A first property, derivable from mass, of a drilling fluid is measured at a point upstream of a cuttings removal device. A second property (also derivable from mass) is measured at a point downstream of the cuttings removal device. A difference between the properties is determined over a period of time to obtain an estimate of the rate of cuttings removal. This allows the property of the drilling fluid before and after cuttings removal to be compared to give an indication of the rate of cuttings removal, which is indicative of the borehole cleaning.