How this instrument works
Air pressure falls as you gain altitude, because there's simply less atmosphere stacked above you pressing down. That falling pressure drags water's boiling point down with it — water boils exactly when its vapor pressure matches the surrounding air pressure, so less surrounding pressure means boiling starts at a lower temperature. At sea level water boils at 100°C (212°F); partway up a tall mountain it can boil at 90°C or lower, even though it still looks and sounds like a rolling boil.
This instrument uses a standard empirical curve fit rather than deriving pressure from first-principles atmospheric physics: pressure in inches of mercury is modeled as P = 29.921 × (1 − 0.0000068753 × altitude)^5.2559, a regression against real barometric data that's anchored to give exactly standard sea-level pressure (29.921 inHg) at zero altitude. That pressure is then fed into a second regression, boiling point (°F) = 49.161 × ln(P) + 44.932, which was itself fit to match water's known boiling point at a range of real pressures.
Both curve fits are calibrated for everyday elevations on Earth's surface and lose accuracy well outside that range — this instrument won't give sensible answers for extreme depths or for altitudes far into the thin upper atmosphere, and it assumes plain water at standard purity, not water carrying dissolved salts or sugars, which raises boiling point through a separate effect (boiling-point elevation) that this calculation doesn't include.
- Enter the altitude in feet above sea level (negative values work too, for elevations below sea level).
- Read the estimated barometric pressure at that altitude, in inches of mercury (inHg).
- Read the predicted boiling point of water at that altitude, given in both Fahrenheit and Celsius.
Worked example — boiling water at Machu Picchu
The Inca citadel of Machu Picchu sits at roughly 7,970 feet (2,430 m) in the Peruvian Andes. Plugging 7,970 ft into the altitude-pressure regression gives a barometric pressure of about 22.25 inHg — well below sea level's 29.92 inHg. Feeding that pressure into the boiling-point regression gives roughly 197.4°F, or about 91.9°C: noticeably below the familiar 100°C, and part of why traditional Andean cooking techniques (like the earthen pachamanca oven) rely on prolonged heat rather than a fast boil.
Water at that elevation is genuinely boiling — bubbling, rolling, doing everything a pot of boiling water does at sea level — it's just doing it roughly 8°C cooler. That's the practical catch of high-altitude cooking: the visual and audible cues for 'it's boiling' are unchanged, but the actual cooking temperature is lower, so pasta, rice, and eggs all take measurably longer to cook through.
Questions
Why does water boil at a lower temperature at higher altitude?
Because atmospheric pressure drops as elevation increases, and water only needs to reach a vapor pressure equal to the surrounding air pressure to start boiling. With less air pressure pushing down at altitude, that threshold is reached at a lower temperature. At Denver's roughly 5,280 ft elevation, water boils at around 95°C rather than 100°C; high in the Andes or Himalayas the drop is considerably more dramatic.
Does high-altitude cooking really take longer?
Yes, and it's a direct consequence of the lower boiling point, not a myth. Since boiling water at altitude is cooler than boiling water at sea level, food cooked by boiling — pasta, rice, boiled eggs, beans — spends longer reaching the same internal doneness, because it's being heated by cooler water. Recipe adjustments for high-altitude baking and boiling exist specifically to compensate for this and for the lower ambient air pressure's other effects on rising and evaporation.
How accurate is this altitude-to-boiling-point formula?
It's a standard empirical regression, not a fundamental physical law, and it's calibrated against real atmospheric and boiling-point data across the range of everyday elevations people actually experience — from below sea level up through high mountain terrain. It reproduces standard sea-level conditions (29.92 inHg, 212°F) almost exactly at zero altitude, which is a good internal sign of its calibration, but like any curve fit it's most trustworthy within the elevation range it was built from and shouldn't be pushed to extreme altitudes.
Does dissolved salt or sugar change the boiling point predicted here?
This calculator models pure water only. Dissolving salt, sugar, or any other solute in water raises its boiling point slightly through a separate effect called boiling-point elevation — a colligative property driven by the number of dissolved particles, not the altitude-pressure relationship modeled here. Salting a pot of pasta water changes the boiling point by only a fraction of a degree in practice, but it is a real, physically distinct effect from what this instrument calculates.
What's the boiling point of water at sea level versus a very high mountain?
At sea level (0 ft, standard pressure 29.92 inHg) water boils at almost exactly 100°C / 212°F. On a very high mountain — Mount Everest's summit at roughly 29,000 ft, for instance — atmospheric pressure has fallen so far that water boils at around 70°C, cool enough that it's genuinely difficult to brew a proper cup of tea, a fact well documented by high-altitude climbers.