Boiling Point at Altitude Calculator

Water does not always boil at exactly 100 degrees Celsius. This temperature is for sea level where the air pressure pushing down on the water is about 101 kilopascals. If you go up in the mountains or drive to Denver, Colorado, the boiling point goes down. That's because there is less weight of air pressing down on the surface of the water.

This calculator allows you to calculate the boiling point and corresponding air pressure for any altitude, or vice versa. You can enter either the air pressure or measured boiling temperature and it will give you the corresponding altitude.

Why does boiling point decrease with increasing altitude?

A liquid begins to boil when its vapor pressure equals the surrounding air pressure. The lower the air pressure, the lower the temperature required to reach that state.

At high altitudes water boils more easily because there are fewer air molecules pushing down on the surface of the water and therefore less heat is required. This means that a pot of water in Boulder will boil at a much lower temperature than an identical pot of water in Boston. The water does boil, but it isn't as hot as you would expect from boiling point at sea level.

How to use this calculator:

Enter the values you have available. If you enter an altitude, you can check the boiling point and local air pressure. You can overwrite the altitude values by entering a barometric pressure from a barometer or weather forecast. Alternatively, you can enter a measured boiling temperature to calculate the corresponding altitude. Regardless of which field you fill in, the other fields will be calculated automatically so you do not have to tell the calculator what field you filled in first.

Please set the units for each field that you normally use. For example, if height is in feet and temperature is in Fahrenheit, then conversion will be automatic. If you want to start over with a different value, first clear the fields entered, then enter new values. When using liquids other than water, please enable the custom liquid function and enter the standard boiling point and vaporization enthalpy of that liquid.

The underlying scientific principles are as follows.

The relationship between altitude and boiling point has two phases. In the first phase, a standard atmosphere model converts altitude to pressure. This model is accurate within about 11 kilometers above sea level.

Here P_0 is the air pressure at sea level (101,325 Pascal), L is the temperature decrease per meter (0.0065 Kelvin) and T_0 is the sea temperature (288.15 Kelvin). The result when the exponent gM divided by RL is approximately 5.2559. In the second step, the pressure is converted into boiling point. For water, the Antoine equation is an accurate and standard approximation formula in the range of 1 to 100 degrees.

The equation is rearranged to calculate temperature where the pressure is given in millimeters of mercury.

For other liquids, the calculator switches to the Clausius–Clapeyron relation. This relation is based on the liquid's standard boiling point and determined by its enthalpy of vaporization.

Denver water, for example.

Denver is about 5280 feet above sea level. When this value is entered into a standard atmosphere model, it yields an air pressure of about 83.4 kilopascals, which is about 82 percent of the air pressure at sea level.

Entering this pressure into the Antoine equation gives a boiling point of about 94.7 degrees Celsius or 202 Fahrenheit. So the maximum temperature of water in a pot in that high city will be more than five degrees less than the 100 degrees Celsius given in textbooks.

Boiling point of water at typical altitudes

The following table shows changes in water at common altitudes. Note that air pressure and boiling point decrease together. At first they drop quickly but as altitude increases the decrease becomes more gradual.

What that actually means

Chefs in high altitude regions adjust their recipes for the lower boiling point of water. At 2,500 meters (8,200 feet), the temperature of water in a pot is about 91 degrees Celsius, so eggs, legumes, dried pasta and rice all take longer to cook. Many cake and bread recipes also require adjustments for high altitudes.

Hikers and people who purify water by boiling must increase the cooking time at high altitude. This is because cold water kills germs more slowly, and boiling is precisely what's needed to kill those germs. Engineers designing pressure cookers, boilers or food processing plants incorporate corrections for altitude directly into their specifications. In any laboratory doing distillation or phase change experiments far from sea level, the textbook values are less important than the local boiling point.

Other liquids than water

Since all liquids are affected by air pressure, the boiling point of any liquid will decrease with increasing altitude. When the custom fluids function is enabled, the calculator can process any fluid if two values are known: the boiling temperature at sea level and the molar enthalpy of vaporization. For example, ethanol has a boiling temperature of about 78.4 degrees Celsius at sea level and an enthalpy of vaporization of about 38.6 kilojoules per mole. In the mountains, its boiling point will be several degrees lower.

How to get accurate values

The more accurate the altitude information is, the more accurate the result will be. Using a GPS device, topographic map or online elevation tool can reduce error to within a few meters. Weather also plays a role; high and low pressure systems passing over can slightly alter actual air pressure, leading to boiling point deviations of a few tenths of a degree from standard atmosphere estimates. Above about 9 km (5.6 mi), simple models start to show errors, so values at extreme altitudes should be considered guidelines rather than guaranteed values.