Volcanic ash clouds are carefully monitored as they present a significant hazard to humans and aircraft. The primary tool for forecasting the transport of ash from a volcano is dispersion modelling. These models make a number of assumptions about the size, sphericity and density of the ash particles. Few studies have measured the density of ash particles or explored the impact that the assumption of ash density might have on the output of a dispersion model. In this paper, the raw apparent density of 23 samples taken from 15 volcanoes are measured with gas pycnometry, and a negative linear relationship is found between the density and the silica content. For the basaltic ash samples, densities were measured for different particle sizes, showing that the density is approximately constant for particles smaller than 100 μm. There is a deviation in density of up to 25% from the operational model currently used by the London Volcanic Ash Advisory Centre (VAAC); by inputting the measured density-size relationship into a numerical simulation, up to 18% difference in ash fallout time was found, with the VAAC model overestimating ash removal times.