Ordering paint by guesswork produces either a shortage that stops a crew or a surplus that expires in the store. The conversion from area to litres is a single formula plus honest loss allowances.
The core formula
Theoretical coverage in square metres per litre equals volume solids percentage multiplied by ten, divided by dry film thickness in microns.
Worked example. A primer at 68 percent volume solids specified at 75 microns dry film thickness. Coverage equals 68 multiplied by 10 divided by 75, which is 9.07 square metres per litre. For 2,000 square metres of steel, theoretical requirement is 2,000 divided by 9.07, about 221 litres.
Then apply loss factors
Theoretical coverage assumes every drop lands at exactly the specified thickness, which never happens. Realistic allowances by method and geometry: airless spray on flat plate work loses roughly 15 to 25 percent through overspray and film variation. Airless spray on complex structural steel, lattice or handrails loses 35 to 60 percent, because much of the spray passes between members. Brush and roller waste less material, perhaps 10 to 15 percent, but reaching specified thickness takes more coats. Windy site conditions push spray losses higher again.
Applying a 40 percent loss factor to the example above: 221 divided by 0.6, roughly 368 litres, so order 380 to allow for touch up.
Do not forget stripe coats
Edges, welds, bolts and corners hold less film than flat surfaces because wet paint pulls away from sharp geometry. Specifications commonly require a brush applied stripe coat to these details before or after each spray coat. Stripe coats consume material that never appears in area based calculations. On heavily detailed structures, add 5 to 10 percent.
Cross check against the practical figure
Many technical data sheets publish a practical coverage figure alongside the theoretical one. If your calculation with loss factors lands far from the manufacturer practical figure, revisit your assumptions. For help reading the rest of the sheet, see our guide to TDS and SDS documents.
MENA Industrial Supplies converts steel areas into quantities as part of every quotation. Send area, specified system and film builds to +971 4 832 44 22 and we will return litres and price the same working day.