What exercise really costs

METs: the unit behind every estimate

Almost every calories-burned figure in circulation traces back to one dataset: the Compendium of Physical Activities, which assigns each activity a MET value — a metabolic equivalent of task. One MET is what you burn lying still, so an activity at 7 METs costs seven times resting metabolism.

The conversion into calories comes from oxygen consumption:

kcal = MET × 3.5 × kg ÷ 200 × minutes

One MET is 3.5 ml of oxygen per kilogram per minute, and a litre of oxygen liberates roughly 5 kcal. Two consequences follow, and both are worth noting. Bodyweight is in the numerator, so the same session costs a 100 kg person about a third more than a 75 kg one. And because it scales with weight, the same workout burns progressively less as you lose weight — the same self-limiting effect that governs BMR.

Gross versus net, and why it matters

Compendium MET values are gross cost. They include the resting metabolism you would have spent anyway during that hour, sitting on the sofa. Since your maintenance figure already charges you for all 24 hours, adding a gross exercise figure on top bills the resting portion twice.

The correction is to subtract the one MET of rest the workout displaced:

net kcal = (MET − 1) × 3.5 × kg ÷ 200 × minutes

This is not a cosmetic adjustment. An hour of hatha yoga at 2.5 METs for an 85 kg person is 223 kcal gross but only 134 kcal net — the gross figure overstates the additional burn by two thirds, because most of what yoga costs is simply being alive for an hour.

The error is proportionally largest for low-intensity activity, which is exactly where people most often expect a bigger number than they get. At 2.5 METs you are discarding 40% of the figure; at 9.8 METs, about 10%.

What common sessions actually cost

Two things stand out. Weight lifting is a much smaller calorie expense than most people assume — its case rests on preserving lean mass, not on energy burned. And the honest spread within a single named activity is wide: "weight lifting" runs 3.5 to 6.0 METs depending on whether you rest three minutes between sets or thirty seconds, which is a 157 kcal difference for the same 45 minutes and the largest single source of error in any of these estimates.

Averaging a week into a day

Training happens on a weekly schedule; a projection runs on daily arithmetic. The right way to reconcile them is to average the weekly total down to a per-day figure rather than model specific days.

Three 45-minute jogs a week for the 80 kg example is 378 × 3 = 1,134 kcal weekly, or 162 kcal a day. That is the figure that belongs in a deficit calculation. Modelling Tuesday and Thursday as spike days would be false precision — over months, the weekly total is what drives the trend, and no model knows which day is leg day.

162 kcal a day is also worth sitting with. Three runs a week feels substantial, and it buys about 0.15 kg a week of additional loss. That is real and worth having; it is not the dominant term people expect it to be.

Why your watch disagrees

Wrist trackers and cardio machines routinely report higher figures than the table above, for several stacking reasons:

• They usually report gross, not net. Immediately 10-40% high as an estimate of additional burn.
• Gym equipment often does not know your weight. If you never entered it, the figure is for a default person who is not you.
• Wrist-based heart rate is imprecise during resistance work.Optical sensors struggle with grip-induced motion, and elevated heart rate from a heavy set does not imply proportionate energy expenditure.
• Validation studies are unflattering. Consumer devices track heart rate reasonably well and energy expenditure poorly, with errors commonly in the 20-30% range and usually in the overestimating direction.

Practical approach: trust your tracker for trends and relative effort, not for calorie arithmetic. And never "eat back" exercise calories from a device reading, because you are adding a number with a 25% error to a food log that already has one, in the same direction.

Why exercise under-delivers for fat loss

Prescribed-exercise trials consistently produce less weight loss than the energy arithmetic predicts. The mechanisms are well documented and none of them is mysterious:

• Appetite compensation. Increased expenditure increases hunger, unevenly across individuals. Some people compensate for nearly all of it without noticing.
• Activity compensation. A hard morning session is often followed by a less active day — fewer steps, more sitting. This is the same NEAT reduction described in the adaptation guide, arriving from a different direction.
• The reward effect. A 400 kcal session followed by a 500 kcal recovery smoothie is a net surplus, and the psychology of having earned it is well studied.

This is not an argument against training. Exercise preserves lean mass during a deficit, improves nearly every health marker independent of weight, and strongly predicts whether lost weight stays lost. It is an argument against treating it as the main lever for fat loss — that lever is intake, and exercise is what makes the result worth having.

Sources

The research this guide draws on, and the claim each one supports. Figures that come from the calculator itself — the NEAT penalty, the danger thresholds, the break model — are documented with their own sources on the methodology page.

1. MET values for common activities, and the conversion to caloriesAinsworth BE, Haskell WL, et al. (2011). 2011 Compendium of Physical Activities: a second update of codes and MET values. Medicine & Science in Sports & Exercise 43(8):1575–1581.
2. The current update of those MET valuesHerrmann SD, Willis EA, et al. (2024). 2024 Adult Compendium of Physical Activities: a third update of the energy costs of human activities. Journal of Sport and Health Science 13(1):6–12.
3. Wrist-worn devices measuring heart rate well and energy expenditure poorlyShcherbina A, Mattsson CM, et al. (2017). Accuracy in wrist-worn, sensor-based measurements of heart rate and energy expenditure in a diverse cohort. Journal of Personalized Medicine 7(2):3.
4. A systematic review of wearable accuracy for energy expenditureFuller D, Colwell E, et al. (2020). Reliability and validity of commercially available wearable devices for measuring steps, energy expenditure, and heart rate: systematic review. JMIR mHealth and uHealth 8(9):e18694.
5. Why exercise trials produce less weight loss than the arithmetic predictsThomas DM, Bouchard C, et al. (2012). Why do individuals not lose more weight from an exercise intervention at a defined dose? An energy balance analysis. Obesity Reviews 13(10):835–847.
6. Total energy expenditure rising less than activity doesPontzer H, Durazo-Arvizu R, et al. (2016). Constrained total energy expenditure and metabolic adaptation to physical activity in adult humans. Current Biology 26(3):410–417.