Evaluation of the atmospheric-pressure change (APC) in a tornado is necessary to assess the integrity of nuclear-related facilities. The Rankine model has been most frequently used to theoretically calculate the APC in a tornado. The result, however, is considered to be overly conservative because the Rankine model wind speed at the ground is larger than that in reality. On the other hand, the wind speed of the Fujita model is closer to that of actual tornadoes but is expressed by more complicated algebraic equations than that in the Rankine model. Also, because it is impossible to analytically derive the APC equation using the Fujita model, numerical computation is required. A previous study employed the finite element method (FEM) for such a purpose. However, a general-purpose FEM code often requires complicated input parameters. In order to conduct parametric studies to evaluate the integrity of facilities in various cases of tornadoes, the finite-difference method code “TORPEC”, which is specialized to analyze the APC, was developed as a convenient design tool. TORPEC is based on Poisson’s equation derived from the Navier-Stokes equation. It also runs on widely available technical calculation software such as Microsoft® Excel VBA or MATLAB®. Taking advantage of such convenience, various calculations have been conducted to reveal the characteristics of APC as functions of the maximum tangential wind speed, axial position and tornado radius. TORPEC is used as a benchmark in the existing paper. The case study results obtained by TORPEC show a constant ratio of the pressure drop of the Fujita model against the Rankine model. This factor can be used to derive the Fujita model result from the Rankine model result without FEM analysis.