How these calorie estimates work
For any activity not covered by its own calculator, from elliptical and weights to tennis and yard work, the calories burned calculator uses the Compendium value for 75 activities.
Running, walking and treadmill calories come from the ACSM metabolic equations, which estimate oxygen use from speed and slope; every litre of oxygen is counted as 4.9 kcal, the figure in ACSM's own table. That is why, for running, the result depends mostly on your weight and the distance, and much less on how fast you went. Hiking and swimming use MET values from the 2024 Adult Compendium of Physical Activities, the reference list used in research.
Cycling and rowing work from power instead. The cycling calorie calculator estimates watts from speed with the physics of rolling and air resistance (or takes your power meter's average), and the rowing calorie calculator uses the Concept2 monitor formula. The running, walking, treadmill, hiking, swimming and stationary bike pages show gross calories and net calories, which leave out what you would burn at rest anyway.
All of these are estimates for an average body. Fitness watches use their own methods, so expect their numbers to differ.