Basal Metabolic Rate Calculator (Mifflin-St Jeor & Katch-McArdle)
The calories your body burns at complete rest. Start every diet plan here with a real BMR number.
What your BMR actually represents
Your basal metabolic rate is the number of calories your body burns to keep you alive when you are doing absolutely nothing. Heart beating, lungs moving air, kidneys filtering blood, liver running its hundreds of biochemical reactions, immune cells patrolling, brain humming along at 20% of your total energy draw — all of that happens at complete rest and all of it costs fuel. For most adults BMR accounts for 60–70% of total daily energy expenditure, which is why getting the BMR number right matters more than counting burpees.
The formulas in this calculator were developed over eight decades of indirect calorimetry research. Harris and Benedict published the first widely used equation in 1919 using a cohort of 239 men and women in Boston. Mifflin and St Jeor revised the math in 1990 using more than 500 healthy adults and a broader age range; their equation is the current gold standard for non-obese adults. Katch-McArdle and Cunningham came out of exercise science labs and base BMR on lean body mass, which is why they work better for athletes and lean physique competitors.
How the Mifflin-St Jeor formula works
The equation reads: BMR = 10 × weight (kg) + 6.25 × height (cm) - 5 × age + s, where s equals +5 for men and -161 for women. The coefficients were fit by regression so that each variable contributes in proportion to its real physiological effect. Weight carries the largest coefficient because lean and fat tissue both consume energy (just at different rates). Height matters because taller bodies have more surface area, more organ mass, and a larger blood volume. Age matters because lean mass slowly declines. Sex matters because men carry on average 10–15% more lean mass at the same weight and height.
A 32-year-old man weighing 180 pounds at 5 foot 10 has a BMR of roughly 1,760 kcal per day. A 32-year-old woman of identical weight and height has a BMR of about 1,600 kcal per day — a 160 kcal gap driven mostly by body composition differences baked into the sex constant. That gap is real and matters more than any calorie cycling trick.
How Katch-McArdle handles lean mass directly
Katch-McArdle reads: BMR = 370 + 21.6 × lean body mass (kg). The equation is simpler because it skips the proxies (height, age, sex) and goes straight to the variable those proxies were trying to approximate. If you have a DEXA scan, a Bod Pod reading, or a reliable 7-site skinfold measurement within 3% of your true body fat, Katch-McArdle is the more accurate number. If your body composition estimate is noisy — smart scale, visual estimate, BIA handheld — you are better off with Mifflin-St Jeor, which at least does not compound your body-fat error.
Cunningham adds a small constant and a slightly higher slope coefficient (500 + 22 × lean kg), and was derived from a cohort of trained athletes. It consistently returns a BMR 100–200 kcal higher than Katch-McArdle at the same lean mass. If you have ten or more years of serious strength training behind you, Cunningham usually matches measured RMR better than any other equation.
Why every formula gives a different answer
BMR varies person-to-person by roughly 300 kcal at the same weight and height because of lean mass, organ size, thyroid activity, and dieting history. No single regression equation can capture that noise, so each one optimizes for a different cohort. Mifflin-St Jeor minimizes error across a broad adult population. Harris-Benedict overestimates for modern sedentary adults because the original cohort was more active. Katch-McArdle and Cunningham reduce error for people whose body composition sits outside the population average.
The practical answer: run all four formulas, take the average, and treat the result as a starting point accurate to plus or minus 100 kcal. Then track two weeks of intake and weight change at the calculated maintenance. If you gained a pound on what the calculator called maintenance, your true BMR is 250 kcal lower than the average and you adjust from there. Eating by a formula without verifying against scale data is the #1 reason people stall on aggressive cuts.
From BMR to a daily calorie target
BMR by itself is not a meal plan. You multiply BMR by an activity factor to get total daily energy expenditure, and then subtract a deficit to get your fat-loss intake. The multipliers: 1.2 for sedentary desk work, 1.375 for 1–3 workouts per week, 1.55 for 3–5 workouts, 1.725 for 6–7 workouts, and 1.9 for physical jobs plus training. Most adults land between 1.3 and 1.5 in real life once you count honest movement.
