BMR calculator (Mifflin-St Jeor)

What is BMR — and why does it matter?

Basal metabolic rate (BMR) is the number of calories your body burns at absolute rest — no movement, no digestion, no cold exposure. It's the floor of your energy needs: the calorie cost of simply being alive. For most sedentary people, BMR represents 60–70% of total daily calorie burn, which makes it the most important number in understanding your overall energy balance.

Knowing your BMR is the starting point for calculating TDEE (total daily energy expenditure), setting calorie targets for fat loss or muscle gain, and understanding why two people with the same weight and height may have meaningfully different calorie needs.

BMR vs. RMR — an important distinction

Strictly speaking, most online "BMR calculators" — including this one — actually estimate resting metabolic rate (RMR), not true BMR. The difference:

• True BMR is measured in a laboratory after a 12-hour fast and at least 8 hours of sleep, in a thermoneutral room. Few people outside research settings ever have this measured.
• RMR is measured at rest but without the strict fasting and sleep conditions. It runs about 10–20% higher than true BMR because post-absorptive digestion and recent activity add a small caloric overhead.

For practical purposes — diet planning, TDEE calculation, macro setting — the distinction rarely matters. Both formulas here estimate RMR, which is what "BMR" means in everyday usage.

Mifflin-St Jeor: the gold standard

The Mifflin-St Jeor equation was published in 1990 by researchers at the University of Nevada. It was designed to correct known overestimates in the older Harris-Benedict equation, which was derived from a small, mostly athletic sample in 1919. A 2005 meta-analysis by Frankenfield, Roth-Yousey, and Compher — published in the Journal of the American Dietetic Association — tested several predictive equations against measured RMR in over 2,000 subjects. Mifflin-St Jeor came out ahead, predicting measured RMR within 10% in 82% of participants.

The Mifflin-St Jeor equations are:

• Male: (10 × kg) + (6.25 × cm) − (5 × age) + 5
• Female: (10 × kg) + (6.25 × cm) − (5 × age) − 161

Harris-Benedict: the original, revised in 1984

The original Harris-Benedict equation (1919) was revised by Roza and Shizgal in 1984 to correct systematic overestimation. The revised formulas:

• Male: 88.362 + (13.397 × kg) + (4.799 × cm) − (5.677 × age)
• Female: 447.593 + (9.247 × kg) + (3.098 × cm) − (4.330 × age)

Harris-Benedict tends to overestimate RMR by 5–15% in non-athletic modern populations, but it was validated primarily on lean, athletic subjects — so it may be more accurate for bodybuilders or athletes who carry significantly more muscle than the average person.

How muscle mass raises BMR

Muscle tissue is metabolically expensive. At rest, each pound of muscle burns approximately 6 calories per day — three times the 2 calories per pound that fat tissue burns. The practical implication: adding 10 pounds of lean muscle through consistent resistance training raises your resting calorie burn by roughly 40–60 calories per day. That's the equivalent of burning an extra 4–6 pounds of fat per year without any other change. This is why maintaining or building muscle mass is the most durable long-term strategy for body composition.

The thyroid connection

The thyroid gland is the primary regulator of metabolic rate. Thyroid hormones (T3 and T4) directly control the speed at which cells burn fuel. Hypothyroidism (underactive thyroid) can suppress BMR by 10–40%, causing weight gain, fatigue, and cold intolerance even at normal calorie intake. Hyperthyroidism does the opposite — elevated BMR, weight loss, heat intolerance. If your measured calorie intake is significantly below your estimated BMR and you're gaining weight, thyroid function is worth evaluating with a physician.

How BMR changes with age

The conventional wisdom that "metabolism slows with age" is partially correct but overstated. A 2021 analysis in Science (Pontzer et al.) of 6,400+ people using doubly labeled water found that BMR stays relatively flat from age 20–60, then declines about 0.7% per year after 60. Most of the age-related BMR decline is attributable to muscle loss (sarcopenia), not a fundamental slowing of cellular metabolism. Adults who maintain muscle mass through resistance training preserve far more of their BMR into later decades.