Use a vapor pressure table for the substance at the desired temperature
Use the Antoine equation if constants for the substance are available
Use the Clausius–Clapeyron equation if you know two vapor pressure–temperature points or the enthalpy of vaporization
Measure the pressure of the vapor in equilibrium with its liquid or solid in a closed container
Use a manometer or pressure sensor to record the equilibrium pressure
Ensure the system is at constant temperature
Wait until liquid and vapor reach equilibrium
Read the equilibrium pressure as the vapor pressure
For mixtures, use Raoult’s law if appropriate
For water, use standard steam tables or water vapor pressure charts
Convert units if needed to match the required pressure scale