At what altitude does performance drop?

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.

Does lower air density help with speed?

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.

How long does acclimatisation take?

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.

Why do teams train at altitude?

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

BikeCalcs

Altitude Calculator — Altitude Effects on Performance

Calculate altitude effects on cycling performance and power.

Altitude

Elevation above sea level

Sea-Level FTP

Functional Threshold Power measured at sea level (watts)

Acclimatised

On = several weeks at altitude (Bassett acclimatised curve); off = arriving within 1–7 days (unacclimatised)

How We Calculate This

Altitude Effects

Air density: Decreases with altitude following the ISA barometric formula — about 0.96 kg/m³ at 2500m versus 1.225 at sea level
Power reduction: Aerobic power as a % of sea level uses the Bassett et al. (1999) regressions. Non-acclimatised: y = 0.178x³ − 1.43x² − 4.07x + 100; acclimatised: y = −1.12x² − 1.90x + 99.9, where x is altitude in km
Oxygen: Effective O₂ falls in proportion to barometric pressure — roughly 15.4% at 2500m versus 20.9% at sea level

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.