Altitude performance calculator

Aerobic power starts to fall far lower down than most people think. In trained endurance athletes, Wehrlin and Hallén (2006) measured in a hypobaric chamber that maximal oxygen uptake already drops between 300 and 800 m, and from there falls roughly linearly: 6.3 % per 1,000 m. In well-trained, unacclimatised cyclists, Clark and colleagues (2007) saw mean power in a 5 min time trial fall by 5.8, 10.3 and 19.8 % at 1,200, 2,200 and 3,200 m.

This calculator applies 6.1 % per 1,000 m, the slope that sums up those three figures, starting at 300 m if you are trained, 600 m if you are an amateur and 1,000 m if you ride recreationally. There is a leap worth knowing about: those studies measure maximal efforts lasting a few minutes, and here they are applied to your FTP. There is no equivalent data for an hour, so treat the figure as an order of magnitude.

Why the fitter rider may suffer more

Gore and colleagues (1996) took trained and untrained cyclists to a simulated altitude of just 580 m. Maximal oxygen uptake fell by 6.8 % on average in the trained group and did not change significantly in the untrained one. The explanation is that a very capable cardiovascular system is already working close to the pulmonary diffusion limit at sea level, and when oxygen pressure falls the blood leaves the lungs less saturated.

Not every study confirms it. Mollard and colleagues (2007) found no difference in the relative drop between trained and untrained men from 1,000 to 4,500 m. That is why the calculator uses the same slope for everyone and only brings forward the altitude at which the drop starts the fitter you are. In practice: if you arrive from elsewhere with 5 W/kg, you will probably feel the difference on the plateau a little more than your 3 W/kg riding partner. It is not a lack of form.

What acclimatisation gives back, and what it does not

The first hours and days are the worst. Between the first and third week part of the performance comes back — ventilation increases, bicarbonate handling improves, plasma volume adjusts — but it isnot fully recovered. Schuler and colleagues (2007) followed eight elite cyclists at 2,340 m: maximal oxygen uptake fell by 12.8 % on the first day, recovered about a fifth of that loss in the first week, just over half by two weeks and about three fifths by three, when it was no longer improving significantly. This calculator applies those proportions: a fifth after a week and three fifths from three weeks. Living at 2,600 m does not give you back the threshold you had at sea level; it brings you closer, and no more recovery is assumed here than those three weeks provide.

That acclimatisation is not erased by moving. The altitude where you live is always read as a resident's, and the days you declare apply only to whatever the destination has above your own altitude: if you live at 3,000 m and are going to ride at 2,600 m, you are not meeting new altitude, you are coming down inside what you have already adapted to. It is a way of splitting acclimatisation across two stretches, not a validated model of anyone's exposure history: someone who has spent months alternating altitudes is outside what this represents.

Why it does not ask for your sex

Because there is no coefficient to apply. Guenette and colleagues (2004) measured oxygen saturation during maximal exercise in trained and untrained cyclists of both sexes: trained men fell to 91.4 % and trained women to 91.3 %, with no meaningful difference. What separated the groups was training, not sex. Asking here would give an appearance of precision the evidence does not support; where it does change the calculation — body composition — it is asked for.

Where it stops working

• The spread between people is wide. These figures are an extrapolation from group averages, not a prediction for you: the studies they come from measured efforts of a few minutes in small groups. Some lose considerably less than the estimate and some lose considerably more.
• It only models the aerobic effect. Very short, explosive efforts are affected far less. Their power barely changes, and the speed they produce can improve because of the lower air resistance.
• Thinner air gives something back. On the flat and downhill, the same power gives more speed. That is why on a climb the loss shows in full and on a flat time trial it is partly compensated.See that effect on the Alto de Patios →
• Coming down does not guarantee a performance boost.The response depends on exposure, recovery and individual adaptation. This tool does not predict a peak-performance window.