BMR plus an activity multiplier equals TDEE. Set a deficit for fat loss (500 kcal below) or a surplus for muscle gain (300-500 kcal above), with daily examples.
A calorie is a unit of energy. In nutrition, the Calorie shown on food labels is technically a kilocalorie, the energy required to raise 1 kilogram of water by 1 degree Celsius. The human body converts food into this energy to fuel every process from heartbeat to movement, and energy balance, the relationship between calories consumed and calories burned, determines whether weight rises, falls, or stays the same.
Total Daily Energy Expenditure, or TDEE, is the full number of calories a person burns in a day. The National Institute of Diabetes and Digestive and Kidney Diseases breaks it into three main parts: resting energy (BMR), which is 60 to 75 percent of the total; the thermic effect of food, about 10 percent; and physical activity, which is the most variable part. Because activity varies so widely, two people of the same size can have TDEEs that differ by hundreds of calories.
Weight is stable when intake equals TDEE. A sustained surplus of roughly 500 calories per day tends to add about 1 pound per week, and a comparable deficit removes it, although the relationship is not perfectly linear over long periods. This energy balance equation is the foundation behind every credible weight-management guideline.
BMR is the energy the body burns at complete rest, the baseline cost of staying alive. Because it cannot be measured without laboratory equipment such as an indirect calorimeter, it is estimated with prediction equations. The Mifflin-St Jeor equation, endorsed by the Academy of Nutrition and Dietetics after its 2005 evidence review, is the most widely used and is accurate to within about 10 percent for most adults.
BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + s ( s = +5 for men, −161 for women )
| Equation | Best Use |
|---|---|
| Mifflin-St Jeor | General population; recommended default |
| Harris-Benedict (revised 1984) | Traditional; tends to overestimate slightly |
| Katch-McArdle | When body fat % is known; best for lean or athletic builds |
Worked example: a 30-year-old woman who is 5 foot 5 (165 cm) and weighs 150 pounds (68 kg). BMR = 10 × 68 + 6.25 × 165 − 5 × 30 − 161 = 680 + 1,031.25 − 150 − 161 = 1,400 calories per day. This is the energy she would burn lying in bed all day, before any activity is counted, and it already covers roughly two-thirds of her total daily burn.
TDEE is estimated by multiplying BMR by an activity factor. The multipliers, drawn from the Harris-Benedict convention, categorize lifestyle from sedentary to highly active. They are population averages, so two people in the same category can still differ based on how much they move outside of formal exercise.
| Activity Level | Multiplier — example TDEE (BMR 1,400) |
|---|---|
| Sedentary (desk job, little exercise) | 1.2 — 1,680 |
| Lightly active (light exercise 1-3 days/week) | 1.375 — 1,925 |
| Moderately active (moderate exercise 3-5 days/week) | 1.55 — 2,170 |
| Very active (hard exercise 6-7 days/week) | 1.725 — 2,415 |
| Extra active (physical job plus training) | 1.9 — 2,660 |
The most common error is overestimating activity. Someone who exercises intensely for one hour but sits for the other 23 is often "lightly active" overall, because non-exercise movement such as walking, standing, and fidgeting frequently contributes more total energy than the workout itself. The American Council on Exercise recommends selecting a category based on a full week of actual movement rather than planned gym time alone.
Once TDEE is known, it becomes the anchor for any weight goal. Maintenance means eating at TDEE. A deficit of 500 calories per day targets roughly 1 pound of loss per week, based on the convention that 1 pound of body fat contains about 3,500 calories. A surplus of 250 to 500 calories per day supports gradual weight gain, commonly used for muscle building.
For the example woman with a TDEE of 2,170 (moderately active), a fat-loss target of 1 pound per week means eating about 1,670 calories per day. If she were sedentary with a TDEE of 1,680, the same 500-calorie deficit would leave only 1,180 calories, which is near the minimum safe intake generally recommended for women. This illustrates why increasing activity, rather than cutting food further, is often the better lever once intake is already low.
Real-world loss runs 15 to 25 percent slower than the linear 3,500-calorie rule predicts, because BMR drops as weight drops and the body becomes more energy-efficient. The NIDDK advises recalculating TDEE after every 5 to 10 pounds of change. The Metriova calorie calculator computes BMR with Mifflin-St Jeor, applies the activity multiplier, and outputs maintenance, deficit, and surplus targets automatically. Sources: National Institute of Diabetes and Digestive and Kidney Diseases, Academy of Nutrition and Dietetics, American Council on Exercise, and Mifflin et al. (1990). This content is educational and is not medical advice.
Calorie Calculator — TDEE & Weight Loss Plan
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