BMR Calculator

What is BMR?

Basal Metabolic Rate (BMR) is the number of calories your body needs to maintain basic physiological functions — breathing, circulation, cell production, and temperature regulation — while at complete rest. Think of it as your body's idle fuel consumption.

This calculator uses the Mifflin-St Jeor equation (1990), which is considered the most accurate for most adults compared to older formulas like Harris-Benedict. It requires weight, height, age, and biological sex.

How to use your BMR

BMR alone isn't enough to plan your diet, it tells you how much energy you burn at rest, but not your total daily needs. To get that, multiply your BMR by an activity factor to calculate your TDEE (Total Daily Energy Expenditure). Your calorie intake should generally stay above your BMR to avoid metabolic slowdown.

When to Use a BMR Calculator

BMR is the essential starting point for creating a calorie deficit. Multiply your BMR by an activity factor (1.2 for sedentary, up to 1.9 for very active) to get your TDEE — your actual daily burn. A sustainable weight-loss deficit is typically 300–500 kcal below TDEE, which translates to roughly 0.3–0.5 kg of fat loss per week.

The Mifflin-St Jeor equation (1990) is the preferred formula for most adults, with a mean error of approximately ±10%. The older Harris-Benedict equation (1919) was derived from a less diverse population and tends to overestimate BMR by 5–15% in modern individuals. Clinical dietitians use Mifflin-St Jeor as their default when prescribing intake targets for weight management patients.

Tracking BMR over time also helps identify adaptive thermogenesis, the metabolic slowdown that occurs during extended caloric restriction. Studies show measured BMR can fall 10–15% below predicted values after several months of dieting, which explains why weight loss often plateaus.

Related tools: Calorie Calculator, Calorie Deficit Calculator, Macro Calculator, and Ideal Weight Calculator.

Limitations and What BMR Doesn't Tell You

BMR formulas assume average body composition. They overestimate energy needs for individuals with high body-fat percentage (fat tissue burns fewer calories at rest) and underestimate for those with high muscle mass (muscle tissue is metabolically active). If you know your body fat percentage, lean body mass-based formulas like Katch-McArdle provide more accurate results.

BMR also doesn't account for NEAT (Non-Exercise Activity Thermogenesis), the energy burned through everyday movements like fidgeting, walking to your car, or doing household chores. Research shows NEAT varies by up to 500 kcal/day between individuals of similar weight, which is why two people can eat the same diet and see very different results.

Finally, BMR calculations are not a substitute for indirect calorimetry, the gold standard in clinical settings, which measures actual oxygen consumption and CO₂ production to determine real metabolic rate. For most people, BMR formulas are a useful estimate; for precision nutrition or clinical weight management, work with a registered dietitian.

Mifflin-St Jeor vs. Harris-Benedict vs. Katch-McArdle

Three formulas dominate clinical and fitness BMR estimation. Understanding when each is most appropriate prevents systematically under- or over-estimating your caloric needs.

Mifflin-St Jeor (1990) — the current standard. Uses weight, height, age, and sex. Mean error versus measured RMR: approximately ±10% in normal-weight adults. A validation study by Frankenfield et al. (2005, Journal of the American Dietetic Association) compared five equations and found Mifflin-St Jeor the most accurate in 82% of non-obese subjects. This calculator uses Mifflin-St Jeor. Use it for: most adults, including those who are overweight. Limitation: becomes less accurate when body fat is very high (BMI above 35) because it cannot distinguish between high-fat and high-muscle weight.

Harris-Benedict (1919, revised 1984) — historically important but now largely superseded. Uses the same four variables as Mifflin-St Jeor. The original equation was derived from a small, non-representative sample of mostly young, lean men. It overestimates RMR by 5–15% in modern sedentary populations. The 1984 revision by Roza and Shizgal improved accuracy modestly but Mifflin-St Jeor remains more accurate for most people. Practical reason to know it: many older apps, textbooks, and clinical databases still use it, so results from these sources will consistently run 100–200 kcal higher than a Mifflin-based calculation.

