Estimate paint by calculating gross wall area, subtracting only openings you will not paint, multiplying by the number of coats, and dividing by the labelled coverage for the selected product. A 45 m² wall area minus 5 m² of openings, with 2 coats at 10 m²/L, requires 8.0 L before a practical allowance for touch-ups or uneven absorption.
Worked example with numbers
A 5 m by 4 m room at 2.5 m high has 45 m² of gross wall area. Suppose one door is 0.9 × 2.0 m and two windows are each 1.2 × 1.0 m: openings total 4.2 m², leaving 40.8 m². Two coats make 81.6 m² of coating area. At 10 m²/L, 81.6 ÷ 10 = 8.16 L, so choose containers using the product label and the project’s allowance rule.
How to make the calculation
List every wall, opening, and surface that receives a different colour or coating. Do not deduct an opening if its trim, reveal, or door will be painted with the same material. Enter coverage exactly as the tin states and keep primer separate from finish paint. The paint calculator supplies the perimeter-height and coats calculation; it does not know which architectural details are included.
Check the units and assumptions
Coverage is the finished surface area one litre or gallon can coat under the product’s stated conditions. Two coats double the coating area even though the wall dimensions do not change. For example, 40.8 m² at 10 m²/L is 4.08 L for one coat and 8.16 L for two. A measurement table is more reliable than estimating all openings by eye.
Common mistakes
Do not divide gross wall area by coverage and forget the second coat. Do not deduct a window twice because its recess will still need paint. Do not treat one brand’s coverage figure as universal. A frequent shortage comes from counting only broad walls while omitting a ceiling, repaired patches, a colour change, or a separate primer coat.
When this result does not apply
The result is a material estimate, not a guarantee of one exact quantity. Porosity, texture, colour contrast, application method, thinning instructions, waste, and the manufacturer’s specified film build change actual use. For exterior coatings or safety-sensitive surfaces, follow the selected product’s data sheet and preparation instructions.
Source for the external fact
Benjamin Moore states that product coverage varies with the surface and lists 300–400 square feet per gallon for a named primer in its technical product information. The named range is why the label, rather than a generic rule, belongs in the calculation.
Related calculators
Check gross wall dimensions with the room-area page, calculate litres with the wall-paint estimator, and compare ceiling dimensions in the room-lighting calculator before assuming one surface total serves every task.