Altitude Sickness & Acclimatization Planner

Altitude sickness can start once you sleep at 2,450 m (8,000 ft), and sometimes lower. CDC’s pace: never from below 1,200 m to 2,750 m overnight; above 3,000 m, at most 500 m higher nightly, plus an extra night per 1,000 m. Machu Picchu: 2,430 m (7,970 ft); Cusco: 3,350 m (10,990 ft).

How to use it

Pick a trip, or build your own: one line per night, with the place you sleep at or the height your tour operator gives. Add where you live and whether altitude sickness has hit you before. You’ll see each night checked against CDC’s pace, the whole trip on CDC’s risk table (low, medium or high), and the quickest plan that keeps the pace to your highest night. Numbers can’t tell you how you will feel, so take it as a check, not medical advice.

CDC’s pace

The pace that keeps altitude sickness least likely:

• Don’t climb to a sleeping altitude of 2,750 m (≥9,000 ft) in a single day; once above 3,000 m (9,800 ft), sleep no more than 500 m (1,650 ft) higher each night; and allow an extra night to acclimatize for every 1,000 m (3,300 ft) of sleeping altitude you gain.
• Don’t go straight from low altitude (1,200 m or <4,000 ft) to a sleeping altitude of 2,750 m (>9,000 ft) in one day.
• Once above 3,000 m (9,850 ft), raise your sleeping altitude by no more than 500 m (1,600 ft) a day, and plan an extra day of acclimatization for every further 1,000 m (3,300 ft) of sleeping altitude.
• Spending at least 2–3 nights at about 2,450 to 2,750 m (8,000–9,000 ft) before going higher protects strongly against acute mountain sickness (AMS).
• Your blood oxygen is lowest while you sleep, so day trips up high with an evening return lower down are much easier on the body.
• No alcohol, and only mild exercise, for the first 48 hours at altitude.

Count a night’s height where you sleep, not the highest point you hike to. Above 3,000 m, only the part of a night’s climb above 3,000 m and above your highest night so far counts, so a night back in Cusco after a night lower down is no gain. An extra night is one where you sleep no higher than you already have. This pace can still be too fast for some travelers and too slow for others.

CDC’s risk table

CDC’s Table 3.5.1 puts a trip at low, medium or high risk of acute mountain sickness (AMS) on three lines, and your trip’s risk is the highest of the three. The table assumes you start below 1,200 m (4,000 ft).

For each line, CDC tells doctors what to consider. At low risk, acetazolamide isn’t indicated; at medium risk, it would be beneficial, so consider it; at high risk, it’s strongly encouraged, and the traveler should carry dexamethasone for emergency use. That’s a conversation for your doctor or a travel clinic, with your plan in hand. Flying in matters most: some common high-altitude destinations require rapid ascent by airplane to more than 11,150 ft (3,400 m), which puts you in a high-risk category for AMS. In some cases, such as Cusco and La Paz, you can descend to elevations lower than the airport to sleep for 1–2 nights and then begin your ascent, which may avoid the need for medication.

Heights of popular high places

“By day” places are summits and ruins you visit and come down from.

The warning signs

Acute mountain sickness: it feels like a hangover: a headache, usually with at least one of loss of appetite, dizziness, fatigue, nausea or, now and then, vomiting. It usually starts 2–12 hours after you arrive up high or climb higher, often during or after the first night, and it generally clears within 12–48 hours if you go no higher.

High-altitude cerebral edema (HACE): unlike AMS, it shows in the mind and balance, with a changed mental state, stumbling (ataxia), confusion and drowsiness, much like being drunk. High-altitude pulmonary edema (HAPE): it starts with chest congestion, a cough, unusual breathlessness on exertion and less stamina; missed and untreated, it moves on to breathlessness at rest and real respiratory distress, often with bloody sputum. In most cases, going down is urgent and a must.

Going down helps: AMS improves fast with a descent of 300 m (1,000 ft) or more, especially with little exertion. If symptoms get worse at the same altitude despite treatment, you must go down.

Worked examples

Cusco first, then Machu Picchu. Flying in to Cusco puts the first night at 3,350 m (10,990 ft): the table’s medium line, and CDC names Cusco among the fly-in places that put travelers in its high-risk group. Machu Picchu and the Sacred Valley first sleeps 2,040 m (6,690 ft) in Aguas Calientes, then 2,792 m (9,160 ft) in Ollantaytambo, and reaches Cusco with no night too fast: low.

A ski week in Breckenridge. Straight from the airport, the first night is at 2,925 m (9,600 ft), CDC’s figure for its lodging: medium. With a night in Denver (1,609 m (5,280 ft)) first, day 1 is under 2,750 m and the trip reads low.

A Kilimanjaro plan with camps at 3,000, 3,850, 3,950, 4,000, 4,650 m. (Type your own operator’s camps: they differ.) Night 2 climbs 850 m above 3,000 m and night 5 650 m, with no extra night at 4,000 m or above: high. CDC’s pace to sleep at 4,650 m takes at least 6 nights.

Common mistakes

• Counting the summit, not the bed. The pace is about where you sleep; a day hike higher, back down to sleep, is a different thing.
• Trusting fitness. Training and physical fitness do not change the risk.
• Thinking children are spared. Children are as likely to get it as adults; people over 50 have slightly less risk.
• Pushing on with symptoms. AMS generally clears within 12–48 hours if you go no higher. Climbing higher while unwell is how mild AMS becomes serious.
• Celebrating the first night. No alcohol for the first 48 hours up high.

Plan the trip

Read Travel Magma’s guide to altitude sickness and climbing high without it, then plan the places: things to do in Cusco, what La Paz is known for, things to do in Quito and things to do in Denver. Pick the month with Machu Picchu’s rainy season and the best time to visit, and pack with the packing list generator.

Sources

• CDC Yellow Book 2026, High-Altitude Travel and Altitude Illness (September 26, 2026), with the Wilderness Medical Society’s guidance it follows
• Heights: CDC’s figure where the chapter gives one (Cusco, La Paz, Lhasa, Everest base camp, Kilimanjaro, Vail’s and Breckenridge’s lodging), otherwise Wikidata’s