Abstract. We measured the mobility equivalent critical dry diameter for cloud condensation nuclei (CCN) activation (d_{c_me}) and the particle mass of size-selected (NH₄)₂SO₄ and NaCl particles to calibrate a CCN counter (CCNC) precisely. The CCNC was operated downstream of a differential mobility analyzer (DMA) for the measurement of d_{c_me}. The particle mass was measured using an aerosol particle mass analyzer (APM) operated downstream of the DMA. The measurement of particle mass was conducted for 50–150-nm particles. Effective densities (ρ_eff) of (NH₄)₂SO₄ particles were 1.67–1.75 g cm⁻³, which correspond to dynamic shape factors (χ) of 1.01–1.04. This shows that (NH₄)₂SO₄ particles are not completely spherical. In the case of NaCl particles, ρ_eff was 1.75–1.99 g cm⁻³ and χ was 1.05–1.14, demonstrating that the particle shape was non-spherical. Using these experimental data, the volume equivalent critical dry diameter (d_{c_ve}) was calculated, and it was used as an input parameter for calculations of critical supersaturation (S). Several thermodynamics models were used for the calculation of water activity. When the Pitzer model was employed for the calculations, the critical S calculated for (NH₄)₂SO₄ and NaCl agreed to well within the uncertainty of 2% (relative). This result demonstrates that the use of the Pitzer model for the calibration of CCNCs gives the most accurate value of S.