Convert between WFT and DFT, calculate paint coverage per gallon, or build a multi-coat system with gallons needed and cost.
Quick answer
DFT = WFT x Volume Solids. Coverage = 1604 / DFT x Volume Solids. Check the manufacturer Technical Data Sheet (TDS) for exact volume solids. Thinning the coating reduces actual volume solids below what the TDS lists.
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Dry Film Thickness (DFT) is the measurement that determines whether a coating system will protect the steel beneath it. Too thin and the coating fails early: pinholes, undercutting corrosion, premature topcoat breakdown. Too thick and you get mud cracking, solvent entrapment, poor adhesion between coats, and wasted material. Getting DFT right is the difference between a coating system that lasts 15 years and one that fails in 3.
For sandblasting contractors, understanding DFT starts before the first coat goes on. The blast profile (anchor pattern) directly affects the first coat of paint. A 3 mil blast profile means the valleys in the steel surface consume roughly 1 to 1.5 mils of the first coat before any measurable film builds above the peaks. That is why primer specs often call for a higher DFT than intermediate or topcoats: part of it fills the profile.
Paint is one of the largest variable costs on a coating job after labor. Knowing the exact gallons needed per coat at a given DFT lets you price materials accurately instead of guessing. The formula is straightforward: theoretical coverage (sqft per gallon) = 1604 divided by target DFT times volume solids. Then apply your efficiency factor for the application method. Airless spray runs 60-80% efficiency. Brush and roller hit 90-95% but take longer.
Structural steel in mild environments typically gets a two-coat system at 6-10 mils total. Marine and offshore structures call for three-coat systems at 12-20 mils: zinc-rich primer for cathodic protection, epoxy intermediate for barrier protection, and urethane topcoat for UV and weather resistance. Tank linings go thicker at 16-32 mils, usually two coats of high-build epoxy. Fireproofing is a different category entirely, running 100-1000+ mils of intumescent or cementitious material.
DFT = WFT x Volume Solids (as a decimal). If you apply 8 mils wet and the coating is 60% volume solids, the dry film thickness will be 4.8 mils. Always check the coating data sheet for the volume solids percentage.
Theoretical coverage = 1604 / DFT (mils) x Volume Solids. A coating with 60% volume solids applied at 4 mils DFT covers about 241 sqft/gal in theory. In practice, expect 60-80% of theoretical due to surface roughness, overspray, and waste.
A common three-coat system is: zinc-rich primer at 3-4 mils DFT, epoxy intermediate at 4-6 mils DFT, and polyurethane topcoat at 2-4 mils DFT, totaling 9-14 mils. The spec depends on the environment (C1 mild to CX extreme per ISO 12944).
The ratio of paint that ends up on the surface versus paint consumed. Brush/roller achieves 90-95%. Conventional spray achieves 30-60%. Airless spray achieves 60-80%. HVLP spray achieves 65-85%. Lower efficiency means you need more gallons to hit the same DFT.
Common causes: incorrect mixing ratio, wrong thinner amount (lowers volume solids), surface profile consuming film (a 3 mil blast profile absorbs the first 1-1.5 mils), or measurement on peaks vs valleys. Measure multiple spots and average.
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