A new mass input:output formula is presented which makes full use of available data which are commonly sub-optimal for mass input:output analysis. Results indicate that some useful information on wet and dry fallout may be obtained from bulk precipitation data only, and that the new formula provides generally more reliable results for catchment chemical mass export under various conditions than other simpler formulae. Care is taken not to assume rates of natural denudation from the results in the light of an inability clearly to separate non-denudational from denudational components of mass export, and natural from man-induced components of denudation.