State 1 (The normal boiling point)
Boiling point
State 2
Boiling point

Boiling Point Calculator

By Bogna Szyk
Last updated: Feb 25, 2021

This boiling point calculator tells you how to calculate the boiling point of most common substances at an arbitrary pressure, basing on the Clausius–Clapeyron relation. Whether you want to analyze water, ethanol or ammonia, simply provide some reference values and this calculator will do the work for you. Read on to learn what exactly is the boiling point definition and what equation can you use to determine its value.

Boiling point definition

Boiling point is simply the temperature at which water starts boiling - in other words, it changes its state from liquid to gas. This temperature is dependent on pressure and the substance latent heat of vaporization. The latter property is unique for each substance - you can be sure that two samples of water will have the same latent heat.

Clausius–Clapeyron relation

Our boiling point calculator uses the Clausius–Clapeyron relation to establish the boiling point of any substance at a given pressure. This equation characterizes phase transitions (such as vaporization) and relates the pressure to boiling point in a following way:

ln(P₁/P₂) = -ΔH/R * (1/T₁ - 1/T₂)


  • P₁ is the pressure at state 1;
  • P₂ is the pressure at state 2;
  • T₁ is the boiling point at state 1 (at pressure equal to P₁);
  • T₂ is the boiling point at state 2 (at pressure equal to P₂);
  • ΔH is the latent heat of vaporization of the substance, measured in J/mol;
  • R is the gas constant, equal to 8.314 J/(K * mol). It is also used by our ideal gas law calculator.

How to calculate the boiling point

  1. Choose your substance. Is it water, or something different? Note down its latent heat of vaporization.
  2. Perform an empirical experiment. Heat up a sample of the substance and check at what temperature it starts to boil. This will be your T₁.
  3. Measure the pressure of the surroundings (preferably using a barometer). Note down this value - it will be your P₁.
  4. Decide for what pressure you want to calculate the boiling point. This value is the P₂.
  5. Insert all of the values into the Clausius–Clapeyron equation to find the boiling point T₂. You can also use our boiling point calculator instead and save yourself some time.

This calculator has the values of P₁ and T₁ set to 1013.25 hPa and 100°C respectively. These values correspond to the normal atmospheric pressure at the sea level and boiling point of water. You need to set different values if you are calculating the boiling point at altitude or are analyzing a different substance.

Bogna Szyk
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