In this study, a novel approach for the determination of the solid mass yield from slow pyrolysis based on a comparison of the volatile matter contents of feedstock and char is presented. The approach was tested with experimental data from the literature and our own measurements. For these experiments, gravimetric data are available to determine the mass yield. The proposed method was compared with conventional ash-based calculations and the gravimetric determination of the yield. It was shown that the new approach not only does perform significantly better than ash-based methods but also approximates the real mass yield of slow pyrolysis under atmospheric pressure quite accurately. These findings may indicate that secondary char formation does not contribute significantly to the mass yield of biomass pyrolysis under conditions found in practical production processes (low heating rates, atmospheric pressure, and medium temperatures).