Hard-sphere colloids are popular as models for testing fundamental theories in condensed matter and statistical physics, from crystal nucleation to the glass transition. A single parameter, the volume fraction (), characterizes an ideal, monodisperse hard-sphere suspension. In comparing experiments with theories and simulation, researchers to date have paid little attention to likely uncertainties in experimentally-quoted values. We critically review the experimental measurement of in hard-sphere colloids, and show that while statistical uncertainties in comparing relative values of can be as low as 10⁻⁴, systematic errors of 3% are probably unavoidable. The consequences of this are illustrated by way of a case study comparing literature data sets on hard-sphere viscosity and diffusion.