Altitude Calculator — Altitude Effects on Performance

Aerobic power starts falling almost as soon as you leave sea level — Bassett et al. (1999) put the unacclimatised loss at roughly 5% by 1000m and about 9% by 1500m. At 2500m expect around 16% (unacclimatised) to 12% (acclimatised); at 3500m around 24% (unacclimatised) to 20% (acclimatised). Use the acclimatised toggle to switch between the two curves.

Yes, lower air density reduces aerodynamic drag, which helps at high speeds. However, the VO₂ max / aerobic-power reduction usually outweighs the aero benefit except in flat time trials and the hour record, where reduced drag can offset the power loss.

Partial acclimatisation takes 1-2 weeks. Fuller acclimatisation to a given altitude takes 3-4 weeks. Even when acclimatised a meaningful power reduction remains — the acclimatised Bassett curve still shows about 12% loss at 2500m versus 16% unacclimatised.

Training at altitude (or sleeping in altitude tents) stimulates red blood cell production, which improves oxygen transport when returning to sea level.