BMR & TDEE Calculator

Two numbers form the foundation of any nutrition plan: Basal Metabolic Rate (BMR)—the calories your body burns at complete rest just to stay alive—and Total Daily Energy Expenditure (TDEE)—the calories you actually burn in a day after factoring in movement, digestion, and exercise. Eat below TDEE and you lose weight. Eat above it and you gain. Match it and your weight stays the same. This is the law of thermodynamics applied to your body.

The Mifflin-St Jeor equation

This calculator uses the Mifflin-St Jeor equation, which a 2005 American Dietetic Association study found to be the most accurate BMR formula for the general population (within ±10% for most adults). The formulas are:

Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161

For a 30-year-old man weighing 75 kg at 175 cm: BMR = (10 × 75) + (6.25 × 175) − (5 × 30) + 5 = 750 + 1,093.75 − 150 + 5 = 1,699 cal/day. That's the calories he'd burn in a coma—not realistic for daily life.

Activity multipliers and TDEE

TDEE multiplies BMR by an activity factor reflecting how much you move. The standard multipliers—known as the Harris-Benedict activity factors—are:

• Sedentary (1.2): Desk job, little to no exercise, mostly sitting.
• Lightly active (1.375): Light exercise or sports 1–3 days per week.
• Moderately active (1.55): Moderate exercise or sports 3–5 days per week.
• Very active (1.725): Hard exercise or sports 6–7 days per week.
• Extra active (1.9): Physical job plus daily hard training (e.g., construction worker who also lifts).

Continuing the example: if our 30-year-old man is moderately active, his TDEE = 1,699 × 1.55 = 2,633 cal/day. That's his maintenance.

Calorie targets for weight loss and gain

A pound of body fat contains roughly 3,500 calories, so a daily deficit of 500 calories produces ~1 lb of weight loss per week; a 1,000-calorie daily deficit produces ~2 lb per week. A daily surplus of 500 calories produces ~1 lb of gain per week—useful for lean bulking. Larger deficits accelerate loss but increase muscle loss, hunger, and rebound risk. The 500-calorie rule is a guideline, not a law—metabolic adaptation means your TDEE falls slightly as you lose weight, so expect to recalculate every 5–10 lbs.

Why BMR formulas can be wrong

Even the best BMR formula has a standard error of roughly 10–15%. A person whose predicted BMR is 1,700 might actually burn anywhere from 1,450 to 1,950 at rest. The biggest sources of error are muscle mass (muscle burns more than fat, so muscular people underestimate; sedentary people overestimate), body composition (formulas assume average lean mass), and metabolic adaptation (your BMR drops after extended dieting). For maximum accuracy, a DEXA scan or indirect calorimetry test at a performance lab can measure BMR directly—but the formula gets most people within 100–200 calories, which is plenty for planning.

Using TDEE in practice

Treat your calculated TDEE as a starting hypothesis. Track your food intake for two weeks, eat at your calculated TDEE, and weigh yourself daily (averaging weekly to smooth out water-weight noise). If your weight stays flat, the formula is accurate for you. If it drops, your true TDEE is higher than calculated; if it rises, lower. From there, subtract or add 250–500 calories for sustainable loss or gain, recalculate every 5–10 lbs, and pair the plan with adequate protein (1.6–2.2 g/kg) and resistance training to preserve muscle.