Biomass is estimated using allometric equations including stem diameter, sometimes height (which itself may be estimated from allometric equations), and sometimes wood density data (when applicable and available).
For the tropics, the most commonly used allometries are those of Chave et al., 2005 (doi: 10.1007/s00442-005-0100-x), which are implemented in the BIOMASS R package (https://cran.r-project.org/web/packages/BIOMASS/index.html) of Réjou‐Méchain et al. 2017 (doi: 10.1111/2041-210X.12753).
For temperate and boreal regions, we recommend the allodb R package (https://docs.ropensci.org/allodb/) of Gonzalez-Akre et al. 2021 (doi: 10.1111/2041-210X.13756).