An empirical formula for calculating the IR extinction of aerosol particles of brass 70 Cu/30 Zn over the IR frequency range was derived based on extensive calculations made on a body-center-like model having a widely varying number of particles ranging from 3.2 x 104 to 2.56 x 105. The expansion coefficients in the empirical formula ere determined by the least squares curve fitting of numerical data obtained using the finite difference time domain (FDTD) method. The formula satisfied the frequency dependence of the extinction in the frequency range from 2.143 x 1013 to 3.75 x 1013 Hz. The expression for the resonant particle size (d0) and the constant coefficients in the empirical formula were derived in terms of the wavelength λ, particle size d, and number of the primary particles N of aerosol particles. Numerical results that were obtained from the empirical formula generally agreed well with those calculated by the FDTD method for aerosol particles of brass 70 Cu/30 Zn with a magnitude of particle size in the range from 0.02 to 0.5 μm. The average error in the extinction calculation was less than 5%. The formula thus provides a simple and inexpensive method for calculating the IR extinction of aerosol particles, which otherwise requires complicated and expensive calculation methods.