Paint Calculator
Estimate how much paint you need to cover the walls of one or more rooms.
Results update as you type.
About this calculator
A paint calculator estimates how much paint you need from room dimensions and the number of coats. It works out the wall area as the perimeter times the wall height — 2 × (length + width) × height — optionally adds the ceiling, subtracts an allowance for each door and window, multiplies by the number of coats, adds a waste margin and divides by the paint’s coverage rate to give the litres or gallons to buy. Coverage defaults to about 10 m² per litre (350 sq ft per US gallon) but can be overridden for the specific paint.
Worked example: a 5 × 4 m room with 2.5 m walls has a wall area of 2 × (5 + 4) × 2.5 = 90 m². Subtract one door (about 1.8 m²) and one window (1.5 m²) for 86.7 m² net; two coats over 10 m²/L with 10% waste is 86.7 × 2 ÷ 10 × 1.10 ≈ 19.1 litres.
Add up to three rooms under advanced options — rooms two and three share the wall height — to estimate a whole floor at once, with a per-room breakdown. Decorators use it to buy the right number of tins and avoid a mid-job trip to the shop.
Frequently asked questions
How much paint do I need per square metre?
One litre of wall paint typically covers about 10–12 m² (a US gallon covers roughly 350 sq ft) in a single coat. This calculator assumes 10 m²/litre or 350 sq ft/gallon unless you override the coverage.
How do I estimate paint for a whole house?
Enter each room’s length and width under advanced options (rooms two and three share the wall height). The calculator sums every room’s wall area, subtracts the doors and windows once, then applies coats and waste — the per-room table shows each room’s share.
Should I paint two coats?
Usually yes. Two coats give an even colour and better durability, especially over a new or contrasting surface, which is why the calculator defaults to two and doubles the paint accordingly.
Does the paint calculator include the ceiling and trim?
It covers the walls by default and adds the ceiling if you choose “walls + ceiling” under advanced options. It does not separately estimate trim, skirting or doors as painted surfaces — those use little paint, but add a small tin if you are painting them a different colour.
Do I need to measure each door and window exactly?
No — the calculator subtracts a standard area for each opening, which is close enough for buying paint. Just count how many doors and windows each room has; the small differences are absorbed by the waste allowance.
How much extra paint should I buy for touch-ups?
The default 10% waste allowance covers spills, roller absorption and uneven coverage, and leaves a little for future touch-ups. Keeping the leftover tin, labelled with the room, means an exact colour match later.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/paint/curl
curl "https://calculator.free/api/v1/paint/?units=metric&length=5&width=4&height=2.5&coats=2"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/paint/?" + new URLSearchParams({
"units": "metric",
"length": "5",
"width": "4",
"height": "2.5",
"coats": "2"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.