Paint calculator: how much paint you really need
This paint calculator works out the litres or gallons of paint for a room: wall sizes, minus doors and windows, times the coats, divided by the coverage on the tin. It then rounds up to whole cans, adds a margin for touch-ups and prices the cans you buy. Switch the material to wall putty to get kilograms and bags instead of litres.
The usual question is “will two 4-litre cans be enough?” The answer depends on three things people skip: the openings (a door takes nearly 2 m² off a wall), the number of coats, and the real coverage on your surface. Fresh plaster and textured walls drink more paint, so coverage matters as much as area.
Everything is calculated in your browser as you type. Switch between metric and imperial (feet, US gallons) at any time.
How to use the Paint Calculator
- Pick metric or imperial. Enter each wall size as width × height × how many walls of that size; “Add wall size” covers different heights.
- Enter the doors and windows and their sizes. The defaults (0.9 × 2.1 m door, 1.2 × 1.2 m window) suit most homes.
- Tick “Paint the ceiling too” and give its length and width if the ceiling gets the same paint.
- Set the coats (two is normal), the coverage from the tin, the can size and an extra percentage for touch-ups.
- Add the price and, for paint, an optional primer coat. For wall putty, choose Wall putty (kg), enter the kg per m² per coat from the bag’s data sheet and pick the bag size. Read the quantity, packs and cost, then copy or share the summary.
Paint Calculator features
Any number of wall sizes
Group walls by size; the calculator multiplies each group and totals them.
Wall putty in kg
Area × coats × kg per m² per coat, plus wastage, rounded up to 5, 20 or 40 kg bags.
Doors and windows subtracted
Openings are removed before the coats are applied, for paint and putty alike.
Coats, coverage and cans
Litres are converted into whole cans of the size you choose (1, 4, 10, 20 L or quart, gallon, 5-gallon) with a touch-up margin.
Primer and ceiling options
Add one primer coat with its own coverage, and include the ceiling when it gets the same finish.
Cost in your currency
Price per litre, gallon or kg becomes a total for the packs you actually buy.
How the paint quantity is calculated
The method painters use on site:
- Wall area = Σ (width × height × number of walls). Four walls of 3 m × 2.7 m give 32.4 m².
- Net area = wall area − doors − windows (+ ceiling if ticked). One standard door (0.9 × 2.1 m = 1.89 m²) leaves 30.51 m².
- Paint needed = net area × coats ÷ coverage. Two coats at 10 m²/L: 30.51 × 2 ÷ 10 = 6.1 L.
- Paint to buy = paint needed × (1 + extra %), rounded up to whole cans. With 10% extra, 6.71 L → two 4 L cans (8 L).
- Cost = litres in the cans you buy × price per litre.
Typical coverage figures
Coverage is printed on every tin and varies with the surface. Use the tin’s figure when you have it; these are typical first-coat values on a smooth, painted wall. Expect 20–30% less on new plaster, putty or rough render.
| Paint | Coverage per coat | Notes |
|---|---|---|
| Interior emulsion (acrylic) | 10–12 m²/L · 350–400 ft²/gal | Most room paints |
| Distemper | 6–8 m²/L | Needs more coats |
| Exterior emulsion | 8–10 m²/L | Less on rough walls |
| Primer / undercoat | 10–14 m²/L | One coat on sound walls |
| Enamel / gloss (wood and metal) | 12–16 m²/L | Thin coats |
| Textured finish | 2–4 m²/L | Check the tin |
Wall putty: how many kilograms?
Putty needed = net area × coats × kg per m² per coat, then add wastage. Take 39.51 m² of wall and ceiling, 2 coats and 0.6 kg/m² a coat: 39.51 × 2 × 0.6 = 47.4 kg, or 52.2 kg with 10% extra, so three 20 kg bags. The doors and windows are deducted exactly as for paint.
The 0.6 kg/m² a coat in the calculator is only an example, equal to about 1.2 kg/m² for two coats. Coverage varies by product, base wall and how thin the coat is applied, and it is higher on rough or uneven plaster. Use the figure on your bag’s technical data sheet, and see the plaster calculator for cement plaster and render. Putty mode is metric only.
Tips before you buy
- Buy the bigger can when the total is close to a size boundary: 10 L costs much less per litre than three 4 L cans.
- Dark to light usually needs three coats or a tinted primer; the coats field handles either.
- Ceilings often get a different, flatter paint, so run the calculator twice when the products differ.
- For odd shapes use the area calculator first, and keep the extra at 5–10%.
Paint Calculator FAQ
How much paint do I need for a 10 × 12 ft room?
With 9 ft ceilings the walls are 2 × (10 + 12) × 9 = 396 ft². Subtract a door and a window (about 35 ft²) for 361 ft². Two coats at 350 ft²/gal need just over 2 gallons.
How many litres of paint per square metre?
Roughly 0.1 L per m² per coat for an ordinary emulsion (10 m²/L). A 12 m² wall takes about 1.2 L per coat, 2.4 L for two coats. Rough or new surfaces need more.
Should I subtract doors and windows?
Yes. A door removes about 1.9 m² (20 ft²) and a window around 1.4 m² (15 ft²).
Does the calculator include primer?
Tick “Add a primer coat” and enter its coverage. It is one coat over the same area, shown separately.
How do I calculate wall putty?
Net wall area × coats × kg per m² per coat, plus about 10% wastage. Choose Wall putty (kg), enter the rate from the bag and the tool rounds up to whole bags.
Is the cost estimate accurate?
It multiplies the quantity in the packs you buy by the price you enter. It excludes labour, brushes, tax and any primer or putty you have not added, so it is a materials estimate, not a quote.
Can I use it for exterior walls?
Yes — enter the outside wall sizes and the exterior paint’s coverage (usually 8–10 m²/L), and allow extra for rough render.