Meet the METs
Most of us think of exercise as burning up calories. But exercise researchers think in metabolic equivalents, or METs.
A single MET is the amount of energy you expend when you're sitting quietly. An activity is rated at 2 METs if it requires twice as much oxygen as sitting, 3 METs if it requires three times as much oxygen, and so forth and so on.
Activities that fall into the 3.0 to 6.0 MET range (some experts draw the line at 5.9) are classified as moderate and those over 6.0 are classified as vigorous. When guidelines say we should aim for 30 minutes of moderate-intensity physical activity five days a week, they mean activity in the 3.0 to 6.0 MET range. Similarly, the alternative of 20 minutes of vigorous activity three days a week means activity rated above 6.0 METs. The formula for figuring out the calories you've burned is METs × weight (in kilograms) × hours of activity.