This report is made for ICAO and the Icelandic Met Office, financed partly by ICAO. Total grain size distribution in explosive volcanic eruptions reports quantitative distribution of all grain that are formed in that eruption. When modelling distribution of volcanic ash in the atmosphere one of most important factor is the quantitative grain size distribution in the whole deposits formed. Fine ash grains (ash) do go further away from the volcano than do the coarse (lapilli). Thus, knowing total grain size calculations and quantitative modelling of how much ash leaves the volcano can be made. This work and its continuation is of prime importance for aviation safety in future eruptions in Iceland.