Accurate measurement of ion current density and ion energy distributions (IEDs) is oftencritical for plasma processes in both industrial and research settings. Retarding field energyanalysers (RFEAs) have been used to measure IEDs because they are considered accurate,relatively simple and cost effective. However, their usage for critical measurement of ioncurrent density is less common due to difficulties in estimating the proportion of incident ioncurrent reaching the current collector through the RFEA retarding grids. In this paper anRFEA has been calibrated to measure ion current density from an ion beam at pressuresranging from 0.5 to 50.0mTorr. A unique method is presented where the currents generated ateach of the retarding grids and the RFEA upper face are measured separately, allowing thereduction in ion current to be monitored and accounted for at each stage of ion transit to thecollector. From these I–V measurements a physical model is described. Subsequently, amathematical description is extracted which includes parameters to account for gridtransmissions, upper face secondary electron emission and collisionality. Pressure-dependentcalibration factors can be calculated from least mean square best fits of the collector current tothe model allowing quantitative measurement of ion current density.