The threat posed by coolants to environment and worker health, escalating manufacturing costs due to coolant usage and stringent regulations enforced by the government for coolant waste disposal trigger research in unconventional coolant delivery techniques. Minimum Quantity Lubrication (MQL) is a technique delivering very small amount of coolant in the form of spray droplets that easily penetrate the tool-work piece interface for improving the cutting action, augmenting tool life and work piece quality. The droplet size which plays an import role in removing the heat and provide a lubrication action in the contact zone is controlled by the spray parameters. In this paper, an analytical model has been developed to determine the Sauter Mean Diameter (SMD) of coolant spray in MQL applied to machining processes, as a function of the spray parameters such as air pressure, viscosity of air, coolant flow rate, surface tension and density of coolant oil, based on the first principles and the conservation methods. The model developed has been validated with the experimental results and it has correlated well with reasonable accuracy. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.