A modified Herning-Zipperer method (H-Z-mod) is still used to calculate the viscosity of natural gas during calibrations. The method can calculate the dynamic viscosity of gas mixtures with a choice of 40 components. In the absence of literature references the method is re-documented in this paper. In addition, the H-Z-mod method has been compared with the RefProp software [3] in the range of -5 ~ 50°C and 1 ~ 101 bar. Using different natural gases, the deviations range between -6% and +7% depending on the gas composition. The viscosity calculation method based on Schley et al. [4] implemented in GasCalc [6], shows deviations ranging between -0.3% and 1.6%. The latter method to be used in the new Closed Loop Pigsar facility, is an order of magnitude more accurate than H-Z-mod. For both methods addition of 20% hydrogen results in an offset of more than 1% and a dispersion increase by more than a factor two.