Calories Burned from MET, Mass and Time

A MET is a metabolic equivalent of task, defined as the energy your body burns sitting quietly: about 3.5 millilitres of oxygen per kilogram per minute, which works out to roughly one kilocalorie per kilogram of body mass per hour. That last form is the useful one, because it makes the arithmetic trivial. An activity rated at 8 METs burns eight kilocalories per kilogram per hour, so a 70 kg person running for half an hour spends about 280 kcal.

The MET values themselves come from the Compendium of Physical Activities, a catalogue assembled by Barbara Ainsworth's group from measured oxygen uptake across hundreds of activities. The catalogue is honest about being an average over the people who were measured. Real expenditure varies with fitness, efficiency, terrain and technique, and the 3.5 mL/kg/min baseline is itself high for many older and heavier adults, so the estimate tends to run generous.

Two further cautions. This is gross energy, including the calories you would have burned anyway just by existing during that hour. And the figure a treadmill or watch reports is the same kind of estimate, which is why two devices strapped to the same person disagree by a hundred calories without either of them malfunctioning.