The AMS Risk Calculator: Estimate Your Personal Altitude Sickness Risk
Acute Mountain Sickness affects between 25% and 85% of climbers who travel above 2,500 meters — depending on rate of ascent, sleeping altitude, prior history, and individual susceptibility. The variation is not random. It’s predictable from a small set of well-studied risk factors. This investigation gives you an interactive risk calculator that estimates your personal AMS, HACE, and HAPE risk based on those factors — built on the 2018 Lake Louise Score, peer-reviewed altitude medicine, and Wilderness Medical Society guidelines. Plus the medical context to use it well.
per 1,000m above 2,500m
prior AMS history
with Diamox prophylaxis
Score (current)
Most altitude sickness articles online give you the same generic warnings: “drink water, ascend slowly, take Diamox, descend if you feel bad.” Useful, but unactionable — they don’t tell you how much risk you carry into your specific trip. The published medical literature actually supports a more useful approach: your personal AMS risk is largely predictable from a handful of well-studied variables — sleeping altitude, ascent rate, prior history, and pre-acclimatization. The Wilderness Medical Society, the CDC Yellow Book, and decades of peer-reviewed altitude medicine all converge on the same risk-factor model. This investigation does what no other altitude-sickness page on the open web does: it gives you an interactive risk calculator built on that model, plus the medical context to interpret the result responsibly. Your number is just the starting point. The recommendations that follow are what actually matter.
This calculator estimates relative risk based on published prevalence data and known risk factors. It is not a medical diagnosis or substitute for professional medical advice. Individual susceptibility to altitude illness varies and cannot be fully predicted. Always consult a physician familiar with travel medicine before any high-altitude trip — especially if you have any pre-existing condition. If you develop symptoms at altitude, follow the gold rule of altitude medicine: do not ascend further with symptoms; descend if symptoms worsen. Severe altitude illness (HACE / HAPE) is a life-threatening emergency requiring immediate descent and medical attention.
The calculator
Enter your trip parameters below. The calculator estimates your AMS, HACE, and HAPE risk and generates personalized recommendations. All inputs are processed locally in your browser. Nothing is stored or transmitted.
Risk model. The calculator uses a transparent points-based estimator built on three layers: (1) a baseline AMS prevalence calculated from the published 13% per 1,000m above 2,500m benchmark from a 91-study, 66,944-patient meta-analysis published in American Family Physician; (2) an ascent-rate multiplier derived from CDC Yellow Book and Wilderness Medical Society guidelines (above 3,000m, sleeping elevation should rise no more than 500m/day); and (3) individual-factor and modifier multipliers derived from documented relative-risk values in peer-reviewed altitude medicine literature. HACE probability scales as a fraction of severe AMS cases, with sleeping-altitude amplification. HAPE probability uses the AAFP-published baseline of ~4% incidence at 4,600m unacclimatized, with documented modifiers. What the calculator is not. It is not a clinical tool. It does not replace consultation with a travel-medicine physician. It cannot predict individual susceptibility — some climbers experience severe AMS at 3,500m while others ascend to 5,500m without symptoms. The calculator tells you the population-level relative risk of someone in your profile, not a guaranteed outcome for you specifically.
What altitude sickness actually is
Acute Mountain Sickness, or AMS, is the syndrome that develops in non-acclimatized climbers ascending to high altitude — typically beginning 6 to 12 hours after reaching elevations above 2,500 meters. It’s the body’s response to reduced barometric pressure: at 5,500m the partial pressure of oxygen is roughly half what it is at sea level, and the cardiovascular and respiratory systems are working overtime to compensate.
The misconception many climbers carry is that altitude sickness is a “lack of oxygen” problem. It isn’t, exactly. The atmosphere is still 20.9% oxygen at the summit of Everest — same as at sea level. What changes is air pressure. Less pressure means fewer oxygen molecules per breath, which means lower blood oxygen saturation, which means the brain and lungs don’t get the supply they’re used to. The body adapts — by breathing faster, producing more red blood cells, and adjusting blood pH — but adaptation takes time. AMS is what happens when you ascend faster than your body can adapt.
The symptoms cluster into a recognizable pattern, codified in 1991 and revised in 2018 as the Lake Louise Score:
| Symptom | Score 0 | Score 1 (mild) | Score 2 (moderate) | Score 3 (severe) |
|---|---|---|---|---|
| Headache | None | Mild | Moderate | Severe, incapacitating |
| GI symptoms | Good appetite | Poor appetite or nausea | Moderate nausea or vomiting | Severe, incapacitating |
| Fatigue / weakness | Not tired | Mild | Moderate | Severe, incapacitating |
| Dizziness / lightheadedness | None | Mild | Moderate | Severe, incapacitating |
The 2018 revision removed sleep disturbance from the diagnostic criteria. Source: Roach et al. (2018), High Altitude Medicine & Biology 19:1-4.
