This paper describes the calibration of Rogowski coils with an integrator for current measurement in the current range of up to 30 kA, at mains frequency. A current loop of ten turns with a current of 3 kA was used for calibration. The magnetic field generated in this way differs from the field of one turn placed at the center and passed with a current of 30 kA. The influence of the unevenness of winding and the cross section of the Rogowski coil may cause a calibration error when using a current loop. The difference in the results between using a current loop and one turn, and the measurement uncertainties are given here. Calculations of the mutual inductances of typical Rogowski coils and their uncertainties are presented in the Appendix.