Solids content is measured by weighing a sample, drying it under defined conditions, and weighing the residue. It tells you how much of what you buy actually becomes coating.
It matters commercially and operationally. Higher solids means more film per kilogram, fewer coats to reach a target thickness, less solvent to evaporate and recover, and lower VOC. It also usually means higher viscosity, which limits how you can apply it.
UV-curable systems are described as 100% solids because reactive diluents polymerise into the film rather than evaporating — a genuinely different position from solvent systems.
When comparing suppliers, compare on a solids basis rather than on price per kilogram. A cheaper resin at 40% solids may be more expensive per kilogram of dry film than a dearer one at 60%.