How much does altitude affect running pace?

Altitude slows running performance primarily by reducing the oxygen partial pressure in the air, which limits the amount of oxygen your blood can carry to working muscles. At 5,000 feet (1,524 m), expect a pace penalty of roughly 3 to 5 percent for aerobic efforts. At 8,000 feet (2,438 m), that rises to 8 to 10 percent or more. The effect is most pronounced in events lasting over two minutes, where aerobic metabolism dominates. Short sprints are barely affected. Acclimatization over 10 to 21 days partially offsets the penalty, but sea-level athletes racing at altitude should adjust expected pace and effort targets accordingly.