Abstract A simple method is presented to calculate the minimum volume of an exponentially-thinning fall deposit for which only one isopach is available. This method exploits the fact that the product of the area enclosed within an isopach ( A) and its thickness ( T) reaches a maximum value ( TA) max for a certain isopach. It is shown that the minimum volume is V min=3.69 TA, which is equal to the actual volume when TA=( TA) max. Calculation of V min for 327 isopachs from 74 fallout deposits shows that it is often just a bit lower than the volume calculated from all the available isopachs. It is therefore proposed that V min provides a useful minimum volume estimate for fallout deposits for which only one isopach is available.