Boiling Point at Altitude Calculator

Water does not always boil at 100°C (212°F). That number assumes standard atmospheric pressure at sea level, 101.325 kPa. Climb a mountain or fly inland to Denver and the boiling point drops, because there is less air pressing down on the water's surface. This calculator returns the boiling point of water for any elevation, along with the atmospheric pressure at that altitude.

Why altitude changes the boiling point

A liquid boils when its vapor pressure matches the surrounding air pressure. Lower the air pressure and the threshold falls. At 3,000 meters there are fewer air molecules pushing down on the surface, so water molecules slip into vapor at a lower temperature. That is why pasta takes longer to cook in Boulder than in Boston: the water is hot, but not 100°C hot.

The calculator use a formula which is created by combining the Barometric Formula (which determines atmospheric pressure at a given height) directly into the Clausius-Clapeyron Equation (which determines boiling point based on pressure), substituting all standard physical constants with their numerical values, we get

• $T_b$: The boiling point at the new altitude/pressure (in Kelvin).

• $T_0$: The standard boiling point of the liquid at sea level (e.g., $373.15\text{ K}$ or $100^\circ\text{C}$ for water).

• $P_0$: Standard atmospheric pressure at sea level ($1\text{ atm}$ or $101,325\text{ Pa}$).

• $P$: The actual atmospheric pressure at your current altitude.

• $R$: The ideal gas constant ($8.3144\text{ J}/(\text{mol}\cdot\text{K})$).

• $\Delta H_{vap}$: The molar enthalpy of vaporization (the amount of heat energy needed to turn one mole of the liquid into gas; for water, this is about $40,660\text{ J}/\text{mol}$).

How to use it

Type your altitude above sea level in whatever units you prefer: meters, kilometers, feet, or miles. The atmospheric pressure and boiling point appear next to it. Toggle between metric and imperial at any time. People come at this number from very different angles, depending on whether they are cooking, hiking, or running a lab experiment.

Where it actually matters

Cooks at elevation have to rewrite recipes. A rolling boil at 2,500 meters sits closer to 91°C than 100°C, so eggs, beans, and pasta need extra time. Backpackers boiling water for safety should hold the boil longer up high, since the lower temperature is less effective at killing pathogens. Engineers designing pressure cookers, boilers, or food-processing equipment bake the altitude correction into their specs. And anyone running a phase-change experiment outside of sea level needs the local boiling point, not the textbook one.

Getting an accurate reading

The closer your elevation, the closer your answer. A GPS, a topo map, or an online elevation finder will land you within a few meters. Weather matters too: a passing low-pressure system can shift the actual boiling point by a degree or so, since the calculator assumes a standard atmosphere. Above roughly 9,000 meters the simple barometric model starts to break down, so treat those readings as a ballpark.

Frequently asked questions

Why does water boil at a lower temperature up high?

Boiling happens when vapor pressure equals atmospheric pressure. Less atmosphere above you means a lower threshold for that match to occur, and a lower threshold means a lower boiling temperature.

How much does the boiling point drop per 1,000 feet?

Roughly 1°F per 500 feet, or about 1°C per 300 meters. The drop is steeper close to sea level and flattens out as you climb, so the rule of thumb gets less reliable above the cruising altitude of a small plane.

Does this apply to liquids other than water?

Every liquid's boiling point depends on atmospheric pressure. The formula here is fit to water specifically, but the same physics governs ethanol, mercury, or anything else with a vapor pressure curve.