How Much Paint Do I Need? Coverage, Coats and Tin Sizes
If coverage is quoted per coat, calculate litres as: paintable area in square metres × number of coats ÷ coverage in square metres per litre. Use the figures for the exact paint you intend to buy. A generic “one tin per room” estimate can leave you short or with unnecessary leftovers.
1. Measure what you will actually paint
For each wall, multiply width by height and add the results. Subtract substantial areas you will not paint, such as doors and windows. Calculate the ceiling separately by multiplying its length by its width. Keep separate totals for surfaces that need different products or colours.
Write the measurements down before choosing tin sizes. A room with fitted cupboards and several doors has less exposed wall than an empty room with the same floor area.
2. Check whether coverage is per coat
Look for the coverage rate and the required number of coats on the label or manufacturer's product page. If the rate describes one coat, multiply your area by the coat count. If the pack already states the area covered with the complete recommended number of coats, do not multiply by that number again.
For example, Ronseal states that its 2.5L Anti Condensation Paint covers approximately 8m² with two coats. That is a finished-area figure, so the two coats are already included. Specialist coatings can have very different coverage from ordinary emulsion.
3. Work through a room example
The following is an illustrative calculation, not a coverage promise for a particular paint.
| Step | Calculation | Result |
|---|---|---|
| Wall perimeter | 4 + 3 + 4 + 3 | 14m |
| Total wall area | 14 × 2.4 | 33.6m² |
| Subtract doors and windows | 33.6 − 3.6 | 30m² |
| Allow two coats | 30 × 2 | 60m² of application |
| Example rate: 12m²/L per coat | 60 ÷ 12 | 5 litres |
This example needs 5 litres before allowing for variation in the surface, application losses or touch-ups. Its 12m² ceiling is not included. If you paint the ceiling, calculate it using the ceiling paint's own coverage and coat count.
4. Turn the estimate into a sensible order
Compare the sizes actually available in the same product, colour and finish. A 5L tin and two 2.5L tins contain the same total volume, but their total prices may differ. Check both options and any delivery charge. Choose enough for the job without assuming that the largest tin is always the best purchase.
A calculated minimum is a planning figure. Roughness, absorbency and the change from the existing colour can affect what the job requires. Check the preparation and coat guidance before adding an allowance; do not thin paint beyond the manufacturer's directions to make it go further.
5. Keep primers and topcoats separate
If your surface needs a primer or undercoat, calculate that product independently. Its coverage and coat count can differ from the finish coat. Likewise, do not combine the quantity for skirting boards with wall emulsion: they may require different products.
Compare paint and plan your preparation
- Browse wall and ceiling emulsions.
- Compare Dulux Matt 2.5L and Dulux Matt 5L, checking that the same colour is offered.
- Read damp seal vs anti-condensation paint if you need a specialist wall treatment.
- For damaged woodwork, see which wood filler to choose.
Calculation method checked on 1 November 2026 against Community RePaint's paint calculation guide. The specialist coverage example comes from Ronseal Anti Condensation Paint. Always use the current figures on your chosen product.