VO2 max is the most oxygen your body can use during hard exercise, measured in milliliters of oxygen per kilogram of body weight per minute (ml/kg/min). This VO2 max calculator estimates it three ways: from a recent race, from a 12-minute run, or from your heart rate.
The race method uses Jack Daniels and Jimmy Gilbert's VDOT formula. It works out the oxygen cost of your race pace and divides it by the share of VO2 max a runner can hold for that long, so the result is an effective VO2 max from performance, not a lab measurement. A 20:00 5K gives 49.8 and a 3:30:00 marathon gives 44.6. The calculator also lists the times the same VDOT points to at other distances: for a 20:00 5K, 41:28 for 10K and 3:11:17 for a marathon.
The Cooper test is Kenneth Cooper's 12-minute run: run as far as you can in 12 minutes on a flat track, then enter the distance you covered. 2,400 meters gives 42.4 and 3,000 meters gives 55.8.
The heart rate method, from Uth and colleagues (2004), multiplies your maximum heart rate divided by your resting heart rate by 15.3. A maximum of 190 and a resting rate of 60 beats per minute gives 48.5. It was derived from well-trained men, so estimates for other people are rougher.
All three methods are estimates; individual results vary, and the methods will not always agree with each other or with a lab test.