Weight Solids and Volume Solids in Coatings: A Worked Comparison

Weight solids and volume solids describe different proportions. A percentage without its basis is incomplete. Comparing two coatings by that number alone can rank them incorrectly before compatibility, application or performance has even been considered.

Understand the denominator

Weight solids expresses nonvolatile mass relative to the original liquid mass under the specified test. Volume solids relates the remaining film volume to the original wet volume under the relevant method. Ingredients can have different densities, so equal masses do not necessarily occupy equal volumes.

Suppose two illustrative liquids each contain 50 g of nonvolatile material in 100 g of product. Both are 50 percent solids by weight. If their nonvolatile ingredients and carriers have different densities, their volume-solids values need not match. There is no universal conversion that uses the weight percentage alone.

Ask for the measurement basis

When a specification lists a solids percentage, request the basis, method and conditions. Reactive coatings can make the interpretation more complicated because curing is not simply solvent evaporation. The supplier should explain what the reported value represents for that formulation.

Do not silently relabel a weight-solids figure as volume solids in a coverage calculator. Likewise, do not multiply a manufacturer's recommended millilitres per square metre by an unspecified percentage and present the result as measured dry-film thickness.

Compare useful outcomes

For purchasing, keep separate columns for solids information, recommended application rate, compatible substrates, application window and the relevant test results. A missing solids basis is a documentation question, not proof that the product is poor.

If one supplier supplies weight solids and another volume solids, compare them within their own documented systems. Ask for compatible measurements if that property is essential to the project. A trial can still compare application labour and finished appearance, provided those observations are not described as proof of chemical equivalence or long-term durability.

Related guidance

• What Solids Percentage Can and Cannot Tell You About a Coating
• How to Check an Independent Nano Coating Laboratory Report
• Wet-to-Dry Film Calculations: When the Volume-Solids Formula Applies
• Coating Viscosity: Why a Thicker Liquid Does Not Prove a Thicker Protective Film

Technical references: Elcometer: Calculating Dry Film Thickness.

For product-specific application instructions, consult NANOLAB technical documentation or ask about your surface and intended use.