Abstract The conversion factor Q, obtained by division of blood alcohol concentration (BAC) by breath alcohol concentration (BrAC) is a widely discussed topic due to its great variance. By Austrian law, regulations frequently require an estimation of a corresponding BAC by a measured BrAC. It is known that Q depends among other things, on the alcohol kinetic state of the person being tested, which mathematically can be transformed to a dependency on the BrAC. Theoretically calculated Q values per BrAC level form a hyperbola shaped curve, thus decreasing with increasing BrAC values. Applying Austrian forensic standards for BAC and BrAC measurements, these calculations were verified in a study under practical conditions with BAC and BrAC data of 390 individuals. Q decreases from 2629 (±455) for BrAC levels <0.1 mg/l to 2229 (±160) for a BrAC range of 0.4–0.5 mg/l and increases again to 2428 (±124) for BrAC levels >0.6 mg/l. Since these results were obtained under realistic practical conditions they can be directly applied in routine forensic expert opinion and can eliminate avoidable variances in the calculation of Q.