Vapor Pressure Calculator

Vapor pressure is the equilibrium pressure exerted by a vapor above its liquid in a closed system at a specified temperature. Molecules continually leave and return to the liquid surface. Once evaporation and condensation occur at equal rates, the vapor is saturated and its pressure is the equilibrium vapor pressure. Raising temperature increases the fraction of molecules energetic enough to escape, so vapor pressure normally rises rapidly rather than linearly. This calculator uses the integrated Clausius-Clapeyron equation to transfer a known vapor pressure from one temperature to another. The relationship uses the natural logarithm of the pressure ratio, the molar enthalpy of vaporization, the universal gas constant, and the difference between reciprocal absolute temperatures. Temperatures entered in degrees Celsius are converted internally to kelvins. Enthalpy entered in kilojoules per mole is converted to joules per mole so that it is compatible with the gas constant. The output retains the pressure unit used by the known value, which is kilopascals in this interface. The equation assumes that the vapor behaves approximately as an ideal gas, the liquid's molar volume is negligible relative to the vapor, and the enthalpy of vaporization remains constant over the selected temperature interval. Those assumptions are often reasonable across a moderate range away from the critical point. Accuracy decreases over wide intervals, near phase transitions, at high pressures, or when association and nonideal behavior are important. If reliable Antoine coefficients or a validated property correlation is available for the substance and range, that correlation may provide a better engineering estimate. A liquid boils when its vapor pressure equals the surrounding pressure. Consequently, the calculator can help explore why boiling temperatures fall at altitude or rise in pressurized equipment, although it does not directly solve for boiling temperature. Use trusted experimental data for the reference point and enthalpy. Do not extrapolate into a region where the substance changes phase or decomposes. For process design, safety relief, environmental exposure, or regulatory calculations, consult authoritative property data and apply an equation of state or approved correlation with documented uncertainty.