How to calculate calories burned during exercise
Two people can do the exact same workout and burn very different calories, which makes any online calculator a rough guess dressed up as precision. Age, weight, gender, how much muscle you carry, even the temperature of the room all nudge the number around. The formula behind those calculators usually comes from a few lab studies, averaged across a handful of people, and then applied to everyone else.
I think the useful part is not the exact number but the pattern. Seeing how a brisk walk compares to swimming helps you plan, and the act of tracking at all makes you pay attention to your body. But I'd want the calculators to be honest about their fuzziness, to say this is a ballpark, not a receipt.
The deeper thing is that metabolism is not a fixed dial. It shifts with sleep, stress, what you ate yesterday. A calorie burned today is not the same as one burned last week. So use the number as a guide, but trust the longer trend more than any single calculation.