AbstractDifferent sources of counting loss are described and the relation Rt = Rm exp (Rt τ) is considered valid for most commercial XRF counting devices. Based on a series of Kα/Kβ measurements and an initial estimate of the Rt (Kα)/Rt (Kβ) value, corresponding τ values can be computed. If the chosen Rt (Kα)/Rt (Kβ) value differs from the correct one, the τ values will not be identical. The computer program iterates the Rt (Kα)/Rt (Kβ) value until the τ values have reached a desired level of agreement. The program is given together with a worked example of the calculation.