A method is proposed for converting number frequencies of unsieved heavy minerals to weight percentages. If the grain count is made by a method, such as the ribbon count method, that gives an unbiased estimate of the number frequency of a mineral, the conversion involves multiplying the number frequency of the mineral in a given microscopically measured size class by a density or density-grain shape weighting factor and by a grain-size weighting factor equal to the cube of the grain diameter. If the grain count is made by the point count method, the grain-size weighting factor is equal to the grain diameter. A study of a natural sand showed that the proposed method gives results comparable in accuracy to the results given by the standard method of making grain counts of closely spaced sieve fractions. For a given degree of accuracy, the proposed method is faster than the standard method, at least when relatively small numbers of grains are counted. Also, the proposed method eliminates the possibility of contamination from sieves. The size distribution data obtained microscopically are easily converted to sieve-size data.