How much paint do you need? A simple calculation

Paintable area is roughly the floor area multiplied by 3.5 for a standard-height room, or measure walls directly as length times height and subtract doors and windows. Divide that by the coverage on the pack, then multiply by the number of coats and add about 10 per cent for wastage. Most emulsions cover 100 to 140 sq ft per litre per coat on a primed smooth surface, and considerably less on a rough or porous one.

The calculation, step by step

• Measure the area. Wall length times height for each wall. Add ceilings separately if you are painting them.
• Subtract openings. Deduct doors and windows over about 20 sq ft. Smaller ones are roughly offset by the extra cutting-in work.
• Divide by coverage. Take the figure from the pack, not from memory. It varies widely between products.
• Multiply by coats. Two coats is standard. A colour change from dark to light needs three.
• Add 10 per cent. For wastage, touch-ups and the litre you will want next year.
• Add the primer and putty separately. These are what people forget, and they are usually a third of the total spend.

A worked example

A 2BHK of about 900 sq ft carpet area, painting interior walls and ceilings.

• Paintable wall area: 900 × 3.5 = approximately 3,150 sq ft
• Less openings, say 200 sq ft: 2,950 sq ft
• At 120 sq ft per litre per coat: 24.6 litres per coat
• Two coats: 49.2 litres, plus 10 per cent = about 54 litres
• So roughly three 20-litre buckets of emulsion
• Plus putty: about 2 kg per 100 sq ft over two coats on new plaster, so roughly 60 kg
• Plus primer: one coat over the same area, about 25 litres

Coverage is not a fixed number

The figure on the pack assumes a smooth, primed, non-porous surface. Reality reduces it. Fresh plaster absorbs more. A rough or sand-faced finish can cut coverage by 30 per cent. Applying over a much darker colour needs an extra coat. Spray application uses more material than a roller, though it is faster.

Exterior surfaces are almost always thirstier than interior ones, and textured exteriors can halve the stated coverage.

Buy in the right pack size

Twenty-litre buckets are meaningfully cheaper per litre than four-litre packs, so for anything above about 25 litres it is worth buying the larger size even if you end up with some left over. Below that, the smaller pack usually wins because unused paint in an opened bucket rarely survives a Goan monsoon in good condition.

What to send us for a quote

Room dimensions or total area, whether the surface is new plaster or already painted, whether you are changing colour significantly, and whether it is interior or exterior. We will come back with quantities and a rate as confirmed for the requirement.