Calories, BMR, and TDEE: How Energy Balance Actually Works
From the Mifflin-St Jeor equation to the activity multiplier, the complete mental model for planning weight change
1. BMR vs TDEE: The Two Numbers You Need
- BMR (Basal Metabolic Rate: calories you burn lying still, awake, fasting. The minimum to keep organs functioning and body temperature regulated
- TDEE (Total Daily Energy Expenditure: BMR × activity factor. What you actually burn through a typical day including walking, working, and exercise
To lose weight, eat below TDEE. To gain, eat above TDEE. To maintain, eat at TDEE. The size of the gap determines how fast — 500 cal/day ≈ 1 lb/week change in stored fat
2. Mifflin-St Jeor Equation Explained
Women: BMR = 10W + 6.25H − 5A − 161
W = weight (kg), H = height (cm), A = age (years)
Mifflin-St Jeor is more accurate than the older Harris-Benedict equation (~5% error vs ~10%). Katch-McArdle is even more accurate when you know lean body mass, but Mifflin-St Jeor is the practical default without body composition testing.
3. Activity Multipliers
1.2 — Sedentary
Office work, no exercise. Most of today's knowledge workers fall here on weekdays
1.375 — Light
Walking, occasional exercise 1-3 days/week. The post-pandemic casual exerciser
1.55 — Moderate
Intense exercise 3-5 days/week. Most recreational athletes land here
1.725 — Very Active
Daily intense training or twice-a-day. Serious athletes and competitive amateurs
4. Why Calories Aren't Everything
Two people eating identical calories can have dramatically different outcomes because of:
- Protein adequacy: 0.7-1g per pound bodyweight preserves muscle during fat loss
- Fiber intake: 25-35g/day supports satiety and gut health
- Ultra-processed vs whole foods: same calories, very different satiety and metabolic response
- Sleep quality: poor sleep disrupts ghrelin/leptin and increases appetite by 200-300 cal/day
- Stress hormones: chronic cortisol promotes abdominal fat storage
5. Privacy for Body Composition Data
🔒 Browser-Only Computation
Calorie data often intersects with weight, body image, and eating-disorder history. ToolWise's Calorie Calculator runs entirely in your browser — your stats, goal, and target never touch our servers or get logged by analytics.
A Calorie Target You Can Trust
Calories are the smallest unit of weight management, but the only unit that fits any goal. The ToolWise Calorie Calculator uses Mifflin-St Jeor BMR plus your activity to compute a defensible target — then pair it with adequate protein, fiber, and sleep for the actual body composition you want.