An analytical model for the filtration efficiency of face masks is developed. The model considers the effect of gap leakage due to the improper fitting of the face mask in the reduction of the effective filtration efficiency. The model is also extended to include the case with multiple face masks. The model results are verified using experimental data from literature. It is shown that the adverse effect of the gap leakage depends on the face mask filtration and leakage characteristics. It is also shown that wearing more than one face mask might not always be beneficial in some scenarios. Recommendations on the best practice in the case of wearing two face masks are given.