Katch-McArdle — the gold standard when you know your lean body mass. Instead of using total body weight, it uses lean body mass (LBM) directly: BMR = 370 + (21.6 × LBM in kg). This eliminates the main source of error in the other formulas — that fat mass does not contribute meaningfully to resting metabolism. A 90 kg person at 10% body fat has an LBM of 81 kg; a 90 kg person at 35% body fat has an LBM of 58.5 kg. Their BMRs are dramatically different, but Mifflin-St Jeor gives them the same answer. Katch-McArdle is more accurate for lean athletes, bodybuilders, or anyone who has had a DEXA scan. Limitation: most people do not know their LBM reliably.

Why your real BMR can differ from the calculator

Formula BMR is a statistical average. Individual biology means your actual resting metabolic rate can be 200–400 kcal above or below what the equation predicts. Here are the main reasons:

Genetics account for up to 40% of variance in RMR between individuals at identical body composition. Twin studies show that BMR heritability is substantial — some people are simply more or less metabolically efficient than average, independently of any lifestyle factor. This is why two people following the identical diet and training program will see different results.

Thyroid function has a large and often underdiagnosed effect on BMR. Hypothyroidism (TSH above 4.0 mIU/L) reduces metabolic rate by 15–30% compared to a euthyroid individual of the same size. This translates to a 300–600 kcal/day difference for many adults. If formula-predicted BMR consistently fails to match your real-world weight outcomes despite careful tracking, thyroid function testing is worth discussing with a physician.

Lean mass composition is the most correctable factor. As described above, formula BMR treats all weight as similarly metabolically active. Two 80 kg people — one at 15% body fat, one at 35% body fat — have different actual RMRs. If you are highly muscular for your weight, your true BMR will be higher than the formula predicts, and you may consistently lose weight faster than expected. If you are higher in body fat, the formula may overestimate your true BMR.

Chronic dieting history — sometimes called 'metabolic damage,' though the more precise term is persistent adaptive thermogenesis. Individuals who have been on repeated extreme diets may have a suppressed RMR that persists for months to years after the diet ends, as documented in studies of The Biggest Loser contestants (Fothergill et al., 2016, Obesity). The practical fix is a prolonged period at maintenance calories with resistance training, not more restriction.

Using BMR in practice

BMR alone does not tell you how much to eat. The practical workflow is: calculate BMR → multiply by an activity factor to get TDEE → apply your goal adjustment. Here is the full translation from raw BMR output to actionable daily targets:

Step 1 — Choose your activity multiplier honestly. Sedentary (desk job, no exercise): 1.2×. Lightly active (1–3 workouts/week, but otherwise sedentary): 1.375×. Moderately active (3–5 real training sessions/week): 1.55×. Very active (6–7 hard sessions/week, or physical job): 1.725×. Athletes training twice daily or doing very heavy manual labor: 1.9×. The most common mistake is choosing 'moderately active' based on workout days while ignoring that most of the remaining 23 hours per day are spent sitting — if this describes you, go one level lower.

Step 2 — Apply your goal. Fat loss: subtract 500 kcal from TDEE for a ~0.5 kg/week rate; subtract 750 kcal for ~0.75 kg/week. Maintenance: eat at TDEE. Lean muscle gain: add 200–300 kcal above TDEE. The calorie surplus for muscle gain should be modest — beyond 300–400 kcal/day above TDEE, most additional calories are stored as fat rather than converted to muscle tissue.

Step 3 — Recalculate after weight change. Every 5 kg of weight lost reduces your BMR by approximately 50–70 kcal/day (from the weight term in the Mifflin equation: 10 × kg). After 10 kg of loss, your TDEE may be 150–300 kcal lower than when you started — meaning the same calorie intake that previously produced a 500 kcal deficit may now produce only 200–250 kcal of deficit. This is why recalculation is not optional.

Step 4 — Calibrate from real data. Track your average weekly weight for 3–4 weeks at your calculated calorie target. If the trend matches your goal rate (0.4–0.6 kg/week for a 500 kcal deficit), the formula is accurate for you. If not, adjust intake by 100–200 kcal in the appropriate direction. Real-world calibration beats any formula because it accounts for your personal genetics, NEAT, and activity multiplier accuracy.

Frequently Asked Questions