Tools & techniques
Wet film, dry film and volume solids: coverage for pros
Every coverage figure on a can comes from one line of arithmetic. Learn it and you can compare paints honestly, check a crew's film build, and predict coverage for any coating.
Paint is solids in a carrier
Liquid paint is two things: the solids, resin and pigment, which become the dry film, and the carrier, water or solvent, which evaporates. The share of the paint’s volume that is solids is its volume solids, printed on the technical data sheet as a percentage. Most interior wall paints fall somewhere between about 30 and 45%; high-build coatings and many industrial products are much higher.
When you apply a wet coat, it shrinks as the carrier leaves. A wet film 4 mils thick (a mil is a thousandth of an inch) of a paint with 40% volume solids dries to 1.6 mils. That relationship is the whole of the theory.
The 1,604 formula
A US gallon is 231 cubic inches. Spread one mil thick, it covers 231 ÷ 0.001 = 231,000 square inches, which is 1,604 square feet. A gallon of 100% solids coating spread to one mil dry would cover 1,604 sq ft. Real paints have less than 100% solids and are applied thicker than one mil, so:
In metric the constant is simpler: one litre spread one micrometre thick covers 1,000 m², so theoretical coverage in m² per litre is 10 × volume solids in percent ÷ DFT in micrometres.
A typical wall paint
40% volume solids, specified at 1.5 mils dry per coat, applied by roller.
- Theoretical: 1,604 × 0.40 ÷ 1.5 = 428 sq ft per gallon
- Wet film needed: 1.5 ÷ 0.40 = 3.8 mils
- Practical, at 90% transfer for a roller: 428 × 0.90 = 385 sq ft per gallon
- For 1,000 sq ft at two coats: 5.19 gallons
- In metric: 10 × 40 ÷ 38 µm = 10.5 m² per litre theoretical, 9.5 practical
Why volume solids is the number to compare
Two paints can both say “350 to 400 sq ft per gallon” on the label and be very different products. Coverage on the label assumes the recommended film; volume solids tell you how much of the can stays on the wall. At the same 1.5 mil dry film:
| Volume solids | Theoretical | Practical by roller | Wet film to apply |
|---|---|---|---|
| 30% | 321 sq ft/gal | 289 sq ft/gal | 5 mils |
| 40% | 428 sq ft/gal | 385 sq ft/gal | 3.8 mils |
| 50% | 535 sq ft/gal | 481 sq ft/gal | 3 mils |
A low-solids paint has to go on thicker, or in more coats, to build the same dry film. That is why a cheaper paint often costs more per square foot of finished wall: you buy more of it, and spend more time applying it. When comparing paints, divide the price by the volume solids to get a price per gallon of solids.
Transfer efficiency
Not all the paint you buy reaches the surface. The share that does is the transfer efficiency. We use 95% for a brush, 90% for a roller, 70% for an airless sprayer and 65% for HVLP, site estimates for typical conditions. The rest is left in the tools or lost as overspray. Spraying a high-build coating at 60% solids and 3 mils dry, the theoretical coverage is 321 sq ft per gallon, and the practical coverage with an airless sprayer only 225. Spraying versus rolling covers overspray in detail.
Measuring wet film on the job
A wet film gauge is a small comb of metal with teeth at different depths, sold for a few dollars. Press it straight into the wet paint, lift it, and see which teeth have paint on them: the film is between the deepest wet tooth and the shallowest dry one. It is the only way to know, during the job, whether the coat being applied matches the specification.
Checking a crew’s coat
The specification calls for 1.5 mils dry of a 40% solids paint, so the wet film should be about 3.8 mils. The gauge reads 4 mils wet.
- Dry film: 4 × 0.40 = 1.6 mils, slightly over specification
- Theoretical coverage at that film: 1,604 × 0.40 ÷ 1.6 = 401 sq ft per gallon
- Practical by roller: 361 sq ft per gallon
From the film thickness calculator in wet-film mode. A reading of 2 mils wet would mean a thin, under-specified coat that will hide poorly and wear early, however good the coverage looks.
What thinning does
Adding water or solvent lowers the volume solids, so the same wet film dries thinner. Thinning a 40% solids paint by 10% leaves a mix of about 36% solids, and the dry film from the same wet coat is about 91% of what it would have been. Coverage per gallon of mix goes up, but each coat hides less, and you may need another. Stay within the label’s thinning limit; the thinning ratio calculator works out the mix and shows the film loss.
Surface and texture
The formula assumes a smooth, non-absorbent surface. Texture adds surface area and porous materials absorb some of the first coat, so real coverage on rough or new surfaces is lower. The paint coverage calculator applies a surface factor on top of the label figure, and paint coverage explained covers the everyday version of this guide.
Primers, high-build coatings and elastomerics
The formula explains why some products cover so little. A high-build primer meant to fill and level a surface is applied at a heavy wet film; its coverage per gallon is lower than wall paint’s even if its solids are higher, because the dry film is thicker by design. Elastomeric wall coatings are the extreme case: applied at around 15 to 20 wet mils per coat so that the dry film is thick and flexible enough to bridge hairline cracks, they cover something like 65 to 100 sq ft per gallon, a fraction of ordinary paint. Spreading them further to save money defeats the product. The figures for each product class are in the coverage rates reference, and the stucco and masonry calculator uses them.
Stains and penetrating sealers are the opposite case. They are designed to soak into the surface rather than build a film, so the formula does not apply to them, and their coverage depends on how porous the surface is; deck stain coverage explains that version of the problem.
Where the numbers are
On a technical data sheet, look for:
- Volume solids, as a percentage, sometimes with a tolerance.
- Recommended film thickness, wet and dry, per coat, in mils or micrometres.
- Theoretical coverage, usually at the recommended dry film; you can check it with the formula.
- Application details: recommended tip sizes for airless, thinning limits, recoat times.
Commercial and industrial specifications are often written in dry film thickness rather than coats, because film thickness is what decides how long a coating protects. For airless application, the data sheet’s tip range is where to start; airless spray tips explained decodes the tip numbers, and the sprayer usage calculator works out the paint and time.
Common mistakes
- Comparing paints by label coverage. Compare volume solids at the same film.
- Confusing wet and dry film. A specification in dry mils needs a wet film two to three times thicker.
- Rolling thin to hit the coverage on the can. The coverage is met; the film is not.
- Thinning to stretch paint. It lowers the solids and the film per coat.
- Forgetting transfer efficiency when spraying. A sprayer leaves a third or so of the paint in the air.