Paint Calculator
Buying paint by guesswork means either a second trip to the store mid-job or half-used cans in the garage for years. This paint calculator works out the exact amount from your wall area, openings and number of coats.
Do not know your wall area? Calculate it first with the Wall Area Calculator, then paste the result here.
Enter valid values to see the results.
Formula
Standard deduction: 1.9 m² (20 ft²) per door and 1.4 m² (15 ft²) per window. Coverage: standard emulsion ≈ 10 m²/L (350 ft²/gal), premium ≈ 12 m²/L (400 ft²/gal), primer on porous surfaces ≈ 8 m²/L (300 ft²/gal).
How to measure
- Measure each wall: width × height, then add them together — or use the Wall Area Calculator.
- Count doors and windows; the calculator deducts standard opening sizes automatically.
- Check the coverage rate printed on your paint can and pick the closest option.
Worked example
Practical tips
- Always paint a test patch — colour looks different on your wall than on the swatch, especially in evening light.
- New plaster or drywall needs a primer coat first, or it will drink twice the paint.
- Buy all cans from the same batch number; slight tint differences between batches show on large walls.
- One generous coat rarely beats two normal coats — thin, even layers cover better and drip less.
Waste allowance
Coverage rates already include normal roller losses. Add 10% only for heavily textured surfaces (rough render, brick) which hold more paint.
Typical paint coverage rates
| Paint type | Metric | Imperial |
|---|---|---|
| Primer / sealer | 8–10 m²/L | 300–350 ft²/gal |
| Standard wall emulsion | 10 m²/L | 350 ft²/gal |
| Premium one-coat | 12 m²/L | 400 ft²/gal |
| Ceiling paint | 9–10 m²/L | 325–350 ft²/gal |
| Wood / metal enamel | 11–12 m²/L | 400 ft²/gal |
Common mistakes
- 1 Forgetting the second coat — most walls need two, and dark-to-light colour changes need three.
- 2 Not deducting openings on rooms with many windows, which overstates paint by 10–15%.
- 3 Using wall-paint coverage for bare plaster or brick; porous surfaces need primer and more paint.
- 4 Rounding down when buying: running out mid-wall leaves a visible seam even with the same colour.
Frequently asked questions
How much paint do I need for a 12×12 ft room?
With 8 ft ceilings the walls are about 384 ft²; minus a door and two windows ≈ 334 ft². Two coats at 350 ft²/gal need about 1.9 gallons — buy 2 gallons.
How many litres of paint per square metre?
About 0.1 L per m² per coat for standard emulsion (10 m²/L). Two coats: 0.2 L/m². A 40 m² room needs roughly 8 litres.
Do I really need two coats?
Almost always yes. The first coat seals and evens the surface; the second gives the true colour and durability. Only premium one-coat paints over similar colours can skip it.
Should I deduct windows and doors?
Yes for accuracy, but many painters keep the extra as a safety margin and for touch-ups. This calculator deducts them and you can add a coat margin instead.
How much does it cost to paint a room?
Paint for an average room (35–40 m² of walls, two coats) is 7–8 litres. At $8–15 per litre that is $60–120 for materials, plus primer if the walls are new.