividing by the spreading rate. The calculator above is wall-only by default, which is the common case. One quirk: if you have a vaulted or sloped ceiling, the surface area is larger than the floor area beneath it — measure the actual slope length, not the floor footprint, or you'll under-buy.
What does "spreading rate" mean and what should I use?
Spreading rate is how much surface area you can cover per unit volume of paint — typically expressed as m² per litre or ft² per gallon. The number on the tin is the manufacturer's figure for ideal conditions: a smooth, primed surface, applied at the recommended thickness. Real-world results are 10-20% lower because of cutting in by brush (which uses more paint than rolling), drips, what stays in the tray, and the imperfect surface you're actually painting. The calculator defaults to 10 m²/L (≈ 400 ft²/gal), which is a realistic figure for an interior emulsion on a previously-painted smooth wall. Drop it to 7-8 m²/L for bare drywall, fresh plaster, or a strong colour change. Drop it to 4-6 m²/L for textured surfaces (woodchip, masonry, exterior stone) or thirsty wood. Specialist paints have very different rates — check the tin and override the default.
How much extra should I buy?
Round up to the next whole can — and prefer the larger can size if you're close to a threshold. Two reasons. (1) Touch-ups: paint settles into walls over months and you'll want some to fix scuffs, picture-hook holes, or kid handprints in the same batch — colour matching to a new tin is never quite perfect because tinting machines vary slightly between batches and the colour also shifts a touch as it ages on the wall. Keeping a sealed half-litre or quart in the cupboard is genuinely useful for years. (2) Running short: if you exhaust the paint mid-wall, the second batch will be a slightly different shade and the join will show in raking light; you also lose 30+ minutes nipping back to the shop. Both costs are far worse than buying one extra can. A practical rule: aim for 10-20% more than the calculator says, rounded up to the next stocked size.