A diagnosis of AMS requires headache + a total score of 3 or higher after a recent ascent. Scores translate to severity rankings as follows:
Mild AMS
3–5 pointsHeadache, mild nausea, fatigue. Most climbers with mild AMS can continue their itinerary with rest, hydration, and possibly Diamox. Do not ascend further until symptoms resolve.
Moderate AMS
6–9 pointsPersistent severe headache, vomiting, marked fatigue, ataxia (loss of coordination). Descent is usually required. Even a modest descent of 300–500m typically produces significant improvement within hours. Continuing to ascend with moderate AMS is the most common pathway to HACE.
Severe AMS
10–12 pointsSymptoms that prevent normal activity. Severe headache resistant to medication, persistent vomiting, inability to walk in a straight line. Immediate descent is mandatory. Severe AMS is the threshold at which life-threatening HACE becomes likely if descent is delayed.
HAPE and HACE — the conditions that kill
AMS is uncomfortable. HAPE and HACE are emergencies. They develop in roughly 1 to 4 percent of climbers ascending above 4,000–4,500m, more often when AMS has been ignored or pushed through. They are also the leading causes of altitude-related death.
HAPE — High Altitude Pulmonary Edema
HAPE is the accumulation of fluid in the lungs caused by altitude-induced pulmonary hypertension. The published incidence at 4,600m in unacclimatized climbers is approximately 4% (per American Family Physician‘s altitude illness review). HAPE typically develops 1 to 4 days after rapid ascent and can develop independently of AMS — meaning a climber with no AMS symptoms can still develop HAPE.
Recognize HAPE by these signs, in approximate order of progression:
- Fatigue, weakness, decreased exercise tolerance noticeably worse than expected at the altitude
- Shortness of breath at rest (the critical warning sign — distinct from breathlessness during exertion, which is normal)
- Dry cough that progresses to a productive cough with pink, frothy sputum (late sign)
- Crackles or gurgling sounds heard in the chest (often heard before the climber notices any subjective change)
- Cyanosis — bluish discoloration of lips, fingertips, or nail beds
- Rapid heart rate and breathing rate even at rest
Treatment for HAPE: immediate descent. Supplemental oxygen, Nifedipine (a calcium channel blocker), and a portable hyperbaric chamber (Gamow bag) are bridging measures, but descent is the definitive treatment. Climbers who descend by 1,000m typically improve dramatically within hours.
HACE — High Altitude Cerebral Edema
HACE is brain swelling caused by hypoxia. It develops most often as a progression of severe AMS — meaning AMS that was not respected with descent. The hallmark symptom is ataxia — the inability to walk in a straight line. Combined with severe AMS symptoms, ataxia is HACE until proven otherwise.
Recognize HACE by these signs:
- Worsening severe AMS symptoms that don’t respond to rest
- Ataxia — assessed by the heel-to-toe walk test (climber walks 10 paces along a straight line)
- Confusion, irritability, behavioral changes that the climber may not recognize in themselves
- Severe headache resistant to medication
- Drowsiness, lethargy, difficulty being roused
- In late stages: hallucinations, coma, death (which can occur within hours of clear ataxia onset)
Treatment for HACE: immediate descent. Dexamethasone (a steroid) is the standard pharmacological adjunct, but descent is the only definitive treatment. The climber may not be able to walk down on their own power and may need to be carried — by their guide, porter team, or via helicopter evacuation.
Descend. When in doubt, descend. When symptoms are not improving, descend. When the next climber up the trail looks at you funny, descend. The mountain will be there next year. Almost every avoidable altitude death in the medical literature traces back to a climber, guide, or party that chose to push higher with symptoms instead of descending. The climbers who summit are not the climbers who push through ataxia or shortness of breath at rest. They’re the climbers who go down and come back another day.
The risk factors the calculator uses (and why)
The calculator above takes inputs across four categories. Here’s what each one represents and why it’s in the model.
Altitude profile (primary driver)
Sleeping altitude is the single most important variable — more important than maximum altitude reached during the day. The body’s adaptation processes happen during sleep; climbing high during the day and sleeping low (“climb high, sleep low”) is the foundational acclimatization principle. A climber who reaches 5,500m during the day and sleeps at 4,200m has a dramatically lower AMS risk than a climber who sleeps at 5,500m. The published Swiss Alps mountaineering data show a clear dose-response: 9% AMS prevalence at 2,850m sleeping altitude rising to 53% at 4,559m.
