Boiling Point at Altitude Calculator (water)

Understanding the boiling point at altitude calculator — concept and principle

Water boils when its vapour pressure equals the surrounding air pressure. Because air pressure falls as you climb, water boils at a lower temperature in the mountains. Enter an altitude and the calculator estimates the air pressure and the boiling point of pure water there. It is useful for hikers and for cooks adjusting recipes at high elevation.

At 2,000 m the air presses less (79.50 kPa), so water boils at 93.4 °C.

How it is calculated

Air pressure from the International Standard Atmosphere (ISA) barometric formula for the lowest layer: P = 101.325 × (1 − 2.25577 × 10⁻⁵ × h)^5.25588 kPa, where h is the altitude in metres.
Conversion to mmHg: P_mmHg = P_kPa × 7.50062.
Boiling point from the Antoine equation for water, log₁₀(P_mmHg) = A − B / (C + T), solved for T: T = B / (A − log₁₀ P_mmHg) − C, with A = 8.07131, B = 1730.63 and C = 233.426 (T in °C).

The principle

The ISA formula describes an atmosphere whose temperature falls by 6.5 °C per kilometre from 15 °C at sea level; integrating the balance between air weight and pressure under that assumption gives the power law above. The Antoine equation is an empirical refinement of the Clausius-Clapeyron relation: vapour pressure rises roughly exponentially with temperature, and the constant C corrects the curvature. Setting the vapour pressure equal to the local air pressure and solving for temperature gives the boiling point.

Worked example

Input: h = 2000 m.
Pressure: 1 − 2.25577 × 10⁻⁵ × 2000 = 0.954885; 0.954885^5.25588 × 101.325 = 79.49519744 kPa.
In mmHg: 79.4952 × 7.50062 = 596.263 mmHg; log₁₀ = 2.775438.
8.07131 − 2.775438 = 5.295872; 1730.63 / 5.295872 = 326.788.
Boiling point: 326.788 − 233.426 = 93.36249078 °C.

Each point is the calculator re-run with only “Altitude (m)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Altitude (m)	Air pressure (kPa)
500	95.46083403
1,000	89.87456043
1,500	84.55599052
2,000	79.49519744
2,500	74.68251228
3,000	70.10852041
4,000	61.64020644

How to read the result

As a rule of thumb, the boiling point drops by about 1 °C for every 300 m of altitude near sea level. The formula gives about 96.7 °C at 1000 m, 93.4 °C at 2000 m and 90.0 °C at 3000 m; near the summit of Mount Everest it is around 70 °C. Lower boiling temperature means food cooks more slowly in boiling water, so pasta, eggs and beans take longer, and a pressure cooker restores higher temperatures.

Common mistakes

Entering altitude in feet; convert first (1 ft = 0.3048 m).
Expecting the exact boiling point on a given day; weather systems change air pressure by a few kPa.
Applying the result to salty or sugary water, which boils slightly higher.

Keep in mind: The ISA describes average mid-latitude conditions and the barometric formula used is valid up to 11 km. The Antoine constants are fitted for roughly 1 to 100 °C. Real local pressure varies with weather, so treat the result as an estimate, not a calibration reference.

Frequently asked questions

Why does food take longer to cook at high altitude?

Boiling water cannot get hotter than its boiling point, so at 2000 m it stays near 93 °C instead of 100 °C, and cooking reactions run more slowly.

Can I use my barometer reading instead of altitude?

Yes. The boiling point depends on actual pressure, so a measured absolute pressure (not the sea-level-adjusted value from weather reports) is more accurate.

Sources: ISO 2533:1975, Standard Atmosphere · U.S. Standard Atmosphere, 1976, NOAA, NASA and U.S. Air Force · NIST Chemistry WebBook, NIST Standard Reference Database 69, "Water: phase change data"

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

Your numbers