All gradation percentiles (e.g. d30) in riprap analysis should be specified in terms of percent-finer by weight (e.g. the d50 is the particle size that for which 50% of the aggregate weight has smaller diameters). Most standard sieve analyses are reported in percent-finer by weight because the weight of material on each sieve is measured. However, particle counts and some surrogate grain size measurements can report grain size in percent-finer by count (e.g. 50% of the particles counted are smaller than the d50).

The example below for a hypothetical sample of 24 stones illustrates the importance of defining percent-finer in terms of the weight of the finer material, not the particle count. The median particle size by count is less than half the median particle size by mass in this example.

Modeling Note:

The documentation accompanying some riprap sizing methods implies that the percent finer refers to the number of stones; however, this assumes that the count is conducted by sampling a single particle at regular intervals, which corrects for the presence of multiple smaller particles that are not counted. Any documentation referring to “half the particles” assumes the term “by weight,” whether or not it is explicitly stated. -Modeling Note provided by Krey Price