During volcanic eruptions, fine ash settles out of the atmosphere to form deposits. Particle aggregation makes it difficult for models to calculate where fine ash will fall. In this study we show that the Ash3d dispersion model can accurately predict where fine ash will land if one assumes a Gaussian size distribution of aggregates, ~ 0.18–0.23 mm in diameter and 600 kg m⁻³ in density. This aggregation scheme has optimally reproduced deposits for four well-documented eruptions.