Ascent rate
The CDC Yellow Book guideline is clear: above 3,000m, sleeping elevation should rise no more than 500m/day on average, with a rest day every 1,000m of cumulative gain. Climbers who fly directly to 3,500m+ (from sea level) are taking the highest-risk approach. Climbers on 9+ day itineraries with acclimatization days are taking the lowest-risk approach. The ascent-rate multiplier in the calculator reflects roughly a 2x risk difference between the fastest and slowest profiles.
Individual factors
Age
Counterintuitively, younger climbers have slightly higher AMS risk than older climbers, per published clinical data. The mechanism isn’t fully understood — possibly related to differences in cerebral blood vessel autoregulation. Over-65 climbers don’t have higher AMS rates but may have other altitude-related cardiac risks worth discussing with a physician.
Prior altitude experience
This is the strongest non-altitude predictor in the model. Recent altitude exposure (within the past 12 months) provides meaningful protection. A climber who has slept above 5,000m in the past year has roughly half the AMS risk of a climber going to that altitude for the first time. The protective effect fades over months — protection from a trip 18 months ago is much weaker than from a trip 3 months ago.
Aerobic fitness
Smaller effect than commonly believed. Marathon runners get AMS at roughly the same rates as average climbers when both groups are on the same itinerary. Acclimatization is largely independent of fitness. The calculator weights fitness modestly precisely because the literature does.
Home elevation
Modest protective effect. Climbers living at 1,500m+ start with a partially adapted physiology and have lower baseline AMS risk than sea-level dwellers — though the effect is smaller than ascent-rate or prior-experience effects.
Risk modifiers
Prior AMS history (~2x risk)
The strongest individual modifier in the model. Climbers who have had AMS in the past are roughly twice as likely to develop it on subsequent trips at comparable altitudes. The reason is unclear but the effect is reproducible across studies.
Migraine history
Modestly elevated risk. Climbers with migraine histories report higher rates of altitude-related headache and AMS-spectrum symptoms, possibly due to underlying differences in cerebral blood vessel reactivity.
Diamox (acetazolamide) prophylaxis (~50% risk reduction)
The strongest protective intervention. Diamox is a carbonic anhydrase inhibitor that acidifies the blood, driving ventilation and accelerating acclimatization. The standard prophylactic dose is 125mg twice daily, starting 1–2 days before reaching 3,000m and continuing until at maximum altitude or descending. Side effects (tingling fingers, increased urination, mild fatigue) are real but generally minor. Diamox is not a substitute for proper acclimatization — but as an addition to a well-paced itinerary, it produces measurable risk reduction.
Pre-acclimatization (~35% risk reduction)
Sleeping in a hypoxic tent or chamber for 2+ weeks before the trip simulates altitude exposure and produces measurable physiological adaptation. The protective effect is real and has been used by elite climbers and increasingly by commercial expeditions (Furtenbach Adventures’ Flash Expedition concept is built on it). It’s expensive — a hypoxic tent rents for $400–$800/month — but for high-altitude trips it’s one of the most cost-effective risk reductions available.
Cardiopulmonary conditions
Pre-existing heart, lung, or anemia conditions increase altitude risk. The calculator flags this category but cannot replace a conversation with a travel medicine specialist. If you have any cardiopulmonary condition, do not book a high-altitude trip without consulting a physician familiar with altitude medicine — not just your primary care doctor.
Strenuous exertion at altitude
The published literature documents that strenuous physical exertion at altitude — climbing rather than trekking, summit pushes rather than rest days — increases HAPE risk meaningfully. Pacing matters. Pole pole, as the Tanzanian guides say.
What actually works to prevent altitude sickness
Combining the calculator output with the literature, here’s what works and what doesn’t:
What works (evidence-based)
- Slow ascent. Above 3,000m, no more than 500m/day sleeping elevation gain. A rest day every 1,000m. This is the single most effective intervention.
- Diamox (acetazolamide). 125mg twice daily, starting 1–2 days before reaching 3,000m. Roughly halves AMS risk in most studies.
- Pre-acclimatization. Hypoxic tent, chamber, or recent prior altitude exposure (within 12 months above 3,000m).
- Adequate hydration. 4+ liters/day at altitude. Dehydration mimics and amplifies AMS symptoms.
- Climb high, sleep low. Reach high altitudes during the day; descend to lower elevations to sleep when possible.
- Recognize and respect symptoms. Mild AMS = stop ascending. Moderate AMS = descend. Severe AMS or any HACE/HAPE signs = immediate emergency descent.
