Hiking at Altitude: What the Calculators Cannot See

Every calculator on this site works from distance, climb, weight, temperature and time. None of them asks how high the trail is. That is a real gap: a route of the same length and climb is a very different day at 3,500 m than at 500 m, because there is less oxygen in each breath. This guide explains what changes at altitude, how to adjust the site's estimates to allow for it, and the warning signs of altitude illness.

What changes as you go higher

Air pressure falls with height, so each breath carries less oxygen. Most people notice the effect on exertion somewhere above about 1,500 to 2,000 m, and altitude illness becomes a real possibility above roughly 2,500 m (about 8,000 feet), although sensitive people can be affected lower. The practical effects for a hiker:

• You are slower. The same climb leaves you more out of breath, and you need more rest.
• You lose more water. Breathing harder in cool, dry mountain air carries away moisture.
• Your appetite may drop, just when your body needs fuel.
• Weather is more extreme. Higher ground is colder, windier and more exposed, and the sun is stronger.
• Judgement can suffer. Tiredness and low oxygen make decisions harder, which is one more reason to make the big decisions, like the turnaround time, before you start.

How much each of these affects you varies a great deal from person to person, and fitness does not protect against altitude illness.

Adjusting the hiking time

The Hiking Time Calculator uses Naismith's rule, which was written for British hills and has no altitude allowance. For a hypothetical route of 10 km with 700 m of climb, starting high:

• Average pace: 3h 10m (Tobler cross-check 2h 32m).
• Leisurely pace, 25% slower: 3h 58m (Tobler 3h 10m).

At altitude, start from the leisurely figure, even if you are normally brisk, and allow more time for breaks than you would lower down. If you are not yet used to the height, slower still is realistic. There is no reliable formula for how much slower; it depends on the height, how long you have spent at altitude beforehand, and you. These figures are estimates, and at altitude they are more uncertain than usual, so plan with a wider margin.

Adjusting the turnaround

With the Daylight Turnaround Calculator, a start at 07:00 and sunset at 19:00:

• Plan made with the average time plus 45 minutes of breaks and the default 25% slower return: turn back by 11:53, projected finish 11:25, margin 395 minutes.
• Plan made with the leisurely time plus 75 minutes of breaks and a 50% slower return: turn back by 11:24, projected finish 13:31, margin 269 minutes.

The second plan is the more honest one for high ground, and its turnaround is half an hour earlier. In many high mountain areas, afternoon storms are also common in summer, which adds a second, earlier deadline; see planning to be down before storms.

Adjusting water

The Water Needs Calculator works from temperature and effort. For five hours at 15°C (59°F) it gives 0.5 litres per hour (2.5 litres) at moderate effort, and 0.65 litres per hour (3.25 litres) at strenuous effort. At altitude, choose the strenuous setting for any sustained climbing, because the same climb is harder work. If it is colder high up, the calculator lowers its estimate (five hours at 5°C and strenuous effort gives 2.6 litres), but the dry air means you still lose a lot through breathing, so do not carry less than you would lower down. Drink regularly and treat any water you collect from streams or lakes.

Signs of dehydration (thirst, dark urine, headache, tiredness) overlap with early altitude illness, so drinking well also makes it easier to tell the two apart. Do not overdo it, though: drinking large amounts of plain water without food or salt carries its own risk, hyponatremia.

Altitude illness: the signs

Acute mountain sickness is the common, milder form. Typical symptoms are a headache with one or more of: nausea or loss of appetite, tiredness or weakness, dizziness, and poor sleep. It often feels like a hangover. If you have symptoms, do not go higher until they have gone. Rest at the same height, and if they get worse, go down.

Two serious forms need immediate action. Warning signs include:

• Confusion, unusual behaviour, drowsiness, or loss of coordination (stumbling, unable to walk in a straight line).
• Breathlessness at rest, a persistent cough, a gurgling chest, or blue lips.

Either of these means descend at once, with someone accompanying the person, and call the local emergency number. Do not wait until morning, and do not let the person go down alone. Descent is the most important treatment; even a few hundred metres can help.

Reducing the risk

• Go up gradually. Spending time at intermediate heights before a high hike gives your body a chance to adjust. Sleeping height matters more than the height you reach in the day.
• Keep the first high hikes short and easy, and build up.
• Eat and drink well, even if your appetite is low.
• Do not push through symptoms to reach a summit.
• Talk to a doctor before a high trip if you have a heart or lung condition, are pregnant, or have had altitude illness before. A doctor can also advise on medication for prevention; that is a medical decision, not one for a hiking website.

Calibrating at height

The most useful adjustment is one you measure yourself. On your first easy hike at a new height, note the distance, the climb and your moving time, then put the distance and climb into the Hiking Time Calculator. If your real time matched the leisurely figure, use leisurely for the rest of the trip, at least until you have spent a few days at that height. If you were slower than leisurely, add your own percentage on top, and use a larger return slowdown in the turnaround calculator too. Do the same for water: if you finished the hike with an empty bottle and a headache, the next day needs more water and probably an easier route. Measured numbers from the trip itself beat any rule written for lower ground.

The rest of the plan still applies

At altitude the basics matter more: warm and waterproof layers, sun protection, a headlamp, first aid, a map and navigation, extra food and water, and a charged phone. Tell someone your route and when you expect to be back, and know the local emergency number, which may differ from home if you are travelling. Check the land manager's current conditions, any permit requirements and the mountain forecast. Then set a turnaround time with a generous margin and keep to it. The calculators give you a starting point; on high ground, the margin is what keeps you safe.