A theoretical analysis of the effect of a flexible diaphragm on the calibration of a pressure cell subjected to one dimensional compression is presented together with the results predicted for a range of cell thickness to diameter ratios, in soils of different moduli. The likely over or under registration of a cell in a given soil can be readily obtained and, more important, the probable sensitivity of the calibration of the cell to the soil modulus is easily seen. The results are restricted to a cell having an active diameter to overall diameter ratio of 0.6, a fairly typical value.