What does the elevation do to air and water?
Get the air pressure, oxygen, air density and boiling point at any elevation, or count the days a climb takes.
This is a planning aid, not a safety assessment or medical advice. Altitude illness can be serious and can strike at any fitness level; learn the symptoms, climb gradually and descend if they get worse. Talk to a doctor before a trip to high altitude if you have a heart, lung or other health condition. How we check our calculators.
Standard atmosphere model, from sea level to 36,000 ft (11,000 m).
The Wilderness Medical Society says every 3 to 4 days above 9,800 ft. A rest day means no climb to a higher sleeping elevation.
What changes as you climb
Air pressure is the weight of the air above you, so it falls as you rise. The mix of gases stays the same, about 20.95 percent oxygen in dry air, so the pressure of oxygen falls in step with the total. Colder air and a lower boiling point of water follow from the same thing. The calculator uses the standard atmosphere, the reference model used by NOAA, NASA and the aviation industry, in which temperature falls 6.5 °C per kilometer and pressure follows p = 101.325 × (1 − 2.25577 × 10−5 × h)5.25588 kPa, with h in meters. Air density is p ÷ (R × T) with R = 287.05 J/(kg·K). The boiling point is the temperature at which water's vapor pressure equals that air pressure, found from the Antoine equation with coefficients published by NIST.
Worked example
At 5,280 feet the pressure is 83.4 kPa, 82 percent of sea level, and water boils at 202.4 °F. At 10,000 feet the figures are 69.7 kPa, 69 percent, and 193.7 °F, and the CDC's 3-minute boil applies. At 14,000 feet pressure is 59 percent of sea level and water boils at 186.3 °F. The table shows the pattern.
| Elevation | Pressure, % of sea level | Water boils, °F |
|---|---|---|
| 0 ft | 100% | 212 |
| 2,000 ft | 93% | 208.4 |
| 5,000 ft | 83% | 202.9 |
| 8,000 ft | 74% | 197.4 |
| 10,000 ft | 69% | 193.7 |
| 14,000 ft | 59% | 186.3 |
Cooking and drinking water at a lower boil
Water cannot get hotter than its boiling point in an open pot, so food that cooks in boiling water, such as pasta, rice and beans, takes longer at elevation. Our kitchen pages cover it for specific cases: see the high altitude baking and canning altitude calculators. For drinking water, the CDC's advice is to boil for 1 minute and for 3 minutes above 6,500 feet. We do not give cooking times here because we found no sourced table for camp stoves and pots; stove performance also varies with fuel, wind and the pot.
Sleeping elevation: the Wilderness Medical Society rule
The 2019 Wilderness Medical Society guidelines on acute altitude illness advise that above 3,000 m (9,800 ft) you should not raise your sleeping elevation by more than 500 m (1,600 ft) a day and should take a rest day every 3 to 4 days. The second mode counts the days from your numbers: one day to reach 3,000 m if you start lower, then steps of 500 m, with rest days only between steps. Climbing 5,000 feet to 14,000 feet takes 5 days with a rest every 3rd day (4 gaining days, 1 rest) and 5 with a rest every 4th. The count is ours, applied to the published rule; it is a minimum and a real itinerary has weather, terrain and the group to consider.
Limits of this method
- Weather moves the numbers. A strong weather system changes the pressure by a few kilopascals and the boiling point by about a degree Fahrenheit. The model gives standard-day values.
- Pressure is not your performance. The percentage tells you about the air, not how you will feel. People differ widely in their response to altitude, and previous acute mountain sickness is a risk factor.
- Sleeping elevation counts. The guidelines are about where you sleep. A day hike to a high pass from a lower camp is not the same as camping on it.
- The boiling point is for open pots. A pressure cooker raises it.
Common questions
What percent of oxygen is there at altitude?
The same 20.95 percent of dry air by volume at every height in the standard atmosphere. What falls is the pressure: at 10,000 feet it is 69 percent of the sea-level value, so each breath brings in about that share of the oxygen. That is why a climb feels like less oxygen even though the air has the same composition.
At what temperature does water boil at my elevation?
It drops about 2 °F for every 1,000 feet. In the standard atmosphere water boils at 202.4 °F (94.6 °C) at 5,280 feet, 193.7 °F at 10,000 feet and 186.3 °F at 14,000 feet. Colorado State University Extension gives 203 °F at 5,000 feet and 193 °F at 10,000, which the formula matches to within a degree. Weather moves the actual value a little: by our calculation about 1 °F for a swing of 2 kPa in air pressure.
How long should I boil water at altitude?
The Centers for Disease Control and Prevention says to bring clear water to a rolling boil for 1 minute, and for 3 minutes at elevations above 6,500 feet (about 2,000 m). The calculator shows the matching figure for your elevation. Boiling does not remove chemicals, and cloudy water should be strained or settled first.
What is the 500 meter rule?
The Wilderness Medical Society's 2019 guidelines on acute altitude illness say that when traveling above 3,000 m (9,800 ft), you should not raise your sleeping elevation by more than 500 m (1,600 ft) a day and should take a rest day, with no climb to a higher sleeping elevation, every 3 to 4 days. It is a limit on where you sleep, because sleeping altitude matters more than the highest point you reach in the day. For a hiker starting at 5,000 feet and aiming for a camp at 14,000, the second mode counts 5 days with a rest every 3rd day and 5 with a rest every 4th.
Is it safe below 9,000 feet?
Safer, not risk-free. The American Family Physician summary of the guidelines says that people with no history of acute mountain sickness who do not sleep above 9,200 feet (2,800 m) are at low risk. The ascent limit in the guidelines starts at 9,800 feet, but acute mountain sickness can strike lower in people who are prone to it. Know the symptoms: headache with nausea, dizziness or fatigue. Descend if they get worse.
How is this different from the density altitude and baking calculators?
The density altitude calculator is for pilots: it starts from the altimeter setting and temperature. The high altitude baking and canning altitude calculators adjust recipes and processing times. This one is for people on foot: what the thin air, the cooler boil and the sleeping elevation mean on a trail.
This is a planning aid, not a safety assessment or medical advice. Altitude illness can be fatal; if you or someone in your group has worsening symptoms, descend and seek help.
Sources: U.S. Standard Atmosphere, 1976 (NOAA, NASA, USAF), the pressure and temperature formulas, checked against the standard atmosphere table; sea-level air composition of 20.9476 percent oxygen as listed by Jefferson Lab and others (the 1976 document not opened); NIST Chemistry WebBook, water, Antoine equation parameters (Bridgeman and Aldrich, 1964); Colorado State University Extension, "High Altitude Food Preparation"; Centers for Disease Control and Prevention, boil water guidance (1 minute, 3 minutes above 6,500 ft); Luks and others, Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness: 2019 Update, Wilderness & Environmental Medicine 30(4S):S3–S18, read through the summaries of G. Rodway (OutdoorEd, 2020) and C. Armstrong, American Family Physician 2020;101(8):505–507. Related: Trail Water Calculator, High Altitude Baking Calculator, Canning Altitude Calculator, Density Altitude Calculator, Clausius–Clapeyron Calculator.