Abstract. Due to their construction, the devices used for the measurement of electromagnetic fields (radiation monitors) must be calibrated individually. The measurement data obtained are documented in a calibration certificate and the user must take them into account in the uncertainty budget he has to draw up. In addition to the uncertainty contributions of the field representation and of the transfer, other contributions occur which depend on the special constructive features of these devices. When the uncertainty budget is drawn up, it depends on the application and the information about the field structure which contributions will have an effect and which will not. The procedure will be explained by an example.