The vapor pressure of water at 25 degrees Celsius is 23.8 mmHg. The formula for calculating the vapor pressure of water is P = exp (20.386 - 5132 /T) mmHg. "P" stands for the vapor pressure in mmHg and "T" stands for the temperature in Kelvin.
Another formula for calculating the vapor pressure of water is log10P = A - B/C+T. The temperature in Celsius is represented by T. The vapor pressure in mmHg is represented by P. Three constants are required to use this formula. For temperatures between 1 and 100 degrees Celsius, A = 8.07131, B=1730.63 and C=233.426. For temperatures between 99 and 374 degrees Celsius, A= 8.14019, B= 1810.94 and C= 244.485.