You can measure your aerobic fitness by the maximum quantity of oxygen your body consumes. This measurement is called VO2 Max and typically uses units of millilitres of oxygen per kilogram of body weight per minute (ml/kg/min). Your MET level is your baseline VO2 consumption at rest, which is set at 1ml/kg/min. The metabolic equivalent (MET) offers a convenient measurement of VO2 levels, as it relates to this baseline. As an example, if your VO2 Max was 12 METs, you would know that your maximum VO2 consumption is 12 times higher than your resting VO2 consumption.
Determine your VO2 levels. This will usually require monitoring in a facility, since you would need access to expensive equipment that measures your oxygen intake during exercise. You can also measure your VO2 Max to find out your maximum aerobic fitness level.
Divide the measured VO2 level by 3.5 to calculate the number of METs that figure represents. As an example, if you measured 28ml/kg/min, then you divide 28 by 3.5, which gives you the measurement as 8 METs.
Compare your number of METs to required levels. Some professions, such as firefighting, require a level be achieved. As a firefighter you may be required to have a MET level of 12, for instance.