A model of the rate at which pit latrines fill was developed and compared with actual fill rates measured in latrines in Ifakara, Tanzania. Model parameters were derived primarily from data collected during the field study in Tanzania, with one fitted parameter. Although the model did not accurately simulate the fill rate of individual pits, it provided a good simulation of the average fill rate of the pits and aided the identification of pits that were performing poorly. Laboratory experiments conducted on samples of sludge from the pits were used to identify the average hydrolysis constant for biodegradation of the organic material. The average hydrolysis constant (0.046 day−1) was higher than that identified in another modelling study (0.0015 day−1), conducted in South Africa. The higher hydrolysis constant provided a simulation of the profile of volatile solids with lower root mean square error in 17/24 of the pits, however, for the other pits a lower hydrolysis constant was more appropriate. A sensitivity study of the model indicated that the pit fill rates were particularly sensitive to the parameter that quantified water accumulation in the pit, as such quantifying water flow through the pit is a priority for future research.