Boiling Point at Altitude Calculator
Pressure falls with altitude and water boils when vapor pressure matches it. The calculator uses the standard barometric formula plus the Clausius-Clapeyron equation.
Result
- Barometric pressure
- 702 hPa
How the calculation works
Two physical laws stack: pressure falls with altitude, and water boils where vapor pressure meets it.
ln(p/101325) = −L/Rᵣ × (1/T − 1/373.15), L = 2.45 MJ/kg
The first line is the standard barometric formula; the second is the Clausius-Clapeyron relation solved for the boiling temperature at that pressure. Output shows both the boiling point and the estimated pressure in hPa.
What the model ignores: weather. The standard atmosphere is an average, not a law - it assumes 1013 hPa and a 15 °C sea-level air column. High- and low-pressure systems move the real boiling point by roughly ±0.5-0.7 °C on any given day, and cold versus warm air columns shift it by about another degree at altitude. That margin is irrelevant for cooking times and water purification, but it means the output is a seasonal average, not a reading for the day you are standing there.
What it means at the stove - fuel planning, cook times and pressure-ish workarounds - is in the altitude cooking guide. The temperature that goes with that altitude is on the altitude temperature calculator.
Planning estimate only - not safety advice.
Frequently asked questions
Why does water boil below 100 °C at altitude?
Boiling happens when the vapor pressure of water equals the air pressure above it. Air pressure falls roughly exponentially with altitude - about 11% per 1,000 m - so the matching temperature falls with it: about 90 °C at 3,000 m and 86 °C at 4,000 m.
Does food cook slower because the water is cooler?
Yes, and it is purely the temperature: chemical cooking rates roughly halve per 10 °C drop. Pasta and rice that take 10 minutes at sea level take noticeably longer at 3,000 m, and beans - cooked at 90 °C or less - slow dramatically. The rule of thumb is about 5-10% longer per 300 m.
Do I need to boil water longer to purify it at altitude?
No. Pasteurization kills pathogens well below boiling; a rolling boil at any altitude is instantly lethal to waterborne pathogens, and one minute of additional boiling (three at very high altitude, mostly as a habit for margin) is the standard advice. The lower temperature is not a safety problem.
Does weather affect the boiling point at altitude?
Yes - through pressure, which is the only variable boiling actually responds to. Altitude is just a convenient proxy for it. A strong high-pressure day versus a deep low moves the boiling point by about ±0.5-0.7 °C at the same altitude, and cold versus warm air columns shift it by roughly another degree. The calculator assumes the standard atmosphere (1013 hPa, 15 °C at sea level), so treat the output as a seasonal average - the deviation is far too small to matter for cooking times or purification.
How much extra stove fuel should I plan?
Two effects stack: the cooler boil slows cooking, and thinner air reduces some stoves' output. Planning +20-30% fuel above 3,000 m, plus more in wind or cold, keeps the margin comfortable. Simmer-lid cooking and heat exchanger pots recover much of it.