Recalculation algorithm for intercomparison of atmospheric spectra with different spectral resolution is presented. It is intended to compare measurements (level 1c data) of infrared Fourier transform spectrometers with different spectral resolution, apodizations and spectral grids. Firstly, the algorithm is dedicated to the routine intercalibration of IKFS-2 and IASI instruments (IASI is used as a reference). Secondly, it is planned to be used for climate investigations based on IKFS-2 (Meteor-M No. 2, 2014) and SI-1 (Meteor-28/29, 1977 and 1979) measurements. Used as an example of IKFS-2 to SI-1 spectral conversion, detailed step-by-step algorithm description is provided. To estimate recalculation error, line-by-line spectra for standard atmosphere convolved with IKFS-2 and SI-1 instrument line shape was used. The difference between modelled IKFS-2 spectra converted to SI-1 spectral data parameters and modelled SI-1spectra does not exceed radiometric noise level of the instruments. Finally, the issues of conversion error and radiometric noise in converted spectra are also discussed.