The estimation of tree biomass in forest ecosystems has a huge interest for the quantification of biomass for energy purposes, to evaluate carbon sequestration and for nutrient cycling studies. The evaluation of biomass at stand level can be easily achieved using the Conversion and Biomass Expansion Factors (BCEFs). These factors allow estimating the biomass of a particular component of the tree or the total biomass based on stand volume information. The main objective of this paper is the study of the conversion and biomass expansion factors in ecosystems of Pinus pinaster Ait. The study is supported by data collected in permanent plots established in the North of Portugal. The results show an allometric relationship between the factors and the dominant height of the stands. The developed models allow estimating the stands’ biomass, for the components of the stem, crown, roots, and for its total, based on information of stand volume and dominant height. These estimates can be used to assess the forest dry wood stocks to be used for pulp, bioenergy or other purposes, as well as the biomass quantification to support the evaluation of the net primary productivity.