Clausius-Clapeyron Calculator: Vapour Pressure, Boiling Point and Heat of Vaporisation

How vapour pressure changes with temperature.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose what you want to find.

Enter the pressure and temperature at the first point, and the other values.

The calculator gives the answer.

Formulas used

  • \ln\frac{P_2}{P_1} = -\frac{\Delta H}{R}\left(\frac{1}{T_2} - \frac{1}{T_1}\right)
  • T_2 = \left[\frac{1}{T_1} - \frac{R\ln(P_2/P_1)}{\Delta H}\right]^{-1}

Worked example

With the default numbers in the calculator above:

  1. Vapour pressure at the new temperature: 70.6361 kPa

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: What do you want to find? The temperature T2 at which the pressure is P2.

  1. Temperature at the new pressure: 363.136 K (89.986 C)

Common questions

What does the Clausius-Clapeyron equation do?

It links the vapour pressure of a liquid to its temperature, using the heat of vaporisation.

Why does water boil at a lower temperature at altitude?

The air pressure is lower, so the vapour pressure needed to boil is reached at a lower temperature.

How accurate is it?

Good over a modest range if the heat of vaporisation is nearly constant. For wider ranges use the Antoine equation or tables.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any thermodynamics or physical chemistry text (the Clausius-Clapeyron equation).

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