V̇O₂ max (also maximal oxygen consumption, maximal oxygen uptake or maximal aerobic capacity) is the maximum rate of oxygen consumption attainable during physical exertion. The name is derived from three abbreviations: "V̇" for volume (the dot over the V indicates "per unit of time" in Newton's notation), "O₂" for oxygen, and "max" for maximum and usually normalized per kilogram of body mass. A similar measure is V̇O₂ peak (peak oxygen consumption), which is the highest rate attained during a session of submaximal physical exercise. It is equal to, or less than, the V̇O₂ max. Confusion between these quantities in older and popular fitness literature is common. The capacity of the lung to exchange oxygen and carbon dioxide is constrained by the rate of blood oxygen transport to active tissue.

The measurement of V̇O₂ max in the laboratory provides a quantitative value of endurance fitness for comparison of individual training effects and between people in endurance training. Maximal oxygen consumption reflects cardiorespiratory fitness and endurance capacity in exercise performance. Elite athletes, such as competitive distance runners, racing cyclists or Olympic cross-country skiers, can achieve V̇O₂ max values exceeding 90 mL/(kg·min), while some endurance animals, such as Alaskan huskies, have V̇O₂ max values exceeding 200 mL/(kg·min).

In physical training, especially in its academic literature, V̇O₂ max is often used as a reference level to quantify exertion levels, such as 65% V̇O₂ max as a threshold for sustainable exercise, which is generally regarded as more rigorous than heart rate, but is more elaborate to measure.

V̇O₂ max is expressed either as an absolute rate in (for example) litres of oxygen per minute (L/min) or as a relative rate in (for example) millilitres of oxygen per kilogram of the body mass per minute (e.g., mL/(kg·min)). The latter expression is often used to compare the performance of endurance sports athletes. However, V̇O₂ max generally does not vary linearly with body mass, either among individuals within a species or among species, so comparisons of the performance capacities of individuals or species that differ in body size must be done with appropriate statistical procedures, such as analysis of covariance.

Accurately measuring V̇O₂ max involves a physical effort sufficient in duration and intensity to fully tax the aerobic energy system. In general clinical and athletic testing, this usually involves a graded exercise test in which exercise intensity is progressively increased while measuring:

V̇O₂ max is measured during a cardiopulmonary exercise test (CPX test). The test is done on a treadmill or cycle ergometer. In untrained subjects, V̇O₂ max is 10% to 20% lower when using a cycle ergometer compared with a treadmill. However, trained cyclists' results on the cycle ergometer are equal to or even higher than those obtained on the treadmill.

The classic V̇O₂ max, in the sense of Hill and Lupton (1923), is reached when oxygen consumption remains at a steady state ("plateau") despite an increase in workload. The occurrence of a plateau is not guaranteed and may vary by person and sampling interval, leading to modified protocols with varied results.

V̇O₂ may also be calculated by the Fick equation: ${\displaystyle {\ce {{\dot {V}}O2}}=Q\times \ (C_{a}{\ce {O2}}-C_{v}{\ce {O2}})}$, when these values are obtained during exertion at a maximal effort. Here Q is the cardiac output of the heart, C_aO₂ is the arterial oxygen content, and C_vO₂ is the venous oxygen content. (C_aO₂ – C_vO₂) is also known as the arteriovenous oxygen difference.