A simple formula is provided to estimate viscosity of glass using parallel plate viscometry well above 10(8) Pa.s, which is the upper limit of the recommended range. The formula is based on computational studies that account for viscoelastic material behavior and frictional slip that justify using linear interpolation to estimate viscosity between established upper and lower bounds. The formula accounts for variations in friction coefficient and cylinder aspect ratio for viscosity from 10(7) to 10(11) Pa.s.