An equation was formulated for estimating the plastic viscosity of cement paste containing an air-entraining admixture, which is a commonly used chemical admixture for concrete. Air entraining admixtures slightly increase the plastic viscosity of neat cement paste. The viscosity equation was derived by incorporating this effect to minimize the difference between the estimation and measurement. The sample temperature and time-related changes were also considered. The ratio of the plastic viscosity estimated from the proposed viscosity equation to the measured plastic viscosity was found to be approximately 1, the anticipated value. Viscosity equations for mortar and concrete were also formulated based on an existing viscosity equation and found to be valid even when the plastic viscosity of the matrix changed.