How do you calculate water vapor pressure?

Water vapor pressure can be calculated using the Clausius-Clapeyron equation, which relates vapor pressure to temperature. The equation is: ln(P2/P1) (Hvap/R)(1/T1 - 1/T2), where P1 and P2 are the vapor pressures at temperatures T1 and T2, Hvap is the heat of vaporization, and R is the gas constant.