Paint Calculator
Several rooms, separate wall and ceiling paint, primer, optimal tins and cost.
| Product | Selected container mix | Paint purchased | Paint cost |
|---|---|---|---|
| Wall paint | 1 × 10 l | 10 l | — |
How it works
Wall and ceiling areas are summed across every included room and measured zone. Each product then uses its own coverage, coats and allowance; primer is calculated over the complete prepared surface.
Container sizes are optimized separately because wall paint, ceiling paint and primer are different products. When all entered sizes have prices, the lowest-cost sufficient mix wins. Otherwise the calculator minimizes purchased volume and then container count.
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How the Paint Calculator works
This paint calculator combines up to three rooms and additional measured zones, calculates wall paint, ceiling paint and primer separately, then selects a purchasable container mix and complete material cost.
Coverage is the variable that moves
The area is straightforward: perimeter times height, minus windows and doors. All the uncertainty sits in the coverage rate. For emulsion on a prepared surface, 10 m² per litre per coat is typical — but that figure halves easily.
Bare plaster and plasterboard absorb far more than a primed wall. Textured paper increases the actual surface area. A dark base under a light colour needs an extra coat. The number on the tin is an optimistic laboratory figure over an ideal substrate, and should be treated as such.
How many coats
Two coats is the realistic minimum for even colour. One coat almost always shows through, particularly where roller passes meet. Three are needed when covering dark with light, or with strongly pigmented colours that cover poorly on their own.
The first coat on a new surface always uses more than later ones, because the substrate absorbs it. Priming pays for itself: it costs less than paint and noticeably reduces how much of it you use.
Walls, ceilings and primer can use different products
Wall and ceiling coatings may have different coverage, sheen, coat count and package sizes. Each room therefore includes its ceiling independently, after which ceiling area can use either the wall product or a separate product. Primer covers the complete prepared surface as its own material line.
All rooms are combined before packages are rounded. Paint left from one room can therefore serve the next instead of every room being rounded to a fresh tin. Accent walls, halls and irregular ceilings can be added as already measured net areas.
How the container mix is selected
Each product accepts three available container sizes. When every entered size has a price, the calculator selects the lowest-cost sufficient combination. If any price is missing, it minimizes purchased volume first and container count second.
Wall paint, ceiling paint and primer are optimized separately because their leftovers are not interchangeable. The purchase table shows the selected mix, purchased volume, surplus and product cost.
FAQ
How much paint does a 20 m² room need? expand_more
At 2.7 m high, a 5 × 4 m room has 18 m of perimeter and almost 49 m² of wall, or about 44 m² after openings. Two coats at 10 m² per litre is roughly 9 litres. Add about 4 more if you are painting the ceiling too.
Should I subtract windows and doors? expand_more
Yes — in an ordinary room that is 4–6 m², more than a litre of paint. Subtracting skirting and architrave is not worth the effort: the difference sits well inside the margin of error on coverage.
What should I do with leftover paint? expand_more
Keep about a litre for touch-ups in a tightly sealed tin. Write the colour name and tint code on it: matching a shade again from a sample is nearly impossible, but any branch will reproduce it exactly from the code.
How do I calculate several rooms using one paint? expand_more
Add the second and third rooms to the same project. Their areas are combined before containers are selected, which usually creates less surplus than rounding three independent purchases.
How is primer included? expand_more
Enable primer and enter its own coverage, coats, package sizes and prices. It covers the selected wall and ceiling area but keeps a separate container plan and cost from the finish paint.