The ability to estimate mean particle size using simple, low-power optical instruments promises to greatly expand coverage of particle size measurements in the ocean and advance understanding of myriad processes from sediment transport to biological carbon sequestration. Here we present a method for estimating the mean diameter of particles in suspension from high-resolution time series of simple optical measurements, such as beam attenuation or optical backscattering. Validation results from a laboratory clay aggregation experiment show a good fit with independent mean particle diameter estimates in the 10-80 μm diameter range, with relative biases of 17%-38% and relative root mean square errors of 10%-24%. In the 80-200 μm range, quantitative validation data were not available, but our mean diameter estimates correlated strongly with particle settling rates.