An algorithm is developed for obtaining grainsize distribution by volume from observations of areas of particles exposed in a plane of cut. The approach follows existing methods except that error estimates are included and a logarithmic scale of grainsize is adopted. One tenth decade class intervals in grainsize are suggested, and there is no limit on the number of orders of magnitude which can be accommodated. The scheme was devised to permit the inventory of magnetic minerals in rocks to be studied, but it may be useful in other branches of petrology. ¿ American Geophysical Union 1990