How the MET method works
A MET, or metabolic equivalent of task, is a multiple of your resting metabolic rate. One MET is what your body burns sitting quietly, defined as 1 kilocalorie per kilogram of body weight per hour. An activity rated at 7 METs burns seven times that. The arithmetic is therefore very simple: calories = MET x weight in kilograms x hours. Because METs scale with body weight, a heavier person burns more doing exactly the same thing.
The values in the activity list come from the Compendium of Physical Activities, a reference table compiled by Ainsworth and colleagues and updated several times since 1993. Walking at 3 mph is 3.5 METs, brisk walking at 4 mph is 4.3, general jogging is 7.0, running at 6 mph is 9.8, moderate cycling is 6.8, lap swimming is 6.0, jumping rope is 11.0, hatha yoga is 2.5 and general weight training is 5.0. The US physical activity guidelines call anything from 3 to 6 METs moderate intensity and 6 or more vigorous.
A worked example
A 180-pound person walking briskly for 30 minutes: 180 lb is 81.6 kg, brisk walking is 4.3 METs, and 30 minutes is 0.5 hours. So 4.3 x 81.6 x 0.5 = 176 calories, about 5.9 calories a minute. Five sessions a week comes to 878 calories, and if nothing else in the diet changes, roughly 13 pounds of fat over a year at the conventional 3,500 calories per pound.
Change one variable at a time to see how it moves. The same half hour spent jogging at 7 METs burns 286 calories, and running at 6 mph (9.8 METs) burns 400. A 140-pound person doing the identical brisk walk burns 137 instead of 176. Doubling the time doubles the number, which is why duration is usually the easiest lever for someone who is not yet fit enough to raise the intensity.
Gross calories, net calories, and how far to trust the number
Strictly speaking the MET result is gross energy expenditure, which includes the calories you would have burned sitting on the couch during that half hour. Subtracting the 1 MET baseline gives net burn, and for the walking example that drops 176 to about 135. For weight-management math the net figure is the honest one, since the resting portion is already counted in your daily maintenance calories.
The compendium describes an average adult, and real people vary. Fitness level, movement efficiency, terrain, wind, altitude, temperature, body composition and whether you are carrying anything can each shift the true cost by 10% or more; the overall error for an individual is commonly 15-20%. Wrist-worn fitness trackers are no better, and research has repeatedly found them off by 20-90% for energy expenditure even when their heart rate readings are good. Use these numbers to compare activities and to see how much work a given effort represents, not to justify a precise dessert. Exercise is also a smaller lever than most people expect: an hour of moderate work is a few hundred calories, while appetite easily moves that much in a single meal.