Weight % vs volume % in polymer formulation

Recipes are usually dosed by weight, but many properties — opacity, colour strength, how much space a filler occupies — depend on volume. When component densities differ, the two percentages diverge, and assuming they are equal is a common formulation error.

Why they differ

Consider a dense mineral (say 2.7 g/cm³) in a light polyolefin (0.92 g/cm³). Add 30% by weight and, because the filler is roughly three times denser, it occupies far less than 30% by volume. A pigment or filler "does its job" largely by volume, so the effective loading is smaller than the weight figure suggests — which affects opacity, cost-in-use and mechanical behaviour.

How to convert

Convert through density. Volume fraction of a component = (mass fraction ÷ its density) ÷ Σ(mass fraction ÷ density) across all components. The blend's own density follows the reciprocal rule: 1/ρ_blend = Σ(weight fraction ÷ ρ_component). MatQuo's blend density calculator applies this, and the masterbatch calculator converts density-adjusted mass % ↔ volume % directly.

Where it bites

Colour and additive dosing, filled compounds, and any cost comparison "per unit effect" rather than per kg. If you set targets by volume but dose by weight (or vice versa) without converting, you will over- or under-shoot — see also let-down ratio.