A submodel for simulating daily grain growth was incorporated in an existing wheat crop growth model. The submodel calculated daily potential grain growth from green leaf area index, plant population and crop stage. Simulated daily potential grain growth was reduced by excessively high or low temperatures or water stress during the period heading to end of anthesis. Actual daily grain growth was calculated using daily potential growth, temperature and crop water status prevailing on the given day. Daily accumulation of grain growth from anthesis to physiological maturity yielded final grain mass. After calibration of the model, it was tested against 19 sets of independent final grain yield data. Statistical validation yielded an index of agreement of 0.91 and a mean absolute error of 20%. The results indicate that the PUTU989 grain growth submodel calculated final grain yield satisfactorily.