- Avoid alcohol for the first 48 hours at altitude. Sedatives suppress respiratory drive when you need it most.
What doesn’t work (popular myths)
- Coca leaves / coca tea. Common in the Andes; placebo at best for AMS prevention. The mild stimulant effect may temporarily mask symptoms but does not address underlying hypoxia.
- “Just be very fit.” Marathon-level fitness does not meaningfully reduce AMS. Acclimatization is independent of cardiovascular conditioning.
- Smoking. The myth that smokers are “pre-adapted” to low oxygen has no clinical support. Smokers do worse at altitude, not better.
- Ibuprofen as prophylaxis. Ibuprofen treats the headache but does not prevent AMS. Diamox is the prophylactic.
- “Drink enough water and you’ll be fine.” Hydration is necessary but not sufficient. You can be perfectly hydrated and still develop HAPE.
Frequently Asked Questions
How accurate is this calculator?
The calculator gives a population-level relative-risk estimate based on published prevalence data. It’s accurate as an indicator of which profile category you fall into — low, moderate, elevated, high, or very high. It is not accurate as a precise individual prediction. Two climbers with identical inputs can have very different actual outcomes because individual altitude susceptibility varies enormously and is not fully captured by any external risk model. Use the calculator as a decision-support tool: if your number is high, take the recommendations seriously. If your number is low, you still need to monitor symptoms at altitude and follow the descent rule.
Should I take Diamox?
This is a medical decision that should be made with your physician, but the published evidence supports Diamox prophylaxis for most high-altitude travel above 3,000m. The current standard protocol is 125mg twice daily, starting 1–2 days before reaching 3,000m, continuing for the duration at altitude (or until descent). Side effects (tingling extremities, increased urination, mild fatigue, an altered taste of carbonated drinks) are real but generally minor and resolve when the medication is stopped. Diamox is not a substitute for proper acclimatization itinerary — you should still book the appropriate route length regardless of medication. If you have a sulfa allergy, alternatives like dexamethasone exist but require closer physician supervision. Always consult a physician before taking any prescription medication.
What’s the difference between AMS, HACE, and HAPE?
AMS (Acute Mountain Sickness) is the common, generally benign syndrome — headache, nausea, fatigue, dizziness — that develops in unacclimatized climbers above 2,500m. Most climbers can manage mild AMS with rest. HACE (High Altitude Cerebral Edema) is brain swelling, usually a progression of severe AMS that wasn’t respected with descent. The hallmark sign is ataxia (loss of coordination). HACE is life-threatening within hours and requires immediate descent. HAPE (High Altitude Pulmonary Edema) is fluid in the lungs from altitude-induced pulmonary hypertension. The hallmark sign is shortness of breath at rest. HAPE can develop independently of AMS — meaning you can develop HAPE without ever having AMS symptoms first. HAPE is the leading cause of altitude-related death and also requires immediate descent. All three respond to descent. Descent is the universal treatment.
How do I recognize HAPE early?
The earliest reliable HAPE warning sign is shortness of breath at rest — being unable to breathe normally even when not exerting yourself, especially when lying down. This is distinct from breathlessness during exertion, which is normal at altitude. Other early signs: unusual fatigue and decreased exercise tolerance noticeably worse than your trip-mates at the same altitude; a dry persistent cough that progresses to a wet cough; crackles or gurgling sounds in your chest when you breathe (often heard by tent-mates before the climber notices). HAPE typically develops 1–4 days after rapid ascent. If any of these signs appear, the response is the same as for severe AMS: immediate descent. Even 500–1,000m of descent typically produces dramatic improvement within hours.
Is altitude sickness related to fitness?
Less than most climbers think. Acclimatization — the process by which your body adapts to lower oxygen — is largely independent of cardiovascular fitness. Marathon runners and Olympic athletes get AMS at roughly the same rates as average climbers when both are on the same itinerary. The strongest predictors of who gets AMS are sleeping altitude, ascent rate, and prior altitude experience — not VO2 max. This doesn’t mean fitness is useless; baseline aerobic fitness lets you handle the workload of climbing without compounding AMS with raw exhaustion. But fitness is a small lever compared to itinerary length.
Can I take Diamox without a prescription?
In the United States and most Western countries, acetazolamide is a prescription medication. In some countries (including Nepal, Tanzania, and parts of South America) it’s available over the counter at pharmacies. We do not recommend taking Diamox without a prescription — not because it’s dangerous in normal use, but because the consultation with your physician is itself valuable. Sulfa allergies, certain kidney conditions, and specific drug interactions matter. A travel medicine consultation costs $100–$200 and provides not just the prescription but the broader pre-trip medical advice you need.
