What this calculator does
Water’s boiling point is not a fixed 100°C: that figure only holds at sea-level atmospheric pressure. As elevation increases, the air above thins and its pressure drops, and water boils at a lower temperature because less pressure is needed to let its vapour escape. This water boiling point calculator works out that temperature from the elevation you enter.
The effect is large enough to matter in practice. On top of a high mountain, water can boil at 15 to 20 degrees below the sea-level figure, which is why cooking times need adjusting at altitude and why pressure cookers exist: they raise the pressure back up so food cooks at closer to the temperature it would at sea level.
The formula
Atmospheric pressure at a given elevation is worked out from the standard barometric formula for the troposphere. That pressure is then fed into the Clausius-Clapeyron relation, which links a liquid’s vapour pressure to its temperature, using water’s molar heat of vaporisation, to solve for the temperature at which water’s vapour pressure equals the surrounding air pressure: the boiling point.
| Term | Meaning |
|---|---|
| Elevation | Height above sea level, the only input this calculator needs. |
| Atmospheric pressure | The weight of the air column above a point, which falls as elevation rises. |
| Boiling point definition | The temperature at which a liquid’s vapour pressure equals the surrounding atmospheric pressure, so it can turn to vapour throughout its volume, not just at the surface. |
The inputs explained
| Field | What to enter |
|---|---|
| Elevation | Height above sea level, in the unit chosen next to it. |
| Elevation unit | Whether the elevation above is in metres or feet. |
When to use it
Adjusting a recipe for altitude
Recipes written at sea level assume water boils at 100°C. At altitude it boils cooler, so food that relies on boiling water, like pasta or boiled vegetables, takes longer to cook and recipes often suggest lowering baking temperatures or extending times as well.
Understanding why a pressure cooker helps at altitude
A pressure cooker raises the pressure inside the pot above the (lower) outside atmospheric pressure, pushing the boiling point of the water inside back up towards, or above, the sea-level figure.
Sanity-checking a thermometer reading
If water at a rolling boil reads well below 100°C on a thermometer, checking the boiling point expected at the local elevation shows whether that is normal physics rather than a faulty thermometer.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the boiling point of water falls with elevation
Boiling point and atmospheric pressure at a range of elevations.
| Elevation | Boiling point of water | Atmospheric pressure at this elevation | Pressure relative to sea level |
|---|---|---|---|
| 0 m | 100.0 °C | 101 kPa | 100.0% |
| 500 m | 98.3 °C | 95 kPa | 94.2% |
| 1,000 m | 96.6 °C | 90 kPa | 88.7% |
| 1,500 m | 94.9 °C | 85 kPa | 83.5% |
| 2,000 m | 93.2 °C | 79 kPa | 78.5% |
| 3,000 m | 89.8 °C | 70 kPa | 69.2% |
| 4,000 m | 86.4 °C | 62 kPa | 60.8% |
| 5,000 m | 82.9 °C | 54 kPa | 53.3% |
| 8,848 m | 69.4 °C | 31 kPa | 31.0% |
Questions
What is the boiling point definition, exactly?
It is the temperature at which a liquid’s vapour pressure equals the pressure of the surrounding atmosphere, allowing bubbles of vapour to form throughout the liquid rather than only evaporating from its surface. Because that surrounding pressure changes with elevation, so does the boiling point.
Why does water boil at a lower temperature at altitude?
Air pressure falls as elevation increases because there is less atmosphere pressing down from above. Since boiling only needs the liquid’s vapour pressure to match that surrounding pressure, a lower surrounding pressure means the liquid reaches that match, and starts boiling, at a lower temperature.
Is 100°C ever wrong for water at sea level?
Local weather causes small day-to-day swings in atmospheric pressure even at a fixed elevation, so the true boiling point can vary by a fraction of a degree either side of 100°C at sea level. The figures here are for standard atmospheric conditions and should be treated as a close approximation, not an exact reading for a specific day.
Does this apply to liquids other than water?
The same underlying relationship, pressure versus vapour pressure, applies to any liquid, but each one has its own vapour pressure behaviour and heat of vaporisation. This calculator is set up specifically for water and should not be used to estimate the altitude boiling point of other liquids.
For how a dissolved solute, rather than elevation, changes a liquid’s boiling point, see the boiling point elevation calculator.