Published equations to calculate total-tract starch digestibility (SD) from the concentration of starch in fecal dry matter (FS) have regression slopes that differ by more than 5 fold. Why these slopes differed and methods to improve the accuracy and precision of prediction were explored. Mathematically, SD and FS are correlated. However, 2 additional factors, starch content of dietary DM (DS) and indigestibility of diet DM (IDM = 100 – DMD, %) alter this relationship. Published literature data (204 diets fed to lactating dairy cows; 191 diets fed to feedlot cattle) that provided or allowed SD, FS, DS, and IDM to be calculated were compiled to examine this relationship. When DS and IDM were ignored, the relationship was imprecise (SD, % = 99.16 – 1.07 FS %; R² = 0.81 for lactating cows; SD, % = 99.98 – 0.44 × FS %; R² = 0.88 for feedlot cattle). Including DS improved precision (SD, % = 100.13 – 0.341 × FS%/DS%, R² = 0.92 for lactating cows; SD, % = 100.29 – 0.257 × FS%/DS%, R² = 0.91 for feedlot cattle); adding IDM increased precision further (SD, % = 100 – 1 × FS%/DS% × IDM%; R² = 1.00; P = 1/∞ for all cattle). Because starch is a component of dietary DM, as SD increases IDM will decrease; hence the relationship of SD to FS is curvilinear, not linear unless the digestibility of non-starch components increases when SD increases. Knowledge of IDM and the dietary and fecal concentrations of any component of the diet including NDF allows apparent digestibility to be computed directly and avoids the need to predict digestibility from in vitro assays. Direct measurement often proves more economical analytically than in vitro procedures and avoids dependence on tabular feedstuff values in publications or computerized diet formulation programs. Combined with intake of DM, direct digestibility measurement can quantify the amount of digested energy available for production by ruminants. As with all analytical procedures, accuracy of digestibility estimates relies on representative sampling and appropriate analyses of diets and feces.