Every calorie-burn number on a treadmill, phone app or fitness watch traces back to a single concept: the metabolic equivalent, or MET. 1 MET equals the energy cost of sitting quietly — approximately 1 kcal per kilogram of body weight per hour, or 3.5 mL of oxygen per kg per minute. Running at 6 mph is 10 MET, meaning your body burns 10 times its resting rate. The formula for total burn is dead simple: kcal = MET × weight(kg) × hours.

The Compendium of Physical Activities, maintained since 1993 and updated in 2011, catalogs over 800 activities with peer-reviewed MET values. Walking at 3 mph is 3.5 MET. Cycling at 12–14 mph is 8 MET. Vigorous swimming: 10 MET. Weightlifting at general intensity: 3.5 MET. High-intensity CrossFit-style circuits: 8–12 MET. These numbers form the basis of every fitness app's calorie calculation, wearable or not.

The accuracy trap: MET tables assume steady-state work at a given intensity in a healthy adult of average body composition. Reality is messier. A well-trained runner uses less oxygen at any given pace than a beginner — burning fewer calories to cover the same distance. Wearables using heart rate + accelerometer + GPS estimate MET dynamically and typically over-report by 15–30% (Stanford 2017 study). Fitness machines are even worse, often 20–40% high.

For fat loss planning, do not eat back all reported exercise calories. If your TDEE calculation already used a 'moderate activity' multiplier, your workouts are baked in. Adding them back double-counts. Best practice: use MET-based numbers for weekly planning, use weekly weight trend for reality-check, and let the scale correct any estimation drift over 2–3 weeks.

The bigger picture matters more than the burn number. A 30-minute run burns 300–400 kcal — one dessert. Diet drives 80% of fat loss because caloric density in food is enormous compared to caloric spend in exercise. Where exercise wins: metabolic health, muscle mass, cardiovascular fitness, sleep quality, mood, and long-term maintenance. Track calories burned to stay accountable, not to earn food.