Process Manufacturing

What Is Solids Content? The Formula, the Oven Test, and Why It Sets Your Real Coating Cost

Two coatings priced the same per gallon can differ by 40% in what a square foot actually costs you, and solids content is the number that explains it. Definition, test method, and the arithmetic buyers skip.

Solids content is the fraction of a coating, adhesive, ink, or slurry that remains on the part after the volatiles evaporate. A 60% solids paint leaves 60% of its mass as dry film; the other 40% is solvent or water you paid for, pumped, sprayed, and then paid again to exhaust from the booth. The number is usually stated by weight (weight solids) or by volume (volume solids), and the two are not interchangeable: volume solids is the one that predicts film build and coverage, because film thickness is a volume phenomenon. Confusing them is the single most common error in coating cost comparisons.

The formula and the oven test

Weight solids is measured by drying a known sample and weighing what survives: percent solids equals dry weight divided by wet weight times 100. The standard methods (ASTM D2369 for coatings, D1259 for varnish-type products) specify roughly one gram of sample at 110 degrees Celsius for 60 minutes, because the answer moves with temperature and time and a benchmark is only useful if everyone runs it the same way. Volume solids comes from the formulation or from ASTM D2697, and it is the input for the coverage equation every estimator should know: theoretical coverage in square feet per gallon equals 1,604 times volume solids divided by dry film thickness in mils. A 100% solids coating spreads 1,604 square feet at one mil; a 50% volume solids coating spreads 802.

Theoretical coverage at 100% volume solids and 1.0 mil DFT: 1,604 sq ft/gal. Multiply by volume solids and divide by target mils. Then multiply by transfer efficiency, because the spray gun keeps a share too.

Why the number sets your real cost

Compare two paints at the same price per gallon, one at 45% volume solids and one at 65%. The 65% product delivers 44% more dry film per gallon, so its cost per covered square foot is 31% lower before you touch the sprayer. The gap widens from there: less solvent means shorter flash times, lower VOC permitting exposure, and less booth airflow doing nothing but carrying vapor away. This is also why quoting coating cost from the purchase order line is a mistake; the honest unit is cost per square foot at specified dry film thickness, and solids content is the bridge between the two. Raw material prices move as well, the producer price index for paints and allied products currently reads 422.95 index (1982=100) (Jun 2026), up about 0.6% from a year ago, so a comparison done a year ago is due for a refresh.

You are not buying gallons. You are buying dry film. Solids content is the exchange rate between the two.

Where estimates go wrong

The four classic solids-content mistakes

The coatings record, six years

Over the 36-year record coatings prices have moved decisively rather than oscillated: from 127.70 at the close of 1990 to 422.95 today, up 231%, and now at the top of its 36-year range. A change of that size across a span this long is a level shift, not a cycle, and planning that assumes a return to the 1990 figure is planning against the whole record.

Enter your wet and dry weights in the solids content calculator to compute percent solids and translate it into coverage and cost per square foot. Run the test math

Published 2026-08-05.