Calculating wall paint correctly: formulas and practical tips
Buying too much is expensive, buying too little forces a reorder – often from a different batch
The formula for paint consumption is simple: liters needed = (wall area − cutouts) × coats ÷ coverage. For a room with 40 m² of wall area, two coats and a paint with 10 m²/L coverage, that works out to 8 liters. Sounds simple – but in practice, mistakes creep in when estimating cutouts and when transferring the coverage value from the label to the real surface.
Coverage in practice: The manufacturer value only applies to smooth, non-absorbent surfaces at a wet film thickness of 100–150 µm. On woodchip wallpaper, fresh plaster, aerated concrete walls or drywall, actual coverage can be 20–40% below the lab value. As a rule of thumb: professionals work with 8–10 m²/L for standard emulsion paint on normal interior walls, and 5–7 m²/L for rough textured surfaces. Always plan a small reserve and buy about 10–15% more than calculated.
Doors and windows as cutouts: A standard interior door (0.90 m × 2.10 m) has an area of 1.89 m². A living room window (1.25 m × 1.40 m) accounts for 1.75 m². In a room with two doors and three windows, cutouts quickly add up to 8–10 m² – at 10 m²/L, that's nearly a liter less paint needed. Precise calculation is especially worthwhile with expensive designer paints.
Watch the batch number: Always buy enough paint from a single production batch (batch number on the can). Paint mixes can vary minimally – not visible on the bucket, but clearly visible on the wall side by side. If a can is missing for the ceiling coat three months later and a new batch has to be bought, color differences are possible.