Abstract Different sources of counting loss are described and the relation R t = R m exp ( R t τ) is considered valid for most commercial XRF counting devices. Based on a series of Kα/Kβ measurements and an initial estimate of the R t (Kα)/ R t (Kβ) value, corresponding τ values can be computed. If the chosen R t (Kα)/ R t (Kβ) value differs from the correct one, the τ values will not be identical. The computer program iterates the R t (Kα)/ R t (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.