The short answer
Calories burned = MET × your weight in kilograms × hours. A 70 kg person running at a moderate pace (MET 10.5) burns about 368 calories in 30 minutes. Heavier people burn more for the same activity, because moving more mass costs more energy.
How the calculation works
Every activity has a MET value: a number expressing its energy cost relative to sitting still. Sitting quietly is 1 MET. Brisk walking is around 6.5. Running at a moderate pace is about 10.5. Those values come from the 2024 Adult Compendium of Physical Activities, a research reference built from measured oxygen consumption across hundreds of activities.
From there the arithmetic is simple:
Calories = MET × weight (kg) × duration (hours)
Three things follow from that formula, and they explain most of the confusion people have about exercise calories.
Body weight scales the result. At the same pace for the same time, a 100 kg runner burns roughly 43 percent more than a 70 kg runner. Moving more mass costs more energy. This is why generic "calories burned running" figures without a weight input are close to useless.
Intensity matters more than most people expect. Walking briskly at 6.5 METs for an hour and running at 13 METs for half an hour burn similar amounts, but the running route gets there in half the time.
Duration is linear. Twice the time is twice the burn, all else equal. There is no bonus tier past some threshold.
What the number does not include
Two caveats that most calculators skip, and both push in the same direction: the figure you see is generous.
It is gross, not net. MET values include the roughly 1 MET of resting metabolism you would have burned anyway during that hour. For a 30 minute session the overlap is modest, perhaps 30 to 40 calories, but for long low-intensity activity it becomes significant. If you are counting exercise as a credit against food, the honest figure is net burn, which is lower.
It is an average across many people. Efficiency varies. Two people of identical weight running the same pace can differ by 15 to 20 percent depending on running economy, technique and fitness. A trained cyclist is more efficient than a beginner, meaning they burn slightly less for the same power.
Neither point makes the estimate useless. It makes it a good tool for comparing activities and a poor one for precise calorie accounting.
The double counting trap
This is the single most common way exercise calories cause weight-loss plateaus.
If you calculated your daily calorie target using an activity multiplier, as our TDEE calculator does, your regular training is already baked into that number. Adding today's workout burn on top counts it twice. Combine that with wearables that tend to overestimate, and a 400 calorie "earned" snack can quietly erase a genuine deficit.
The cleaner approach: set your target from your typical week including training, then leave it alone. Use the burn figures here to understand relative effort between activities, not as a daily allowance to spend.
Why the burn side is the easier half
Exercise is the visible, satisfying part of the equation, which is exactly why it gets more attention than it deserves. A hard 45 minute session might account for 400 to 500 calories. A single restaurant meal can carry more than that, and takes twenty minutes to eat.
That asymmetry is not an argument against training, which does plenty that calorie burn does not capture: preserving muscle in a deficit, cardiovascular health, sleep, mood. It is an argument for paying at least as much attention to intake, which is the side almost everyone tracks badly.
CalAlly exists for that side. You log a meal by describing it out loud, the AI breaks it into ingredients, calories and macros, one tap confirms it, about 3 seconds. And the AI coach reads your real data, so it can tell you when the extra sessions are not landing because the intake side quietly moved.