The boundary layer transition is predicted using the law of mass conservation. The increasing ratio of the boundary layer thickness to the laminar boundary layer thickness at the transition is introduced. Formulas and equations for laminar and turbulent boundary layers are used to predict the transition Reynolds number. The effect of the momentum deficit at the leading edge is also incorporated in this approach under compressible flow conditions. The calculated transition Reynolds number shows reasonable agreement with the experimental results. Under the compressible flow conditions, the calculation simulates the bucket of the transition Reynolds number with a Mach number, the change in the transition Reynolds number due to wall cooling, and the increase in the transition Reynolds number with increase in the bluntness Reynolds number.