What if I’m climbing with a child?
Children are equally susceptible to AMS as adults — possibly slightly more so. The diagnostic challenge is that young children may not be able to articulate symptoms clearly, so AMS in children can present as fussiness, refusal to eat, lethargy, or unusual sleepiness. The acclimatization rules are the same as for adults: gradual ascent, no more than 500m/day sleeping elevation gain above 3,000m, descend if symptoms develop. Children under 5 are sometimes recommended against high-altitude trips by pediatric travel medicine specialists; consult a physician. Diamox can be prescribed for children at weight-adjusted doses, but this is a physician decision.
How long should I plan for acclimatization on a 5,000m+ trip?
The published guideline is roughly: 2–3 days at 2,500–3,000m (or staged at progressively higher altitudes); thereafter no more than 500m/day sleeping elevation gain, with a rest day every 1,000m of cumulative gain. For a 5,500m trip starting from sea level, this works out to roughly 7–10 days minimum from the start of the climb. Most successful Kilimanjaro itineraries are 7–8 days; most successful Aconcagua itineraries are 16–20 days; most successful Everest expeditions are 7–9 weeks. The fastest path to the top is the second-fastest itinerary. Climbers who try to compress the schedule almost always end up on a slower path overall — either by failing the first attempt and returning, or by needing emergency descent partway up.
This is a planning tool, not a medical tool. Used at trip-planning time, it helps you understand which interventions matter most for your specific profile — whether you should add days to your itinerary, whether Diamox prophylaxis makes sense, whether pre-acclimatization is worth the investment, whether your trip is realistic given prior history. Used at altitude, the calculator is irrelevant. At altitude, what matters is the symptoms in front of you and the rule the literature has settled on for fifty years: do not ascend further with symptoms; descend if symptoms worsen; treat HACE and HAPE as the life-threatening emergencies they are. The climbers who come home are the ones who respect the mountain and respect their own bodies’ signals. Use this calculator to plan well. Then leave it behind when you start walking.
Sources and Verification
This investigation was built from peer-reviewed altitude medicine literature and authoritative clinical guidelines:
- Roach, R.C., Hackett, P.H., Oelz, O., Bärtsch, P., et al. (2018). The 2018 Lake Louise Acute Mountain Sickness Score. High Altitude Medicine & Biology 19:1-4. The current consensus diagnostic framework; revision removed sleep disturbance from the criteria.
- Hackett, P.H., & Roach, R.C. High-Altitude Illnesses: Physiology, Risk Factors, Prevention, and Treatment. NCBI / PMC. Foundational review of risk factors including ascent rate, sleeping altitude, prior history, and individual susceptibility.
- StatPearls / NIH (2025). Acute Mountain Sickness — clinical reference. Documents prior AMS history as ~2x risk multiplier; documents 25-43% AMS prevalence at 2,500-4,300m.
- CDC Yellow Book 2024. High Elevation Travel & Altitude Illness. Provides the operational guidelines including the 500m/day sleeping elevation rule above 3,000m.
- Wilderness Medical Society Practice Guidelines. Prevention and Treatment of Acute Altitude Illness — current consensus on Diamox dosing, descent criteria, and HAPE/HACE management.
- American Family Physician (2018). Single Question Is Useful for Identifying Acute Mountain Sickness — meta-analysis of 91 studies (n=66,944) establishing the 13% AMS prevalence increase per 1,000m above 2,500m baseline used in this calculator.
- American Family Physician (2010). Altitude Illness: Risk Factors, Prevention, Presentation, and Treatment — published HAPE incidence figure of approximately 4% at 4,600m unacclimatized.
- Hackett, P.H. (1976). Foundational study of 278 unacclimatized hikers ascending to 4,243m, documenting AMS prevalence at 53% — the medical baseline still used today.
- Frontiers in Physiology (2024). Recent advances in predicting acute mountain sickness — review of multidimensional cohort studies and prediction models.
Calculator methodology. The risk model is a transparent points-based estimator using published prevalence baselines and documented relative-risk multipliers. It is not a clinical instrument. Risk multipliers are derived from the relative-risk values documented in the sources above, with conservative weighting where the literature shows variability between studies. The calculator code is open and inspectable — anyone wanting to audit the math can view source on the calculator page. Annual review. The calculator and its underlying model are reviewed annually against newly published altitude medicine literature. Next review: November 2026.
Published May 12, 2026 · Last reviewed May 2026 · Next scheduled review: November 2026
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