Biomass and biochar products are often described in scientific literature by weight but handled and sold in the commercial space by volume. While density connects these two concepts and can be used to convert between these metrics, the definition and application of density is not always straightforward. We describe different kinds of densities that can be used for biomass and biochar: skeletal density, bulk density, and envelope density. We summarize how these densities are measured and how they affect our understanding of “how much.”