How are calories calculated?

Using the MET method (Metabolic Equivalent of Task):

Calories (kcal) = MET × weight (kg) × time (hours)

MET values are sourced from the Compendium of Physical Activities (Ainsworth et al., 2011). 1 MET = 1 kcal/kg/h = the energy cost of sitting at rest.

Important caveats

These are estimates. Individual calorie burn varies significantly based on fitness level, body composition, and technique.
The formula counts gross calories, including basal metabolic rate. Net calories (above rest) are slightly lower.
For running, terrain, heat, and carried weight all affect actual burn.