Identify the surface and follow the current instructions for your exact products. Do not sand or strip uncertain old coatings. Read the safety guide before disturbing materials.
Measure each surface separately
For each rectangular wall, multiply its width by its height to find its area in square metres, then add the walls together. Measure the ceiling separately. Deduct substantial unpainted openings if useful, but remember window reveals and recesses may add paintable area.
Doors, skirtings and cabinets use different finishes and should have their own estimate. A room’s floor area is not the area of its walls.
Use the tin’s coverage, not a universal number
Divide the total paintable area by the product’s stated coverage per litre, then allow for the number of coats specified. Porous or textured surfaces, strong colour changes and practical losses may require more. Primers and finishing paints need separate calculations.
For example, 40 square metres at a stated 10 square metres per litre would require about 4 litres for one coat under those conditions. Two coats would be about 8 litres before any allowance. This is an illustration, not a coverage claim for your paint.
Plan drying as well as working
Allow separate time for cleaning, repairs, drying, preparation, priming and each coat. The quoted recoat time usually assumes particular conditions. Low temperatures and high humidity can extend it.
Schedule the room around how you use it. Kitchens, bathrooms and doors may need longer before normal service than before they feel dry. Buy enough compatible material from a consistent batch where possible and keep the labels.
Common mistakes
- Budgeting for one coat when the system specifies more.
- Forgetting ceilings, reveals and primer.
- Rushing a recoat to fit a planned finish time.
If the surface is too damaged or uncertain to measure a straightforward scope, resolve the preparation and repair needs before ordering all the materials.
Sources & further reading
Original general guidance from Aethos, informed by the resources below. These references cover relevant safety, materials or techniques, not a specification for your property. Product examples are not paid recommendations.
- Farrow & Ball: paint finishes (opens in a new tab)
Different products and sheen levels for different surfaces. Check the current product advice sheet.
- Zinsser: AllCoat Exterior Satin (opens in a new tab)
A product example covering exterior substrates, preparation and application conditions, not a universal recommendation.
- This Old House: how to paint a room (opens in a new tab)
Preparation and room painting technique. US products and terminology do not always translate directly to UK specifications.