How Oxygen Availability Declines at High Altitude
As elevation increases, barometric pressure decreases, reducing the number of oxygen molecules inspired with each breath—a state called hypoxia. Although air composition remains ~21% oxygen throughout the troposphere, its partial pressure drops markedly.
Calculations utilize the International Standard Atmosphere (ISA) barometric model: P = 1013.25 × (1 − 0.0000225577 × h)^5.25588, where h is altitude in meters. This equation accurately mirrors barometric gradients across the troposphere.
Blood oxygen saturation (SpO₂) declines with altitude: sea-level baseline (95–99%) can fall below 80% on high American summits like Denali (20,310 ft / 6,190 m) or Mount Whitney (14,505 ft / 4,421 m). Above 26,000 ft (8,000 m) is the extreme "death zone" where supplemental oxygen is required.
Reference values for UK peaks such as Ben Nevis, Scafell Pike and Snowdon are pre-defined in the tool. Use this calculator before your climb to plan your acclimatisation time and safety equipment.