A new method for calibration of water vapor Raman lidars based on first principles is proposed. The calibration constant of a Raman lidar is derived from a set of water vapor and nitrogen Raman backscatter signals measured with the lidar receiver in a cell filled with reference humidity mixture. The reference humidity mixture is prepared gravimetrically. The water vapor mixing ratio is calculated directly as ratio of the mass of water to the mass of air or nitrogen. Since mass is a fundamental quantity, this method yields an absolute value of water vapor mixing ratio which translates to the lidar calibration constant.