The demand for seaweeds has intensified in recent decades and will most certainly continue to expand. Several methods exist to evaluate the biomass of seaweeds in the field but most of them are destructive. The objectives of this study were (1) to develop and evaluate allometric equations for estimating seaweed biomass in the field for some harvested species and (2) to provide uniform calculated dry/wet biomass ratios to estimate the relative water content of these seaweeds. Sampling and measurements of more than 350 seaweed individuals were carried out for 8 species of commercial interest. Our models were fitted for both power and linear equations and were tested for different explanatory variables. While the power equation was found to be the best for predicting biomass of all species, we found that the best descriptive biometric variable varies according to seaweed morphology. Species with a bushy morphology were best described by the volume, while long stringy species were best described by the length and flat species by the surface. This study attempts to provide nondestructive tools that could be used by professional seaweed harvesters, their employers as well as scientists and public regulators, to assess the harvest potential of a field of seaweed in a nondestructive approach.