Measures of passage rates (kp) of potentially digestible NDF (pdNDF) and indigestible NDF (iNDF) are needed to model the process of rumen fiber digestion. Our objective was to develop and validate models to predict kp of iNDF and pdNDF. Nineteen flow influx studies with lactating dairy cows were compiled (n = 73, treatment means). Three empirical models to predict kp iNDF were developed. The following parameters were included in all 3 models: milk yield (MY, kg/d), DIM, iNDF incubation time (h), diet NDF (%). The 3 models differed by one parameter: Model 1 (M1) also included diet iNDF:NDF ratio, Model 2 (M2) included diet iNDF(%), and Model 3 (M3) included ratio of diet concentrate proportion to diet NDF. Models were validated with an independent data set from 2 experiments (n = 64, individual animals). M1, M2 and M3 were used to predict kp iNDF of individual animals in the independent data set. The relationship between kp iNDF and kp pdNDF was best described with a segmented regression with kp iNDF as an independent variable and kp pdNDF the dependent variable. When kp iNDF <2.45%/h, kp of pdNDF was constant, but when kp iNDF >2.45%/h, a regression: kp pdNDF = −0.011 + 0.942 x kp iNDF (%/h) described the passage rates of iNDF and pdNDF. Concordance correlation coefficient (CCC) of kp iNDF predictions and observed kp iNDF was 0.40, 0.44, and 0.35, for M1, M2, and M3, respectively. When kp pdNDF calculated from M1, M2, and M3 was compared with observed, CCC was 0.33, 0.36, and 0.26, respectively. Rumen NDF digestibility was calculated as kd/(kd+kp) x pdNDF, where kp was predicted by M2 and adjusted by the segmented regression, kd (degradation rate) was estimated by in vitro incubation (24, 48 and 72 h), and pdNDF was estimated using 240 h in situ incubation. When M2-predicted NDF digestibility was compared with observed, CCC and root mean square error of prediction (RMSEP) was 0.60 and 7.6%, respectively. When an equation using kp iNDF was used (Kriszan et al., 2010), CCC and RMSEP were 0.51 and 8.0%, respectively. These data suggest that kp pdNDF can increase the accuracy and precision of rumen NDF digestibility predictions.