This online calculator computes saturation vapor pressure for a given temperature and atmospheric (barometric) pressure. Theory and formulae are given below the calculator.
Let's start from a couple of definitions which you can find on Wikipedia:
A vapor is a substance in the gas phase at a temperature lower than its critical point. This means that the vapor can be condensed to a liquid or 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 occurs 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 vapor's density and pressure increase. When the escape and return process 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 vapor. When the vapor is said to be saturated, the vapor pressure is called the saturation vapor pressure (or saturated vapor 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.