VO2 max, the maximum volume of oxygen your body can use during intense exercise, is one of the most respected indicators of cardiovascular fitness, but measuring it precisely requires a lab test with specialized breathing equipment while exercising to true exhaustion, not something most people can access. Field-based and formula-based estimates fill that gap, trading some accuracy for something you can calculate from numbers you already have.
What the number actually represents
VO2 max is expressed as milliliters of oxygen consumed per kilogram of body weight per minute (ml/kg/min). A higher number means your body can deliver and use more oxygen during maximal effort, which generally correlates with better cardiovascular endurance, since oxygen delivery to working muscles is a core limiting factor in sustained aerobic performance. Elite endurance athletes can post VO2 max values well above 70 ml/kg/min, while sedentary adults commonly fall in the 25-40 range, though normal ranges vary by age and sex.
Estimating it from resting heart rate
One common non-lab method, the Uth-Sørensen-Overgaard-Pedersen estimation, uses the ratio of maximum heart rate to resting heart rate, VO2 max ≈ 15.3 × (max HR ÷ resting HR), relying on the physiological relationship between heart rate efficiency and aerobic capacity: a fitter cardiovascular system generally has a lower resting heart rate and a proportionally larger ratio between max and resting values. Since max heart rate itself is usually estimated using 220 minus age, this method stacks one estimate on top of another, which is part of why it's a rough approximation rather than a precise figure.
Estimating it from performance data instead
An alternative approach estimates VO2 max from real-world performance, such as time to complete a set distance run (like the widely used Cooper 12-minute run test, or various race-time-based formulas), on the logic that faster sustained running speed directly reflects greater aerobic capacity, since you need more oxygen delivery to sustain a faster pace. Performance-based estimates tend to correlate somewhat better with lab-tested VO2 max than resting-heart-rate-based formulas for people who are already reasonably fit, though both remain estimates rather than exact measurements. The VO2 Max Calculator can use either input pathway depending on what data you have available, and pairing a VO2 max estimate with tracked calories burned during cardio sessions gives a broader picture of aerobic training load over time.