How it is calculated
The formula is calories ≈ MET × weight(kg) × hours. A MET, or metabolic equivalent of task, expresses how much energy an activity uses relative to sitting quietly, which is defined as 1 MET.
Worked example: running at about 9.7 km/h is rated 9.8 METs. A 70 kg person running for 30 minutes burns roughly 9.8 × 70 × 0.5 ≈ 343 calories. A second example: brisk walking at 5 METs for 45 minutes at 80 kg gives 5 × 80 × 0.75 = 300 calories.
Where MET values come from
They are drawn from the Compendium of Physical Activities, a reference catalogue first published in 1993 and substantially updated in 2011, which assigns MET values to hundreds of activities based on measured oxygen consumption in research settings.
The compendium's own authors are clear about its limits: the values are population averages measured in specific groups under defined conditions, intended primarily for standardising physical activity in research, not for precise individual prediction. Using them to estimate your personal burn is a reasonable extrapolation, not the purpose they were built for.
Why this figure — and your tracker — overestimate
Three separate issues, and they compound.
The gross versus net problem. The MET calculation gives your total energy expenditure during the activity, including the calories you would have burned anyway just being alive for that half hour. If your resting rate is about 1.2 kcal per minute, 30 minutes of existing costs roughly 36 calories regardless. The genuinely additional burn from your run is therefore about 343 − 36 ≈ 307. A modest correction here, but it grows with duration and matters for low-intensity activities, where a substantial share of the "burn" is just baseline metabolism.
Individual variation. Fitness, technique, terrain, gradient, temperature, and body composition all shift real expenditure. Better-trained people are more economical and often burn less doing the same work.
Device optimism. Wearables and gym cardio machines commonly overestimate energy expenditure substantially — validation studies routinely find errors of 20–40%, almost always in the flattering direction. Treadmills that do not know your weight are guessing outright.
You cannot outrun your diet
The arithmetic is unkind. Burning 300 calories takes half an hour of genuine effort; eating them back takes about ninety seconds and a pastry. This is why exercise alone is a poor primary tool for fat loss, and why studies of exercise-only interventions produce disappointing weight results.
There is a further wrinkle: energy compensation. People frequently move less for the rest of the day after training, and appetite often rises, so the net deficit is smaller than the workout suggests. None of this is an argument against exercise — it is an argument against using it as your main lever for weight change. Your calorie intake does most of that work.
Exercise for what it is actually good at
Cardiovascular fitness, insulin sensitivity, blood pressure, bone density, mood, sleep quality, cognitive function, and the muscle mass that determines how well you function in later life. WHO guidelines recommend adults get 150–300 minutes of moderate-intensity activity per week, or 75–150 minutes of vigorous activity, plus muscle-strengthening on two or more days.
Those benefits are large, well evidenced, and mostly independent of whether you lose any weight at all. If the scale is the only reason you exercise, you are undervaluing it considerably.