An equation is presented for the calculation of the shear viscosity of hydrous (0-12.5 wt% H₂O) leucogranitic melts from 10² to 10¹³ Pa·s. The equation is a multiple nonlinear least-squares regression of a data set of 111 viscosity determinations in the literature. It is based on the Vogel-Fulcher-Tammann (VFT) form and thus accounts for the very important non-Arrhenian temperature dependence of the viscosity. This is possible because of the inclusion of data obtained recently in the high-viscosity region.

The equation includes a logarithmic dependence of the three adjustable parameters of the VFT equation on the H₂O content of the melt. The root-mean-standard deviation for the 111 data points included is 0.46 log₁₀ units. In comparison with earlier methods based on the Arrhenian approximation of the temperature dependence of viscosity, the present method provides significant improvement in the accurate prediction of viscosity. We recommend its use in all petrologic calculations involving hydrous, metaluminous, leucogranitic melts. It is an especially powerful method for calculating the high viscosities thought to be relevant to erupting silicic volcanic systems and magmatic hydrothermal granitic and pegmatitic systems at the brittle-ductile transition.