Your session
An estimate from average MET values. Individual burn varies widely.
MET × weight × hours
A MET is a multiple of resting energy use: walking briskly is about 3.8 METs, so it costs roughly 3.8 times what sitting still does. Multiply by body weight in kilograms and by hours, and you have the estimate. A heavier body burns more for the same activity, because moving more mass costs more.
Exercise burns less than the food it replaces
Half an hour of brisk walking for a 70 kg adult comes to about 130 calories — less than a can of soft drink. This is not an argument against exercising; it is an argument against treating exercise as a licence to eat. The health case for movement has almost nothing to do with the calorie count.
The MET table describes averages
Fitness, technique and terrain all shift the real figure, sometimes by a third. Watches and machines estimate too, usually generously. Treat any of these numbers as an order of magnitude rather than a measurement.
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Frequently asked questions
Q. Does this include what I would have burned anyway?
Yes, it is gross rather than net. Sitting for that half hour would have cost something too, so the calories genuinely added by the exercise are a little lower than the figure shown.
Q. Why is my watch giving a different number?
Because it uses heart rate and its own model rather than a MET table. Both are estimates. Fitness trackers are widely found to run high on calorie burn, so the difference is usually in that direction.
Q. How much exercise does losing a kilogram of fat take?
Roughly a 7,700 calorie deficit. At 300 calories a session that is around 25 sessions with no change in eating — which is why diet does the heavier lifting in weight loss, and exercise does it for everything else.