Increasing use of woody plants for greenhouse gas mitigation has led to demand for rapid, cost-effective estimation of forest carbon stocks. Bole diameter is readily measured and basal area can be correlated to biomass and carbon through application of allometric equations. We explore different forms of allometric equations and analyse the potential to use of equations for individual trees to derive stand-level equations, where the basal area and the average diameter are used as explanatory variables. To test the relationships derived from published allometric equations, we used data from a forest inventory in the oak–pine forests in La Primavera (Mexico). Results show that in two forests with the same species and basal area, there will be more carbon where trees are larger. Allometric equations for individual trees can be transformed into stand-level equations. The values of average diameter weighted by the basal area for these equations can be based on a small sample of large trees once the local relationship between tree size and tree density per hectare is known. This approach could considerably reduce field data requirements in comparison with inventory methods based on enumeration of all trees for estimation of biomass and carbon.