Here, we present a calibration of the Neutral cluster and Air Ion Spectrometer (NAIS, Airel Ltd.) for the size and concentration of ions in the mobility-diameter size-range 0.98-29.1 nm. Previous studies raised accuracy issues in size and concentration determination and highlighted the importance of used data inversion algorithm. Therefore, we investigated the performance of the NAIS by using five inversion methods. The presented results illustrate that the size information given by the NAIS is very accurate, regardless of the version of the data inversion. The number concentrations determined by the NAIS were 15%-30% too low especially at the lower end of the measurement size range (< 5 nm), whereas concentrations at diameters 19.6 nm and larger were overestimated by up to 8%. With the correction presented in this study, the uncertainty of the ion concentration measurement of the NAIS can be reduced to less than 10%, allowing the NAIS to be used in quantitative ion cluster studies and more accurate determination of formation and growth rates.