Expressions are derived relating counting rate measured with quantum detectors to the dosage rate in roentgens. The number of quanta/cm2per roentgen is strongly energy dependent. In the x‐ray region 5 to 50 kv, however, the variation of the quantum counting efficiency with wavelength of some counters (e.g., argon Geiger counter) matches fairly closely the energy dependence factor, and hence such detectors can be used asr‐meters.