Application
How to measure volume solids of paint
There are several ways to measure the volume solids of paint, from a calculation that takes minutes to methods that wait for the film to dry. Below are the methods coatings labs use, compared side by side.
Volume solids is the share of a wet coating that stays on the substrate as dry film once the solvents have left. It decides how much area a litre of paint covers at a given dry film thickness, which makes it one of the numbers behind the price per square metre. It is on almost every technical datasheet, and the manufacturer has to stand behind it.
The value calculated from the formulation is fast, but it is not always what the paint delivers. Solvent can stay trapped in the film, pigment can pack differently and a batch is never mixed exactly as written. A measured value tells the lab what the paint it actually made will cover.
Volume solids methods labs use today
Calculation from the formulation
The lab adds up the volume of every solid raw material and divides it by the total volume of the paint. It takes minutes and needs no test panel. It only holds if the data for every raw material is right and no solvent stays trapped in the film, so it gives an estimate of the volume solids.
ISO 3233-1
ISO 3233-1 determines volume solids by weighing a coated test panel. The result comes once the film has dried.
ASTM D2697
The ASTM method, common on protective and marine coating datasheets. It also works by weighing a coated sample, and the result comes once the film is dry.
Wet and dry film thickness
Since the coated area does not change while the solvents leave, volume solids is the dry film thickness divided by the wet film thickness. A paint applied at 135 µm wet that ends at 100 µm dry has 74% volume solids. With a wet film comb and a dry film gauge it is cheap and quick, but a comb reads in coarse steps and the two readings are rarely taken at the same spot, so it is mostly used as a quick check on site.
Volume solids methods compared
| Method | Measures the real film | When you get the result | Needs to know what is in the paint |
|---|---|---|---|
| Calculation from the formulation | No | Minutes | Yes |
| ISO 3233-1 | Yes | Once the film is dry | No |
| ASTM D2697 | Yes | Once the film is dry | No |
| Wet film comb and dry film gauge | Yes, roughly | Once the film is dry | No |
| AMI | Yes | The next day, with the full drying curve | No |
How AMI measures volume solids the next day
AMI’s compact benchtop instrument measures volume solids from the wet and dry film thickness of the same spots. It uses terahertz light, which passes through paint and is reflected at each boundary between layers. From these reflections it measures the thickness of the film without touching it.
The operator scans a few selected spots right after application, while the paint is still wet. The panel goes back into the curing cabinet, and the next day the same spots are scanned again. The ratio of the two readings is the volume solids. Each scan takes about a minute and the operator spends a few minutes on the whole test.
Nothing is weighed, so the result does not depend on who runs the test. There is no need to tell us what is in the paint, so the same test works on new formulations, competitor products and every type of coating, solvent-borne or waterborne, at any volume solids level.
We compared AMI’s results on commercial epoxy, polyurethane and acrylic coatings with the values the manufacturers publish on their datasheets. The difference is smaller than the uncertainty the datasheets themselves state.
Volume solids the next day or after a full week?
Most of the solvent leaves the film in the first 24 hours. The difference between the reading after one day and after a full week is usually small and follows a predictable trend, so the next-day result already shows which version of a formulation is better.
A weighing method gives one value once the film is dry. AMI’s instrument can scan the same spots as often as needed, so the lab sees the whole drying curve and knows when the film has actually stopped changing. A film that holds on to solvent shows it in the curve, which a single weighing at the end cannot show.
When a datasheet value has to follow the agreed drying schedule, the lab simply takes the last scan at the end of that schedule. The next-day result is enough to choose between formulations.
Which volume solids method for which job
- An early estimate while designing a formulation: calculation from the formulation
- A value that must follow a standard method: ISO 3233-1 or ASTM D2697
- A quick check on site during application: wet film comb and dry film gauge
- Measured values on many formulations, with results the next day: AMI
Further reading: Measuring under the coating for a faster digital lab, European Coatings, 2 October 2026.
Frequently asked questions
Why does measured volume solids differ from the calculated value?
The calculation assumes that every raw material behaves as its data says and that all the solvent leaves. Trapped solvent, air and the way pigment packs in the film make the real value differ, and only a measurement on the film shows it.
Can I measure volume solids without knowing the formulation?
Yes. Methods that measure the film work on the paint itself. With AMI you apply the paint to a test panel and scan it, with no need to know what is in the paint.
How long does it take to measure volume solids?
A calculation takes minutes. Methods that measure the film need it to dry first. With AMI the result is ready the next day, and the drying curve shows whether the value is still changing.
Can volume solids be measured on waterborne coatings?
Yes. AMI’s instrument measures thickness and not weight, so waterborne and solvent-borne coatings are measured in the same way.
