Abstract: Loop inductance is a key parameter in the exploding foil initiation system (EFIS). Calculation of inductance is the foundation in studying small sized EFIS. A simplified physical model was given for EFIS. Three assumed conditions were set up. Using Laplace transform, a formula for the calculation of inductance of EFIS through current curve was deduced. The formula was checked by experimental data and curves from Quantic Inc. The result shows the precision with which the inductance value has attained. The formula promoted here is a useful tool on the study of EFIS.