About Coating Coverage Calculator — WFT, DFT & Volume Solids
The coating coverage calculator does the film-build arithmetic behind every industrial coating datasheet. The anchor is an exact identity, not a rule of thumb: a US gallon is exactly 231 cubic inches, so spread at 1 mil (0.001 in) it covers 231,000 square inches — 1,604.17 ft². Multiply by the coating's volume solids and divide by the dry film thickness you need, and you have the theoretical spread rate; divide DFT by solids and you have the wet film thickness to check with a wet-film gauge as you spray.
Real jobs never hit theoretical: overspray, surface profile, container residue, and irregular geometry eat 10–50% of the material depending on the method (airless spray on flat steel loses far less than conventional spray on lattice). The loss factor here is deliberately YOUR input — enter your estimator's number and the calculator turns it into practical coverage and gallons. This is the industrial WFT/DFT calculation; interior room painting from a can's stated spread rate lives in the paint coverage calculator.
How It Works
- Read the volume solids off the product datasheet — the percentage of the wet coating that remains as dry film after the solvents leave (e.g. a typical high-solids epoxy runs 65–80%). If you thin the coating, enter the thinner percentage: 10% thinner turns 50% solids into 45.5% effective solids and raises the WFT you must apply.
- Enter the specified dry film thickness (mils or micrometres — the toggle converts at the exact 25.4 µm/mil and clamps to the field caps). The calculator returns the wet film thickness = DFT ÷ solids: the number you verify with a notch gauge while the film is still wet, because fixing DFT after cure means abrading and recoating.
- Read the theoretical coverage: 1,604.17 × solids ÷ DFT in ft²/gal, with the m²/L equivalent from exact conversions. This is the 100%-transfer-efficiency figure every datasheet quotes.
- Apply your loss factor. Typical planning allowances run around 10–20% for brush/roller on smooth surfaces, 20–40% for airless spray depending on geometry and wind, and more for conventional spray or open lattice — but the honest number comes from your own transfer-efficiency experience, so the field is editable rather than preset. Practical coverage and gallons (rounded up) follow.
- Use reverse mode to audit a spec or a bid: enter a known spread rate and the solids, and the calculator returns the DFT that rate actually delivers — the fastest way to catch a coverage claim that cannot meet the specified film build. DFT measurement and acceptance criteria follow SSPC-PA 2.
Worked Example
A tank exterior is specified at 4.0 mils DFT with a 65% volume-solids epoxy. Wet film: 4 ÷ 0.65 = 6.2 mils — the gauge reading to hold while spraying. Theoretical coverage: 1,604.17 × 0.65 ÷ 4 = 260.7 ft²/gal (6.4 m²/L). The estimator carries 30% losses for airless spray on curved plate: practical = 260.7 × 0.70 = 182.5 ft²/gal, so the 1,000 ft² shell needs 1,000 ÷ 182.5 = 5.48 → 6 gallons. Run it backward: a bid claiming 400 ft²/gal at 65% solids delivers only 1,604.17 × 0.65 ÷ 400 = 2.6 mils — it cannot meet the 4-mil spec.
Formulas
- Exact mil-gallon identity (derived)
231,000 in² ÷ 144 = 1,604.17 ft²·mil/gal; coverage = 1,604.17 × VS ÷ DFT- Film build and losses
WFT = DFT ÷ VS; practical = theoretical × (1 − loss); gal = area ÷ practical- Reverse mode
DFT = 1,604.17 × VS ÷ spread rate
Standards & References
- Exact-constant derivation: 1 US gallon = 231 in³ (NIST SP 811 / definitional), 1 mil = 0.001 in, 1 mil = 25.4 µm, 1 ft² = 0.09290304 m², 1 gal = 3.785411784 L — the 1,604.17 ft²·mil/gal figure is derived, never transcribed
- SSPC-PA 2 — procedure for determining conformance to dry coating thickness requirements (measurement context)
- AMPP/SSPC estimating practice — loss factors are job-specific user inputs, not standardized constants
Frequently Asked Questions
How do I calculate coating coverage from volume solids?
Theoretical coverage in ft²/gal = 1,604.17 × volume solids (as a fraction) ÷ DFT in mils. The 1,604.17 is exact — a 231 in³ gallon spread 1 mil thick covers 231,000 in² = 1,604.17 ft². A 65% solids coating at 4 mils covers 260.7 ft²/gal theoretically; real coverage is that times (1 − your loss factor).
What is the difference between wet film thickness and dry film thickness?
DFT is the cured film the spec demands; WFT is what you measure with a notch gauge while applying. They are linked by volume solids: WFT = DFT ÷ solids, because only the solids stay behind when the solvent evaporates. A 4-mil DFT with 65% solids needs 6.2 mils wet — and if you thin the coating 10%, the effective solids drop and the required WFT rises to 6.8 mils.
What loss factor should I use for spray painting?
There is no single standard number, which is why the field is editable. Planning practice commonly carries roughly 10–20% for brush and roller on smooth surfaces, 20–40% for airless spray depending on part geometry and wind, and more for conventional spray or open structures like lattice and pipe racks. Your own measured transfer efficiency beats any rule of thumb — put in the number your jobs actually achieve.
Does thinning paint change its coverage?
Thinning does not change the theoretical dry-film coverage per gallon of ORIGINAL coating — but it lowers the effective volume solids of what is in the pot (50% solids with 10% thinner becomes 45.5%), so you must apply a thicker wet film to reach the same DFT. The calculator handles this: enter the thinner percentage and watch the WFT rise while the theoretical coverage per original gallon stays put.
How is this different from the paint coverage calculator?
That tool estimates interior room painting — wall areas, doors, windows, and a can's stated spread rate. This one is the industrial film-build calculation: it derives coverage from volume solids and specified DFT via the exact mil-gallon identity, converts WFT for gauge checks per SSPC-PA 2 practice, and audits spread-rate claims in reverse. Use it for epoxies, urethanes, floor coatings, and anything with a specified film thickness.