Altitude Boiling Point Calculator | Water Temperature at Elevation

What the calculator does

The altitude boiling point calculator finds the temperature at which water boils at a given elevation above sea level. Water boils cooler as altitude increases, because there is less air pressing down from above. At sea level water boils at about 100 °C (212 °F); the tool's standard-atmosphere result is 99.97 °C. On Mount Everest's summit, 8,848 m up, it boils at about 70 °C (158 °F).

Enter an elevation and the tool returns the boiling point in Celsius and Fahrenheit, plus the local air pressure. This matters for cooking, food safety, brewing, and lab work, all of which depend on how hot boiling water actually gets.

Why water boils cooler at altitude

Water boils when its vapor pressure — the push of steam trying to escape the liquid — equals the pressure of the air pressing down on it. At sea level, air pressure is high, so water has to get all the way to 100 °C before its vapor pressure can match it. Higher up, the air is thinner and pushes down less hard, so water reaches that balance point at a lower temperature.

Boiling point of water by altitude

How to calculate boiling point at altitude

The calculation has two steps: turn altitude into air pressure, then turn air pressure into a boiling point.

Step 1: altitude to air pressure

Air pressure drops in a predictable way as altitude increases. The tool uses the International Standard Atmosphere (ISA) barometric formula, which holds through the troposphere, from sea level up to about 11 km. In that layer temperature falls at a steady rate and pressure follows a power law:

$P_{\text{inHg}} = 29.921 \times \left(1 - 6.8753\times10^{-6} \times h_{\text{ft}}\right)^{5.2559}$

Here $h_{\text{ft}}$ is altitude in feet, and $P_{\text{inHg}}$ is the local pressure in inches of mercury (a unit meteorologists use for barometric pressure).

Step 2: air pressure to boiling point

Water boils once its vapor pressure equals the surrounding air pressure, so the boiling point at a given altitude is just the saturation temperature of water at that pressure. A simple straight-line fit between pressure and temperature looks tempting, but the real curve bends: a line anchored at sea level drifts several degrees too low by the time it reaches high altitude.

To avoid that error, the calculator uses the IAPWS-IF97 formula, the international standard equation for water and steam properties published by the International Association for the Properties of Water and Steam. It gives the exact saturation temperature for any pressure between water's triple point (where ice, liquid, and vapor can all exist together) and its critical point (where the distinction between liquid and gas disappears):

$T_s(\text{K}) = \text{IAPWS-IF97 Region 4 saturation-temperature equation, } T_s(P)$ $T(°C) = T_s(\text{K}) - 273.15, \qquad T(°F) = T(°C) \times \tfrac{9}{5} + 32$

The full equation combines ten fitted coefficients with a pressure term raised to the power 0.25, then solves a quadratic. IAPWS publishes it for pressures from 611.213 Pa up to 22.064 MPa. It is not something to work out by hand, but it reproduces laboratory-measured steam tables almost exactly, which is why it is the equation power-plant and steam-table engineers rely on.

Worked example

Denver, Colorado, sits at about 1,609 m (5,280 ft), earning it the nickname "the Mile High City."

1. Convert to feet: 1,609 m × 3.28084 = 5,280 ft.
2. Plug into the pressure formula: $P = 29.921 \times (1 - 6.8753\times10^{-6} \times 5{,}280)^{5.2559} \approx 24.64$ inHg, which converts to about 83.4 kPa.
3. Look up the saturation temperature for 83.4 kPa using the IAPWS-IF97 equation: about 94.6 °C (202.3 °F).

So water in Denver boils at roughly 94.6 °C instead of 100 °C — about 5.4 °C cooler than at sea level.

Accuracy and limits

• Below about 11 km: the pressure-to-temperature step is essentially exact, since it is the same equation used to build official steam tables. The main source of error is the altitude-to-pressure step, which assumes the standard atmosphere. Real weather moves local air pressure up and down, which shifts the true boiling point.
• Above 11 km: altitude leaves the troposphere the barometric formula was built for, so results become an extrapolation. The calculator flags this case.
• Dissolved solids: salt or sugar in the water raise its boiling point slightly. The tool assumes pure water.
• Below sea level: places like the Dead Sea shore (about 430 m below sea level) have higher air pressure than sea level, so water there boils slightly above 100 °C — about 101.4 °C (214.5 °F).

Why the boiling point matters

• Cooking: at 2,000 m, water tops out around 93 °C. It still looks like a rolling boil, but it is cooler, so pasta, rice, and eggs need longer to cook. A pressure cooker traps steam and raises the internal pressure, which pushes the boiling point back above 100 °C. This is why high-altitude cooks use them.
• Food and water safety: boiling still kills most waterborne pathogens, but the CDC advises a rolling boil for 1 minute, or 3 minutes at elevations above 6,500 feet (about 2,000 m), since the lower temperature needs more time to work.
• Labs and industry: autoclaves, distillation columns, and reflux setups calibrated at sea level do not behave the same way at elevation and may need adjustment.

Frequently asked questions

Why does water boil cooler at altitude?

Boiling starts when water's vapor pressure matches the surrounding air pressure. There is less air overhead at altitude, so less pressure to match, so water reaches that point at a lower temperature.

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

Roughly 1 °C (about 1.8 °F) per 1,000 ft near sea level. The tool gives 98.96 °C at 1,000 ft. The drop gets slightly steeper higher up: the tool gives about 1.04 °C per 1,000 ft near 25,000 ft. At low pressure, each kilopascal of pressure lost lowers the boiling point more.

What temperature does water boil at on Mount Everest?

About 70 °C (158 °F) at the 8,848 m summit, compared with 100 °C at sea level. That is one reason melting snow for water takes so long on the mountain.

Does water boil hotter below sea level?

Yes. Below sea level, air pressure is higher than the sea-level average, so water needs a slightly higher temperature before its vapor pressure can match it. At the Dead Sea shore, water boils at about 101.4 °C.

Does this affect cooking times?

Yes. Since the water is cooler, food takes longer to cook by boiling. Recipes for high altitude often add cooking time or use a pressure cooker.

How accurate is this calculator?

The pressure-to-temperature step matches official steam tables. The altitude-to-pressure step assumes a standard atmosphere up to 11 km. Weather changes local air pressure and can shift the real result, so for critical work measure the actual barometric pressure instead of relying on altitude alone.

References

1. International Association for the Properties of Water and Steam (IAPWS). Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (IAPWS R7-97(2012)). IAPWS-IF97 release (PDF) — Region 4 saturation-temperature equation Ts(p) with coefficients n1 to n10 (Table 34), β = p^(1/4), valid from 611.213 Pa to 22.064 MPa.
2. NASA Glenn Research Center. Earth Atmosphere Model, metric units. NASA atmosphere model — the troposphere runs from the surface to 11,000 m; temperature falls linearly and pressure follows p = 101.29 × [(T + 273.1)/288.08]^5.256.
3. U.S. Centers for Disease Control and Prevention. Boil Water Advisory. CDC boil water advisory — bring water to a rolling boil for 1 minute; above 6,500 feet, boil for 3 minutes.