Water Does Not Always Boil at 212 °F

A pressure cooker runs the same physics backwards

If lowering the pressure lowers the boiling point, raising it raises the boiling point — and that is the whole mechanism of a pressure cooker. Hold the inside at one bar above the outside and water boils at 249 °F instead of 212 °F; at the 15 psi that domestic cookers are built around, 250 °F. Thirty-seven degrees does not sound like much until you remember that reaction rates climb steeply with temperature: it is the difference between an hour of simmering and twenty minutes. It is also why the same appliance is used to sterilise: 250 °F held under pressure is the standard condition for an autoclave, and it is unreachable in an open pan at any altitude.

Weather moves it too, just less

The calculator takes a pressure directly as well as an altitude, which is the honest way to use it, because the altitude route assumes a standard atmosphere and the real one moves. A deep low can take the sea-level pressure down toward 28.4 inches of mercury and a strong high can push it past 30.5, which moves the boiling point 2.8 °F down and 1.1 °F up — small next to altitude, but not zero, and it is why a thermometer in a pan of boiling water is a rough barometer. If you have a reading from a barometer or a weather station, enter that instead: the answer stops being an estimate of your location and becomes a statement about the pan in front of you.

Frequently asked questions

Do I need to change recipes at altitude?

Anything cooked in water needs more time, because the water is cooler — pulses and grains are the usual culprits, and above about 3,000 feet extension guides start recommending a pressure cooker rather than longer simmering. Baking is a separate problem with the same cause: lower pressure lets leavening gases expand more, so cakes rise faster and can collapse, and liquids evaporate quicker. Frying and roasting are barely affected, since they never involve boiling water.

How accurate is the altitude route?

Good to a fraction of a degree for everyday use, and it is worth knowing what it assumes. The pressure at a given altitude comes from the standard atmosphere — a fixed sea-level pressure and a fixed temperature profile — so a cold day or a passing depression will shift the real value slightly. The step from pressure to boiling point uses the Clausius–Clapeyron relation with water's enthalpy of vaporisation held constant, which is accurate over the range a kitchen or a mountain will ever produce and drifts only at extremes far outside it.

Does salting the water change the boiling point?

It raises it, and by far too little to matter. A tablespoon of salt in a large pan lifts the boiling point by a fraction of a degree — a rounding error next to the several degrees that altitude is worth. Salt is added for taste and for what it does to the food, not to make the water hotter.