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Three models are proposed to evaluate stem volume, following the assumption: the metric units of the independent variables in the proposed models should be structured to express the same metric unit of the dependent variable, because the same volumetric units for both sides of the model will provide better allometric relationship between them. They were tested in stands of black wattle in the state of Rio Grande do Sul, Brazil. Stem volume (v) using combined variables squared diameter times height (d²h), squared diameter at breast height (d²), and height (h) was proposed in the models to fit the coefficients by means of ratio estimates. The model, in which the form factor was combined with the exponent of the diameter variable, presented the best performance to estimate the tree volume. The other two additional volumetric models did not produce unbiased distribution of the residuals. The effectiveness of the proposed models was compared with the traditional Schumacher-Hall model, which showed a slightly lower performance regarding the resulting statistics. The proposed models and methods are shown to be efficient and accurate, enabling the successful modeling of the stem volume of black wattle in forest stands of ages varying between 1 and 14 years.