Paint quantity: surface area and coats before the number of tins
A room’s floor area is not its wall area, and litres depend on coverage and coat count. Start with the surfaces being painted rather than the room’s size alone.
Measure the surface that will receive paint
A rectangular wall’s area is length ×height. Add the intended surfaces and subtract openings or portions that will not be painted. Include the ceiling only when it is part of the job.
For a rectangular room 4 ×3 metres and 2.8 metres high, wall area before openings is 2 ×(4 +3) ×2.8 =39.2 m², while floor area is only 12 m².
Get coverage and coats from the chosen product
Base litres =net area ×coat count ÷coverage per litre for one coat. Dulux guidance separates surface dimensions, required coats and the coverage stated on the container.
Products and surfaces vary; a default number is not a universal paint standard. If a product sheet gives coverage for a complete coating system, do not multiply by the coats again without checking its meaning.
References: Dulux — How to calculate paint requirements
The estimate and the purchase are different totals
A reasoned extra allowance can represent surface or application variation, but it is an input assumption, not a guaranteed percentage. Check the product and surface instead of treating an allowance as universal advice.
Whole-container count rounds up after dividing required litres by container size. Purchase quantity can therefore exceed calculated need; comparing container sizes does not require changing the measured area.
Worked example: from wall area to containers
Assume 48 m² of surface with 6 m² excluded, leaving 42 m². Two coats at an illustrative coverage of 12 m² per litre per coat require 42 ×2 ÷12 =7 litres before any allowance.
If the user chooses an illustrative 10% allowance, the requirement becomes 7.7 litres. With 2.5-litre containers, 4 whole containers supply 10 litres. Both 10% and the coverage of 12 are example assumptions, not recommendations for every surface or product.