Boiling Point Altitude Calculator
Water does not always boil at 100 degrees Celsius. It boils when its vapour pressure matches the surrounding air pressure, and that pressure drops as you climb. This calculator combines two pieces of physics: the barometric formula from the U.S. Standard Atmosphere, which gives the air pressure at your altitude, and the Clausius-Clapeyron equation, which converts that pressure into a boiling temperature. Enter your altitude in metres and, if you wish, refine the enthalpy of vaporisation. The result is the boiling point of water in Celsius and Fahrenheit, plus the local pressure.
Boiling point at altitude formula
Barometric: P = 101,325 * (1 - 0.0000225577 * h)^5.25588
Clausius-Clapeyron: 1/Tb = 1/T0 - (R / Hvap) * ln(P / P0)
Boiling point (C) = Tb - 273.15
Boiling point (F) = boiling C * 9/5 + 32
(T0 = 373.15 K, P0 = 101,325 Pa, R = 8.314462618 J/(mol K))
The barometric formula uses U.S. Standard Atmosphere constants for the troposphere. The Clausius-Clapeyron relation then converts the lowered pressure into the temperature at which water's vapour pressure equals it, the boiling point.
Physics and cooking context
- At sea level (101,325 Pa) water boils at 100 degrees Celsius, the reference point used here.
- The barometric constants are from the U.S. Standard Atmosphere 1976 troposphere model.
- The gas constant R is the exact SI value 8.314462618 joules per mole per kelvin.
- The default enthalpy of vaporisation, about 40,660 J/mol, is the NIST value near water's normal boiling point; it is editable.
- Lower boiling temperatures mean longer cooking times at altitude; pressure cookers raise pressure to reverse the effect.
Boiling point at altitude: frequently asked questions
Why does water boil at a lower temperature at altitude?
Water boils when its vapour pressure equals the surrounding air pressure. Air pressure falls with altitude, so less heat is needed to reach that pressure, and water boils cooler. At sea level water boils at 100 degrees Celsius; in Denver, around 1,600 metres, it boils near 95 degrees Celsius.
How does this calculator find the boiling point?
First it computes the atmospheric pressure at your altitude using the barometric formula from the U.S. Standard Atmosphere. Then it inverts the Clausius-Clapeyron equation, which links a liquid's vapour pressure to temperature, to find the temperature at which water's vapour pressure equals that local air pressure.
What is the Clausius-Clapeyron equation?
It is a physical relationship describing how the vapour pressure of a liquid changes with temperature, governed by the enthalpy of vaporisation. Rearranged, it gives the boiling temperature at any pressure relative to a known reference point, here water boiling at 100 degrees Celsius at one standard atmosphere.
Why can I edit the enthalpy of vaporisation?
The default is the measured enthalpy of vaporisation of water, about 40,660 joules per mole near its boiling point, from NIST. We expose it as an input so you can use a more precise value for your conditions or model a different liquid. The number drives how steeply boiling point falls with altitude.
How does altitude affect cooking?
Because water boils cooler at altitude, foods cooked in boiling water take longer, and recipes often need adjusting at high elevations. Pressure cookers counter this by raising the internal pressure, which raises the boiling point back up and speeds cooking, the reverse of the altitude effect.
Official sources
- NOAA / NASA: U.S. Standard Atmosphere 1976.
- NIST Chemistry WebBook: Water thermodynamic data.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.