How This Calculator Works
The Mifflin-St Jeor equation (1990) is the gold standard for estimating Basal Metabolic Rate (BMR). It uses height, weight, age, and sex to calculate the calories your body burns at rest. Multiplying BMR by an activity factor gives your Total Daily Energy Expenditure (TDEE).
BMR 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
TDEE = BMR × activity multiplier (1.2 sedentary to 1.9 very active). A deficit of 500 kcal/day produces roughly 0.5 kg fat loss per week.
Activity Level Guide
| Level | Multiplier | Description |
|---|---|---|
| Sedentary | 1.2 | Office job, little or no exercise |
| Lightly Active | 1.375 | Light exercise 1–3 days/week |
| Moderately Active | 1.55 | Moderate exercise 3–5 days/week |
| Very Active | 1.725 | Hard exercise 6–7 days/week |
| Extra Active | 1.9 | Physical job + twice-daily training |
Studies consistently show that people overestimate their activity level. When uncertain, choose one level lower than your first guess — the result is usually more accurate.
Macronutrient Split
The calculator uses a balanced 40/30/30 split (carbs/protein/fat) as the starting point. Protein at 30% (roughly 0.7–1.0 g per lb of body weight) supports muscle retention during calorie restriction. Carbohydrates at 40% fuel exercise and brain function. Fat at 30% maintains hormones and absorption of fat-soluble vitamins A, D, E, and K.
For weight loss: consider shifting to 35/35/30 (lower carb, higher protein). For athletes: 50/25/25 (higher carb for glycogen). These are starting templates — adjust based on how your body responds over 3–4 weeks.
Calorie Deficit and Rate of Loss
One pound of body fat contains approximately 3,500 kcal. A daily deficit of 500 kcal produces roughly 1 lb (0.45 kg) per week. This assumes the deficit comes from fat, not muscle. Adequate protein and resistance training help preserve lean mass during a cut.
Why TDEE Changes Over Time
As you lose weight, your TDEE decreases — a lighter body burns fewer calories. Metabolic adaptation also reduces BMR slightly during sustained deficits. Recalculate every 4–6 weeks and adjust your intake accordingly. Most people plateau not because of a "broken metabolism" but because their calorie needs have dropped and their intake hasn't kept pace.
Safety Thresholds
Health guidelines advise against dropping below 1,200 kcal/day for women or 1,500 kcal/day for men. Very low intakes can reduce lean mass and basal metabolism. Aim for a modest, sustainable deficit rather than extreme restriction. If your TDEE result is already close to these minimums, focus on increasing activity rather than cutting food further.