Empirical correlations over a diverse range of biomass feedstocks have been developed for prediction of resultant biochar properties using experimental data of more than thirty slow-pyrolysis batch reactions. Biochar was produced under a standard set of conditions; including 550 degrees C higher heating temperature (HHT), 7 degrees C/min heating rate, atmospheric pressure, and 40 min residence time. Analysis of the experimental results complimented with literature data showed that calculating biochar yields based on a conservation of the ash method was a valid approach to estimating solid yields in cases where gravimetry is difficult or inconvenient. Increasing the proportion of feedstock ash is observed to increase the biochar yield semilinearly, with the greatest deviation from this observation in low-ash feedstocks. The developed empirical correlation is provided here. When considering highly diverse feedstocks of varying origins, including extensive literature data, the dominating influence of feedstock ash content on the biochar produced is clearly observed in this work. A second-order polynomial relationship between the starting feedstock ash and final biochar ash content was observed, even when including literature results for biochar obtained under widely varied experimental programs. This result suggests a dependency between the amount of the organic material removed from a feedstock and the amount of feedstock ash initially present. This dependency appears to overshadow expected reliance on the heating rate or HHT (within the range of 1-15 degrees C/min and 450-800 degrees C respectively). Empirical correlations have been developed and verified and will be of use to the ones doing greenhouse gas, mass and energy balances, or business case modeling for slow-pyrolysis processes utilizing a range of feedstocks, particularly in the case of high-ash and waste-derived sources.