How to work out how much paint you need

The question, as one formula

How much paint to buy depends on four things: the actual surface to be painted, the coverage rate of the paint you pick (how many m² a litre covers), the number of coats needed, and a safety margin. The calculator applies this chain in order, without ever asking you for a technical value you're not expected to know.

Step 1 — Actual surface, not gross surface

We start from the gross wall surface (width × height), then subtract the openings: doors, windows, anything that won't be painted. A standard door alone is roughly 1.8-2 m² — on a small room, missing it can throw the estimate off by 10-15%.

If your openings add up to more than the gross surface you entered, the calculation is blocked rather than returning a negative or zero result: that's a sign a value was mistyped somewhere, not a real-world case to handle.

Step 2 — Wall type, and what the calculator doesn't measure

Two walls of the same size can absorb very different amounts of paint. A fresh, porous, or freshly rendered surface soaks up more than an old, sound, well-adhered coat of paint — the first coat simply doesn't spread as far. In practice, the wall's condition also affects whether you need a primer (new, very absorbent, patchy or repaired surfaces). The calculator doesn't assess condition: since no published, numeric rule applies across all paints, it recommends a primer every time and leaves the product's data sheet to refine that advice.

The wall type, on the other hand, is asked for every calculation: it narrows the list to paints that suit that surface. If yours isn't listed, the “Other” option still lets you run the calculation, with a note that the estimate may be less precise.

Step 3 — Coverage and number of coats: why they're automatic

Coverage rate (m²/L) and number of finish coats aren't fields to fill in. Coverage comes from the paint you pick; coats follow a precautionary rule of two finish coats, whatever the surface. For a one-coat paint, the result also shows the quantity for a single coat, to be relied on only if the product's data sheet promises it for your wall. Making these values editable would produce results cut off from the paint you actually selected.

The coverage rates in our database are category averages, not product-by-product figures, reviewed by the site's publisher. Their origin and the reference ranges published by manufacturers are set out in our methodology.

Step 4 — From consumption to final quantity

Once coverage is known, consumption per coat follows directly: it's the inverse of coverage (1 ÷ coverage rate). Multiplied by the actual surface and the number of coats, it gives the quantity before margin. The safety margin is applied last, to that total — never to the coverage rate or the number of coats themselves, which stay technical values, not adjustment knobs.

Illustrative example (sample values)

To make this concrete, using example figures — not a specific product's real numbers: a 20 m² net wall, a sample coverage rate of 10 m²/L, and 2 coats give a consumption before margin of 20 × (1/10) × 2 = 4 L. With a 10% margin, the final quantity would be 4.4 L, rounded up to the available tin sizes (here, a single 5 L tin covers the need).

What this estimate doesn't replace

The calculator gives you a reference quantity, not a guarantee. A tin's real-world coverage also depends on application technique (roller, sprayer, brush), dilution, surface texture, and the person doing the painting. For a specific product you've already chosen, the manufacturer's data sheet remains the most reliable source.

Using the calculator

The method above is exactly what the tool applies: no extra steps, no technical knowledge needed beforehand.

Open the calculator