Basal Metabolic Rate

Your BMR is the minimum caloric engine running your body 24/7 — breathing, pumping blood, maintaining body temperature, building and repairing cells. Everything else you eat is fuel for activity and digestion on top of this baseline. Get the baseline wrong and every calorie target you build on it is off. The BMR calculator uses the Mifflin-St Jeor equation — validated to within 10% of measured BMR in 82% of healthy adults — to give you a reliable starting number for your energy needs.

BMR Formulas: Mifflin-St Jeor vs. Harris-Benedict

The two most widely used BMR equations:

Mifflin-St Jeor (recommended):

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

Revised Harris-Benedict (1984):

• Men: BMR = 13.397 × weight(kg) + 4.799 × height(cm) − 5.677 × age + 88.362
• Women: BMR = 9.247 × weight(kg) + 3.098 × height(cm) − 4.330 × age + 447.593

Example (35-year-old woman, 65 kg, 165 cm): Mifflin BMR = (10 × 65) + (6.25 × 165) − (5 × 35) − 161 = 650 + 1,031.25 − 175 − 161 = 1,345 kcal/day. Use this online calculator for your personal BMR. The calorie calculator extends this to your full TDEE with activity level.

From BMR to Daily Calorie Target: The Activity Multiplier

BMR × activity factor = Total Daily Energy Expenditure (TDEE):

• Sedentary (desk work, no exercise): × 1.2
• Lightly active (light exercise 1–3×/week): × 1.375
• Moderately active (exercise 3–5×/week): × 1.55
• Very active (hard exercise 6–7×/week): × 1.725
• Extremely active (physical job + daily training): × 1.9

Your TDEE is your maintenance calorie level — eat this to maintain weight. Subtract 500 kcal/day to lose approximately 0.5 kg/week; add 250–500 kcal/day for lean muscle gain. The TDEE calculator and calorie calculator apply these multipliers automatically.

Factors That Affect Your BMR Most Significantly

Understanding what drives your BMR helps you optimize it:

• Lean muscle mass: the biggest variable — each kg of muscle burns approximately 13 kcal/day at rest; fat tissue burns only 4.5 kcal/kg/day. Two people of the same weight can differ by 200–300 kcal/day in BMR based on body composition alone.
• Age: BMR drops 1–2% per decade after 20, primarily from muscle loss (sarcopenia). Resistance training counteracts this effectively.
• Sex: men typically have 5–10% higher BMR than women of the same size due to greater lean mass and hormonal differences.
• Thyroid function: hypothyroidism reduces BMR by 20–40%; hyperthyroidism raises it — both significantly affect calorie needs

The lean body mass calculator and metabolic calculators provide the complete metabolic assessment toolkit.

BMR and Weight Loss: The Math Behind the Calories

One kg of body fat ≈ 7,700 kcal. To lose 0.5 kg/week: create a deficit of 7,700 ÷ 2 = 3,850 kcal/week = 550 kcal/day below TDEE. To lose 1 kg/week: 7,700 ÷ 7 = 1,100 kcal/day deficit — aggressive and typically unsustainable without muscle loss. Never eat below your BMR chronically — doing so forces your body to break down muscle for energy, reducing BMR further and creating a cycle of metabolic adaptation. Most nutrition professionals recommend not eating more than 500–750 kcal below TDEE for sustainable weight loss. Consult a registered dietitian for a personalized plan.