Body Energy
How much energy does your body use in a day? Your basal metabolic rate keeps you alive at rest; everyday activity adds 20–90% on top. Estimated with the Mifflin–St Jeor equation.
Where the Energy Goes
About two thirds of the energy an average person uses goes to simply staying alive: keeping warm, the heart beating, the brain thinking. That is the basal metabolic rate. Activity — from walking to the fridge to running a marathon — adds the rest, captured here by an activity factor.
Your body runs at about 100 watts on average, a little more than an old light bulb. Eat more than you use and the surplus is stored; about 7 700 kcal makes a kilogram of body fat.
Energy per Day by Activity Level
kcal/dayKey Relations
Mass m in kg, height h in cm, age a in years; s is +5 for men and −161 for women. Published in 1990 and among the most accurate simple equations.
The physical activity level (PAL) multiplies the basal rate: 1.2 for mostly sitting up to 1.9 for very heavy work.
2000 kcal a day is a steady 97 W.
Sources & Verification
- M. D. Mifflin, S. T. St Jeor, L. A. Hill, B. J. Scott, S. A. Daugherty & Y. O. Koh, “A new predictive equation for resting energy expenditure in healthy individuals”, American Journal of Clinical Nutrition 51(2), 241–247 (1990).