Vapor Pressure Calculator (Clausius-Clapeyron)
The vapor pressure of a liquid rises steeply as it warms, and the Clausius-Clapeyron equation is the standard tool for predicting exactly how much. The physics behind it is intuitive: heat gives molecules more energy, so more of them can break free of the liquid surface and enter the gas phase, pushing the vapor pressure up. The equation ties that temperature dependence to a single property, the enthalpy of vaporization, the energy it takes to boil one mole of the liquid. This calculator uses it to find the vapor pressure at a new temperature when you already know the pressure at one reference temperature. Enter the known vapor pressure and its temperature, the enthalpy of vaporization, and the new temperature you are interested in. Temperatures must be in kelvin because the equation works with absolute temperature, and the tool uses the gas constant of 8.314 joules per mole per kelvin. It returns the predicted vapor pressure at the new temperature. The result is most accurate over modest temperature ranges, where the enthalpy of vaporization stays nearly constant. Every figure is computed deterministically from the standard form of the equation, with a worked example below that reconciles exactly to the calculator.
The Clausius-Clapeyron equation predicts vapor pressure at a new temperature: P2 = P1 exp[-(dHvap/R)(1/T2 - 1/T1)]. For water at 101,325 Pa and 373.15 K with dHvap 40,700 J/mol, the vapor pressure at 363.15 K is 70,603.33 Pa.
Clausius-Clapeyron formula
P2 = P1 x exp( -(dHvap / R) x (1/T2 - 1/T1) )
P1 = known vapor pressure, T1 = its temperature (K)
T2 = new temperature (K)
dHvap = enthalpy of vaporization (J/mol)
R = 8.314 J/(mol K)
The bracketed term measures how far the new temperature sits from the reference. The exponential turns that into the ratio between the two vapor pressures.
Worked example
Water has a vapor pressure of 101,325 Pa at 373.15 K, with an enthalpy of vaporization of 40,700 J/mol. Find the vapor pressure at 363.15 K.
- 1/T2 - 1/T1 = 1/363.15 - 1/373.15 = 0.00275365 - 0.00267989 = 0.00007376.
- dHvap / R = 40,700 / 8.314 = 4,895.36.
- Exponent = -(4,895.36 x 0.00007376) = -0.36110.
- P2 = 101,325 x exp(-0.36110) = 101,325 x 0.69680 = 70,603.33 Pa.
These are the calculator's default inputs, so the result above matches the widget exactly.
Vapor pressure calculator: frequently asked questions
What does the Clausius-Clapeyron equation describe?
It relates the vapor pressure of a liquid to temperature. As temperature rises, more molecules have enough energy to escape the liquid, so vapor pressure climbs steeply. The equation lets you predict the vapor pressure at one temperature if you know it at another, given the enthalpy of vaporization.
What is the enthalpy of vaporization?
It is the energy needed to turn one mole of liquid into vapor at constant temperature, written as the heat of vaporization. For water it is about 40,700 joules per mole near the boiling point. A larger value means vapor pressure changes more sharply with temperature.
Why must temperatures be in kelvin?
The equation involves the reciprocal of absolute temperature, so temperatures must be on the kelvin scale, which starts at absolute zero. Using Celsius would give nonsense because the reciprocal of a value near zero or negative is undefined or meaningless. Add 273.15 to a Celsius temperature to get kelvin.
Is the equation exact?
It is an approximation that assumes the enthalpy of vaporization is constant over the temperature range and that the vapor behaves ideally. Over modest temperature spans it is accurate. Over wide spans the enthalpy itself varies, so the prediction drifts from measured values.
What is the Clausius-Clapeyron formula?
P2 equals P1 times the exponential of negative (the enthalpy divided by the gas constant) times (one over T2 minus one over T1). P1 and T1 are the known pressure and temperature, T2 is the new temperature, and the gas constant R is 8.314 joules per mole per kelvin.
Official sources
- Thermophysical properties, the gas constant and vapor pressure data: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.