Calorie Expenditure: From BMR to a Daily Target, One Formula at a Time

Energy balance in three lines

Every weight-management plan reduces to the same equation: intake above expenditure means weight gain, intake below expenditure means weight loss, and intake equal to expenditure holds weight steady. The hard part is estimating expenditure accurately, and that's what this simulation's three-stage model does — BMR, then TDEE, then a calorie goal.

Basal Metabolic Rate: the engine at idle

Basal Metabolic Rate is the energy your body burns at complete rest — breathing, circulation, cell repair, keeping temperature stable. This calculator uses the Mifflin-St Jeor equation, generally considered more accurate for modern populations than the older 1919 Harris-Benedict formula:

men:   BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161

BMR naturally declines with age as muscle mass shrinks (a process called sarcopenia), and men typically post a higher BMR than women at the same weight and height, mostly because of greater average muscle mass — muscle tissue burns more energy at rest than fat tissue, which is part of why strength training raises long-run expenditure.

From BMR to TDEE

Total Daily Energy Expenditure multiplies BMR by an activity factor: 1.2 for a sedentary desk job, 1.375 for light activity, 1.55 for moderate exercise, 1.725 for very active, and 1.9 for extra-active manual labour or serious training. Because the multiplier applies to the whole BMR, it compounds — someone with a 1,600 kcal BMR moves from a 1,920 kcal TDEE at Sedentary to a 3,040 kcal TDEE at Extra Active, an 1,120 kcal swing from activity level alone.

TDEE = BMR × activity_factor
  sedentary 1.20 · light 1.375 · moderate 1.55 · very active 1.725 · extra active 1.90

A large share of that variation is not structured exercise at all but NEAT (Non-Exercise Activity Thermogenesis) — fidgeting, standing, walking around the house, taking the stairs — which research suggests can account for anywhere from 50% to 80% of daily calorie expenditure outside of a dedicated workout.

Setting a calorie goal

Roughly 7,700 kcal are stored per kilogram of body mass, so a sustained daily deficit or surplus of 500 kcal works out to about 500 × 7 ÷ 7,700 ≈ 0.45 kg (1 lb) of change per week — a moderate, broadly sustainable rate that this calculator applies directly to the Lose and Gain goals. The Thermic Effect of Food adds a further wrinkle: protein burns roughly 20-30% of its own calories just being digested, versus 5-10% for carbohydrates and 0-3% for fat, which is one reason higher-protein diets are often recommended alongside a calorie deficit.

Frequently asked questions

Try it live

Everything above runs in your browser — open Calorie Expenditure Calculator and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.