We have performed an accurate frequency calibration of the 4s 2 1 S 0 → 4s4p 1 P 1 principal resonance line of the neutral calcium atom at 423 nm. Doppler-free cw excitation on a Ca atomic beam was performed by utilizing a Sagnac geometry in the alignment of the excitation beams. From frequency calibrations against a frequency comb, stabilized to a global positioning system (GPS) disciplined Rb standard, the transition frequency is determined at 709 078 373.01(35) MHz for the main 40 Ca isotope. Slightly lower accuracies are obtained for the transition frequencies of the less abundant isotopes. The achieved fractional uncertainty of 5 × 10 −10 exceeds the requirements for including this transition in investigations that aim to probe a possible variation in the fine-structure constant α on cosmological time scales.