The model inputs for a sediment source in PTM require a standard deviation (in phi) and a median grain size in mm. In order to generate a distribution of particle sizes you would also need a mean particle size. How does PTM (or perhaps an SMS pre-processor) calculate a mean grain size from a median grain size and a distribution? As an example I have been running a simple model to look at what the grain size distribution in the model is like and I get results like this. Model Input: median grain size: 0.088 mm St Dev. (phi): 1.22 Statistics of Model Output median: 0.0879 (mm) mean: 0.1497 (mm) How is the mean calculated?