This online calculator computes saturation vapour pressure for given temperature and atmospheric (barometric) pressure. Some theory and formulae are given below the calculator
Let's start from couple of definitions which can easily be found at wikipedia or whatever:
A vapour is a substance in the gas phase at a temperature lower than its critical point. This means that the vapour can be condensed to a liquid or to a solid by increasing its pressure without reducing the temperature. Evaporation is a type of vaporization of a liquid that only occurs on the surface of a liquid.
If evaporation takes place in an enclosed area, the escaping molecules accumulate as a vapor above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapor increases. When the process of escape and return reaches an equilibrium, the vapor is said to be "saturated," and no further change in either vapor pressure and density or liquid temperature will occur.
Vapour pressure is the pressure exerted by a vapour. When the vapour is said to be saturated, the pressure of that vapour is called the saturation vapor pressure (or saturated vapour pressure).
This calculator uses formulae obtained from Guide to Meteorological Instruments and Methods of Observation, World Meteorological Organization, 2008.
So, saturation vapour pressure in the pure phase is
Saturation vapour pressure of moist air is
where pressure function is
Units of temperature are degrees centigrade and units of pressure are hectopascals (hPa). 1 hectopascal = 100 pascals.