Calculate your
total daily energy expenditure

TDEE stands for Total Daily Energy Expenditure — the total number of calories your body burns in a 24-hour period, accounting for everything from keeping your heart beating to the energy cost of your workouts. It is the single most important number in any nutrition strategy. Eat below it to lose weight; eat above it to gain; match it to maintain.

Every TDEE calculator starts with your Basal Metabolic Rate (BMR) — the calories your body burns at complete rest just to sustain organ function. The formula used here is the Mifflin-St Jeor equation, published in the American Journal of Clinical Nutrition in 1990 (PMID 2305711) and consistently validated as the most accurate predictive equation for resting energy expenditure in the general population.

The Mifflin-St Jeor BMR formula:

• Male: (10 × weight_kg) + (6.25 × height_cm) − (5 × age) + 5
• Female: (10 × weight_kg) + (6.25 × height_cm) − (5 × age) − 161

Each variable has a clear physiological basis. Weight matters because lean tissue is metabolically active — more mass means more calories burned at rest. Height correlates with body surface area, which determines heat dissipation demands. Age accounts for the gradual decline in lean mass and metabolic rate that occurs from early adulthood onward. The sex constant (+5 for males, −161 for females) reflects the average difference in lean mass distribution between biological sexes.

Once you have your BMR, the calculator multiplies it by an activity factor (the Harris-Benedict physical activity coefficient, sometimes called the PAL or physical activity level) to estimate TDEE:

• 1.2 — Sedentary: desk job, little or no deliberate exercise
• 1.375 — Lightly active: light cardio or resistance training 1–3 days per week
• 1.55 — Moderately active: moderate training sessions 3–5 days per week
• 1.725 — Very active: hard training 6–7 days per week or a physically demanding job
• 1.9 — Extra active: physically demanding occupation combined with structured hard training daily

An important caveat: static equations like Mifflin-St Jeor are population averages. A 2005 meta-analysis by Frankenfield et al. found that predictive equations overestimate resting energy expenditure by 10–15% in sedentary, overweight individuals. If you are sedentary and find that eating at your calculated TDEE causes gradual weight gain, your true TDEE is likely lower than the formula predicts — which is exactly why tracking real-world outcomes is more accurate than any equation.

This is where Zenith's adaptive TDEE differs. Instead of locking in a static formula estimate, Zenith measures your actual weekly weight trend and back-calculates your real energy balance from the data. If you're eating 2,400 kcal/day and losing 0.3 lb per week, your true TDEE is roughly 2,550 kcal — regardless of what the formula says. Zenith refines this estimate every week as more data accumulates, so your calorie target becomes more accurate over time, not less.