How to Calculate Calories Burned (Formula, Calculator, and Why Your Watch Is Wrong)

Marcus weighs 82 kg. He runs for 30 minutes at 6 mph, which the Compendium rates at 9.8 METs.

Step 1, calories per minute. 9.8 × 3.5 × 82 ÷ 200 = 14.1 calories a minute.

Step 2, multiply by duration. 14.1 × 30 = 422 calories. That is his gross burn.

Step 3, subtract what he would have burned anyway. Sitting still for those same 30 minutes would have cost him 43 calories. So the run genuinely added 379 calories, not 422.

Almost nobody does that third step. It gets its own section below.

Your body burns calories whether you exercise or not. A 300-calorie workout does not mean 300 extra calories.

Gross burn is the total energy the activity cost. Net burn is gross minus the calories you would have spent anyway, lying on the sofa, over the same period. Only the net figure is a genuine addition to your deficit.

Marcus burned 422 gross calories on his run. Sitting still for 30 minutes would have cost him 43. So his run added 379.

The gap is small for hard exercise and embarrassing for easy exercise. Thirty minutes of yoga at 80 kg is 105 gross calories, but 42 of those were happening anyway, so the net is 63. Fully 40% of the number on the screen was never extra.

This is the single biggest reason people burn plenty and lose nothing. They earn a gross number, eat against it, and the real deficit was never as large as the screen said. Working out the right target in the first place is what the guide to calculating a calorie deficit is for.

Apple Watch does not use the MET formula. It combines heart-rate data, motion from the accelerometer and gyroscope, and the personal details you entered (age, sex, height, weight) in a model Apple does not publish.

It reports two numbers, and the difference between them causes most of the confusion. Active calories are what your movement added. Total calories are your resting burn plus your active burn. If your watch and this calculator disagree wildly, check which of the two you are reading.

Calibration matters more than people realise. Apple asks you to walk or run outdoors with GPS for around 20 minutes so the watch can learn your stride and fitness. Without that, it is guessing from a generic model.

It is still an estimate. Apple has never published the model, so anyone quoting a precise accuracy percentage for it is inventing one. Treat the number as directional.

Fitbit works differently again. It runs your BMR continuously as a baseline and adds calories on top from heart rate and motion.

That is why your Fitbit already shows several hundred calories burned when you wake up, which reliably confuses people. It is not broken. It is counting the energy you spend simply being alive, the same 1 MET this page keeps subtracting.

Because that baseline is always running, a Fitbit number is closer to a gross, all-day figure than to the extra burn from a workout. Do not read your daily Fitbit total as calories you have earned back.

Heart-rate estimates are least reliable in strength training. Your heart rate spikes between heavy sets, but heart rate is a poor proxy for energy cost under a barbell, so the figure tends to drift.

Do not eat back your exercise calories, or eat back at most a small fraction of them.

Three errors stack in the same direction. Your tracker reports gross rather than net. Wrist-based heart-rate models tend to overstate. And if you got your daily target from a TDEE calculator, your activity level is already baked into it, so eating exercise calories back counts them twice.

Take a 300-calorie treadmill reading. Perhaps 40 of those were resting burn, so 260 net. Perhaps the machine overstated by 15%, so call it 220. Eat 300 back and you have wiped out most of a 500-calorie deficit and added a little on top.

Set your target from your maintenance calories and leave it alone. Exercise is what moves the maintenance figure over time, not a coupon you redeem at dinner.

1. Using gross calories to plan a deficit. The resting burn inside that number was always going to happen. Use the net figure.
2. Eating back what the machine reported. Gym cardio machines and wrist trackers both tend to overstate, and your TDEE already includes your training.
3. Reading Total calories when you meant Active. On Apple Watch these are very different numbers, and Total includes everything you burned lying in bed.
4. Trusting heart rate during strength training. Heart rate spikes between heavy sets, but it is a weak proxy for energy cost when lifting.
5. Treating a MET value as exact for you. METs are population averages. Your fitness, technique, and terrain all move the real figure.