For a melt flow with viscous heating, a temperature-independent viscosity model is simple to use in estimating its temperature profile. In this model, a constant viscosity is chosen to represent a temperature-dependent one. However, it is still not clear about what value should be selected for the constant viscosity. In this short communication, we derive a relation to find it, followed by validation using existing numerical examples. (C) 2018 Elsevier Ltd. All rights reserved.