Boiling Point at Altitude Calculator
This calculator works out the boiling point of water at any altitude, along with the air pressure that sets it. Water boils when its vapour pressure rises to match the pressure of the air pushing down on it. At sea level that balance is reached at 100 degrees Celsius, but as you climb the air thins and its pressure falls, so the water reaches the balance sooner and boils at a cooler temperature. The effect is real enough to matter in the kitchen: at altitude, food that cooks in boiling water takes longer, because the water is simply not as hot. Enter your elevation in metres and the calculator estimates the local air pressure with the barometric formula, then converts that pressure into a boiling temperature using the Antoine equation for water, a well-established relationship between pressure and boiling point. It returns the boiling point in degrees Celsius plus the air pressure in kilopascals and as a percentage of sea-level pressure, so you can see how much thinner the air has become. As a rule of thumb, the boiling point drops by roughly 1 degree for every 300 metres you gain. Trampers, high-country cooks, home brewers and science students use it to adjust cooking times and to understand why a pressure cooker, which raises the internal pressure, restores a higher boiling point. Note this is the pure-water boiling point: dissolved salt or sugar nudges it up slightly.
Enter 0 for sea level. Use negative values for below sea level.
At 1,000 m the air pressure is about 88.7% of sea level, so water boils at roughly 96.7 °C, about 3.3 degrees below the sea-level boil. Expect cooking to take a little longer.
Boiling point for pure water, from the barometric formula and Antoine equation. Weather changes the actual pressure day to day. Estimate only.
How it works
The calculator runs in two steps. First it estimates the air pressure at your altitude using the barometric formula: pressure equals 101.325 kPa times (1 minus 0.0000225577 times the altitude in metres) raised to the power 5.25588. Then it converts that pressure to a boiling temperature with the Antoine equation for water, which links vapour pressure and temperature. The pressure is turned into millimetres of mercury and fed into the equation, returning the temperature at which water's vapour pressure equals the local air pressure, which is the boiling point. At sea level the formula returns almost exactly 100 degrees Celsius, confirming the method.
Worked example
Suppose you are at 1,000 metres. The barometric formula gives an air pressure of about 89.87 kilopascals, which is 88.7 percent of the sea-level value of 101.325 kilopascals. Feeding that pressure into the Antoine equation returns a boiling point of about 96.7 degrees Celsius, roughly 3.3 degrees below the sea-level boil. Climb to 3,000 metres and the pressure falls to about 70.11 kilopascals, dropping the boiling point to around 90 degrees, so a soft-boiled egg or a pot of pasta needs noticeably longer.
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