Theoretical values for estimating only. Confirm against the mill certificate or a stamped calculation before ordering or fabricating.
Coating is quoted by area and bought by volume, and the gap between them is the spread rate. Get it wrong and you either run short mid-job or carry a drum of expired paint.
The calculation
Volume needed = surface area ÷ spread rate × number of coats. A 100 m² surface at 10 m²/L and two coats needs 20 litres.
Spread rate is the number that actually matters
Manufacturers quote a theoretical spread rate on a smooth, non-absorbent surface at a stated dry film thickness. Blasted steel, edges, welds and lattice work all increase consumption — 10 to 30% is normal, which is why a wastage allowance is applied.
Primer, intermediate and topcoat
A three-coat system needs three separate calculations, because each coat has its own spread rate and dry film thickness. Add them for the total.
Why zinc-rich primers are different
Zinc-rich primers are usually specified by dry film thickness rather than spread rate, and the volume solids content varies widely between products. Check the technical data sheet before ordering.
Frequently asked questions
How many square metres does 1 litre of paint cover?
That is the spread rate and it is printed on the tin — commonly 10 to 14 m² per litre for wall paint, and 5 to 8 m² per litre for an industrial primer on blasted steel.
How much paint do I need for 100 m² with two coats?
At 10 m² per litre, 10 litres per coat and 20 litres for two coats. Add about 10% for spray loss and touch-up, so order 22 litres.
How do I convert litres to gallons?
Multiply litres by 0.264 to get US gallons, or by 0.220 for imperial gallons. The calculator shows US gallons directly.
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