Dry Film Thickness Calculator
Estimate cured coating thickness from wet film thickness, volume solids, and thinner ratio. Returns DFT, effective volume solids, and theoretical coverage for paint and protective coatings.
Welding Toolkit runs this math offline
Worked example
WFT = 6 mils, volume solids = 50%, no thinner added.
Coating volume solids reference
| Coating type | Typical VS % | Recommended DFT (mils) | Notes |
|---|---|---|---|
| Alkyd primer | 40–48 | 2.0–3.0 | General-purpose primer, easy application |
| Alkyd topcoat | 45–52 | 1.5–2.5 | Architectural or maintenance finish |
| Epoxy primer | 55–70 | 3.0–5.0 | Corrosion protection on steel |
| Epoxy intermediate | 60–75 | 4.0–8.0 | Build coat for total system thickness |
| Polyurethane topcoat | 50–60 | 2.0–3.0 | UV-stable, good gloss retention |
| Zinc-rich primer | 65–78 | 2.5–4.0 | Sacrificial primer for steel |
| Coal tar epoxy | 68–75 | 8.0–12.0 | Immersion or buried service |
| Inorganic zinc | 60–68 | 2.0–3.5 | Tight DFT control, mud-crack risk |
Common mistakes
- Ignoring thinner. 10% thinner drops effective solids by 9% — the DFT calculation must account for it.
- Confusing WFT with DFT on the spec. Specifications are written around DFT. Applicators target WFT that produces the specified DFT.
- Treating theoretical coverage as practical. Transfer efficiency cuts coverage 20–40%. Add overspray and profile absorption.
- Skipping the PDS. Volume solids, thinning limits, and DFT ranges live on the manufacturer's product data sheet. Nominal ≠ project.
FAQ
What is dry film thickness?
The cured coating thickness remaining on the substrate after solvents evaporate and the film sets. DFT governs corrosion protection, appearance, and long-term coating performance.
What is volume solids in paint?
The percentage of the applied wet volume that remains as solid film after drying. Higher volume solids yields higher DFT at the same WFT.
How accurate is a calculated DFT?
A theoretical planning estimate based on stated solids and uniform application. Real results vary with thinner, spray efficiency, surface profile, and environment — field measurement is always required.
What tools measure DFT in the field?
Magnetic or electronic coating thickness gauges for cured film. WFT combs for immediate wet-film checks during application.
What happens if DFT is too thin or too thick?
Too thin leaves the substrate under-protected. Too thick can cause cracking, poor cure, solvent entrapment, reduced adhesion, or topcoat problems.
Sources
Log every pass before the CWI signs off
Welding & Sheet Metal Toolkit bands kJ/in against your WPS, stacks the pass log, and prints the day-sheet citing AWS D1.1 / ASME IX. Offline. Pay once.
Related
CalcSpec is a planning estimator for coating specifications. Field DFT measurement per SSPC-PA 2, AMPP, ISO 19840, or the governing project specification is always required for acceptance.