Sonic flow of liquid-gas mixtures at drill bit nozzles is believed to be responsible for some complications in underbalanced drilling (UBD), owing to pressure discontinuity and temperature drop at the nozzles. It is vitally important to avoid this flow condition in UBD by selecting optimal nozzle size and fluid flow rates. However, there is no method that can be used for prediction of the sonic flow condition. This paper introduces an analytical model for calculating the critical pressure ratio when two-phase sonic flow occurs. The analytical model can be coded in a spreadsheet. The critical pressure ratio charts are also provided for field engineers. Use of the model and charts in selection of bit nozzle size and two-phase flow rates will reduce complications in UBD.