Most sediment and geotechnical analyses that quantify soil or rock gradations deal with rock and soil size in terms of diameter. However, riprap design often specifies rock size in terms of the weight of the rock because rock weight is a more direct measurement of the protection provided.

The Riprap Calculator converts between size and weight calculations in several places. The Size/Weight toggle in the Edit menu sets the approach in most of the interface:Weight mode reports the 30th and 100th percentile particle in weight units (W30 and W100) but converts the W50 and W100 to d50 and d100 to compute the layer thickness (Max (d100, 1.5d50)), which it reports in length units.

The Riprap Calculator uses the following equations to covert between Size and Weight: Where W% is the weight percentile, gs is the unite weight, and dxx is the grain class percentile.