Boiling point at altitude
What water boils at where you are, and what that water can no longer cook — because above a certain height adding minutes stops being the answer. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
You can boil an egg on Everest, by a third of a degree
The story everybody tells is that water boils too cool on Everest to cook an egg. It is very nearly true, and it is wrong.
Water boils at 70.3 °C on the summit. An egg yolk sets at 70 °C. The highest point on Earth clears the line by about a third of a degree.
The altitude at which water genuinely stops being able to set a yolk is 8,925 m — seventy-six metres higher than the summit. There is nowhere on land where it fails, and Everest misses it by the height of a tower block. That figure was solved for on the real curve, not read off a chart.
The real limit is the white, and it is much lower down
An egg white becomes a tender solid at 65 °C, which is why the yolk is the binding constraint for a soft-boiled egg. But ovalbumin — the main protein in the white, 54% of it by weight — does not coagulate until about 80 °C, and that is what turns a tender white into a firm one.
Water stops reaching 80 °C above 5,987 m. That is reachable land: people live and cook above 5,000 m, and climbers cook far higher.
So above about six thousand metres a boiled egg white will set and never firm up, however long you leave it. This is the part worth knowing, because every high-altitude cooking chart in existence tells you to add minutes — and minutes cannot fix a temperature that is never reached.
The rule of thumb is better than it deserves to be
The common advice is that water boils one degree cooler for every 285 metres of altitude. The relationship it is approximating is exponential, so a straight line through it ought to drift badly.
It does not. It is 0.2 °C out at 1,000 m, 0.9 °C at 5,000 m, and its worst error anywhere in range is 1.44 °C, at the very top of the troposphere. We went looking for a fault in it and did not find one, which is worth saying as plainly as if we had.
And water does not boil at 100 °C anyway
At exactly one standard atmosphere, Antoine's equation gives 99.997 °C and the IAPWS-IF97 steam tables — the international standard formulation, fitted to measured data — give 99.974 °C.
Neither gives the round number. The 100 everybody learned is a definition the temperature scale has since moved away from, and the two best available formulations both land just underneath it. The difference is far too small to matter to anything you are cooking. It is here because this page is about what the numbers actually say.
What this does not know
Today's weather. The altitude-to-pressure step is the International Standard Atmosphere, which describes an average day. Real sea-level pressure varies by several per cent, which moves the boiling point by more than a few hundred metres of altitude would.
Anything above 11,000 m. That is where the troposphere model this uses stops being defined, so the page stops too rather than extrapolating a formula past the range its authors gave it.
Your pan. A lid, a pressure cooker, or dissolved salt and sugar all change the boiling point, and none of them are in here.
Where this comes from
Antoine equation constants for water from NIST (A = 8.07131, B = 1730.63, C = 233.426). Pressure from the International Standard Atmosphere troposphere model. Protein temperatures from Harold McGee. Everest's height is 8,848.86 m, the figure agreed by the joint China–Nepal survey announced on 8 December 2020.
The boiling curve here is graded against IAPWS-IF97 — a genuinely different formulation rather than the same equation typed twice — at 112 altitudes from sea level to 11,000 m. The worst disagreement is 0.084 °C.