Studies of field-dependent microwave losses are useful for the detection and characterization of superconducting materials. The measurements are made in a conventional ESR spectrometer, but until now the data interpretation has been mainly qualitative. A calibration procedure using just a frequency counter, DC voltmeter, and ball bearing is described. This allows a quantitative determination of magnetic losses to similar to 5 % accuracy without the need to determine the cavity Q, power level, or field strength.