BMR: Your Body's Energy Foundation
Your Basal Metabolic Rate (BMR) is the number of calories your body requires to maintain basic physiological functions while at complete rest — heartbeat, breathing, maintaining body temperature, cell production, and organ function. It represents the minimum energy needed to keep you alive and typically accounts for 60–75% of the total calories you burn each day. Understanding your BMR is the foundation for any structured approach to nutrition, whether your goal is weight loss, maintenance, or muscle gain.
The Mifflin-St Jeor Formula
Several BMR formulas exist, but the Mifflin-St Jeor equation, published in 1990, is consistently rated as the most accurate for modern sedentary and lightly active populations in meta-analyses. The formulas are: Male BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5. Female BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161. The only difference is the constant at the end, reflecting the fact that women generally have slightly lower BMR per unit of body mass due to higher body fat percentage and lower muscle mass.
From BMR to TDEE
Your Total Daily Energy Expenditure (TDEE) is BMR multiplied by an activity factor. Sedentary (desk work, no exercise): × 1.2. Lightly active (light exercise 1–3 days/week): × 1.375. Moderately active (moderate exercise 3–5 days/week): × 1.55. Active (hard exercise 6–7 days/week): × 1.725. Very active (hard daily exercise or physical labor): × 1.9. Most people overestimate their activity level — if in doubt, use the level below what you think is accurate and adjust based on real-world results.
BMR vs Harris-Benedict
The original Harris-Benedict equation (1919, revised 1984) is also widely used and is what many older fitness tools and textbooks reference. Mifflin-St Jeor tends to be more accurate for most modern adults, particularly those who are overweight. For very muscular individuals, the Katch-McArdle formula (which uses lean body mass) may be even more accurate, but it requires a body fat percentage measurement. For general use, Mifflin-St Jeor is the best single-formula choice.
Factors That Affect BMR
Several factors can raise or lower your BMR. Muscle mass is the most important: muscle tissue burns more calories at rest than fat tissue, which is why resistance training is beneficial even for non-athletic weight management goals. Age gradually lowers BMR due to muscle loss (sarcopenia). Body size — taller and heavier people have higher BMR. Hormones, particularly thyroid hormones, regulate metabolic rate significantly. Extreme calorie restriction can lower BMR by up to 15–20% as the body adapts, which is why very low calorie diets often produce diminishing results over time.
This calculator is for general estimates only and is not medical or dietary advice. Individual metabolic rates can vary significantly.