Every honest exercise calorie number routes through one small unit: the MET, or metabolic equivalent of task. Understanding it takes two minutes and permanently changes how you read fitness displays.
The definition
1 MET = the energy cost of sitting quietly ≈ 3.5 ml O₂ per kg per minute ≈ 1.2–1.4 kcal per minute for a typical adult
An activity rated "5 METs" costs five times the resting rate. The scale is intuitive at the edges: sleeping ~0.9, desk work ~1.5, walking ~3–5, running ~8–12, and the all-out sprint ceiling ~20+ (sustainable only for seconds). The formula that turns it into calories:
kcal/min = MET × 3.5 × weight(kg) ÷ 200
Where MET values come from
The Compendium of Physical Activities (Ainsworth et al., first published 1993, updated since) cataloged hundreds of activities by measuring oxygen consumption across groups of adults in laboratory and field conditions. Its values are the closest thing exercise science has to ground truth — and the source most researchers would defend. Two honest caveats are built into it: values are population averages (your efficiency differs ±10–20%), and several activities span wide ranges (yoga 2.0–6.0) where style and effort dominate.
Why your watch disagrees
| Source | Typical error | Why |
|---|---|---|
| Compendium + your weight | ±10–20% | Population average applied to you |
| Gym machine display | +15–40% | Default weights, gross-burn math, optimism |
| Wrist wearable | +20–90% (varies) | Heart rate ≠ energy; calibrations favor flattery |
| Metabolic cart | Reference | Direct oxygen measurement — the ground truth |
The wearable problem is structural: heart rate rises with effort, emotion, heat, caffeine, and hydration status — it correlates with energy cost but is not the same quantity. Devices trained on population data then err in whatever direction their calibration favored. Validation studies routinely find overestimation, sometimes dramatically.
The net-vs-gross distinction
Gross burn (everything during the session) includes the ~1.2–1.4 kcal/min you burn while alive. Net burn subtracts it. For an hour of brisk walking at 75 kg: gross ≈ 300 kcal, net ≈ 220. For energy-balance purposes — the only place calories matter — net is the honest currency. Devices and machines rarely make the subtraction, which is a quiet second source of their optimism.
Using METs without the math
- 3–6 METs = moderate intensity (the health-guideline zone: brisk walk, easy cycle) — you can talk but not sing.
- 6+ METs = vigorous — speech breaks into phrases.
- 150 weekly minutes of moderate (3–6 MET) or 75 of vigorous is the guideline dose — METs are literally the unit the guidelines are written in (they call it MET-minutes).
Our activity calculator applies the Compendium directly to your weight; the reality check discounts the machines. The unit behind every number is now yours to audit.