Abstract A computer simulation model is developed to estimate the bulk density within the cultivated layer of a swelling clay soil. The model calculates the untilled density of the surface 100 mm of soil and estimates the changes caused by tillage and subsequent weathering. Field work to collect data for developing and validating the model is described. The untilled density can be estimated solely from the water content of this soil. The increase in porosity caused by a tined scarifier, cultivator, grain drill, blade plow or rod weeder also depends on the soil water content. It is established that changes in density following tillage can be estimated from the initial tilled value, from changes in soil water content and accumulated rainfall and from time after tillage. However, relationship between density and time after tillage varies with the structure produced by different implements. In tests of the model, satisfactory values of tilled density are obtained for tined scarifier/cultivator and for blade plow/rod weeder tillage plots. Comparison of the simulated and measured densities gives r 2 values of 0.51 for the scarifier/cultivator treatment and 0.75 for the alternative blade plow tillage system.