From TDEE, subtract 300–500 kcal for moderate fat loss, 500–750 for aggressive fat loss, and cap total intake at a floor of 1,200 kcal for women and 1,500 kcal for men. If hitting your target weight by a target date requires a deeper cut than those floors allow, use the goal weight by date tool to extend the timeline instead of crushing the intake.
Adaptive thermogenesis: BMR is not a constant
Left uncorrected for, adaptive thermogenesis is the single biggest reason diets fail after 8 weeks. When you cut calories your body suppresses BMR 5–15% below what the equation predicts — a protective response driven by T3 and leptin downregulation and by the loss of metabolically active tissue. After 8–12 weeks of dieting, recalculate BMR at your new weight and expect the measured number to be another 100–250 kcal below formula.
The fix is a diet break (5–10 days at maintenance), more protein, and resistance training. Aerobic-only dieting produces the steepest BMR suppression because it drives the largest lean-mass loss. A lifter losing fat with high protein and 3-4 resistance sessions per week suppresses BMR half as much as a runner eating the same deficit.
BMR myths that waste your time
Eating five small meals per day does not raise BMR. The thermic effect of food scales with total food eaten, not with meal frequency — six 400-kcal meals produce the same TEF as three 800-kcal meals. Cold showers raise energy expenditure by 3–5% for about 20 minutes, which nets 30–60 kcal of extra burn, not the hundreds claimed on social media. Green tea raises BMR 2–4% for three hours. Spicy food the same. These effects are real but small; stacking all three gets you maybe 80 extra kcal per day.
The only two levers that meaningfully change BMR over months are lean body mass and thyroid hormone status. Lift heavy, eat enough protein, sleep 7–9 hours, and get bloodwork annually if your energy or mood is off. Everything else is rounding error.
When to trust the calculator, when to measure
For general diet planning the calculator is accurate enough. If you are a competitive athlete, have a known thyroid issue, or have lost more than 20% of starting weight on a previous diet, pay for a metabolic cart test at a local university lab or performance clinic. The test costs $100–200, takes about 45 minutes, and returns a measured RMR to within 2% accuracy. Anyone with 10+ years of dieting history should measure at least once because cumulative adaptive thermogenesis can suppress BMR 25% below formula predictions.
For everyone else, pair this calculator with the TDEE tool, the calorie deficit calculator, and the lean body mass estimator. The combination gives you a realistic starting point, a reality check after three weeks of tracking, and a path to recalibrate when the scale stalls.
Frequently Asked Questions
What is BMR and how is it different from RMR?
BMR (basal metabolic rate) is the energy your body burns in a fully post-absorptive state, lying still, in a thermoneutral environment, right after waking. RMR (resting metabolic rate) is measured under less strict conditions — after a light meal, a few minutes of movement, normal room temperature — and runs about 10% higher than true BMR. In everyday use the numbers are used interchangeably because the difference is within formula error.
Which BMR formula is the most accurate?
For the general population the Mifflin-St Jeor equation is the most accurate published equation, with a mean error of about 5%. If you know your body fat percentage within 3%, the Katch-McArdle formula is more accurate because it predicts BMR from lean body mass alone. Very lean, heavily muscled, or very obese individuals should rely on Katch-McArdle or Cunningham rather than Mifflin-St Jeor, which tends to underestimate the first group and overestimate the last.
Why do two people of the same weight have different BMRs?
Muscle is metabolically more active than fat. Two 180-pound adults with body fat of 10% and 35% can have BMRs 200–350 kcal apart. Sex, age, thyroid hormone status, recent dieting history, sleep quality, and genetic variation in mitochondrial efficiency all shift the number further. That is why every serious BMR calculator now offers both a weight-based estimate and a lean-mass-based estimate side by side.
How much does BMR drop with age?
About 1–2% per decade after age 25 in adults who do nothing to preserve muscle. The drop is driven almost entirely by loss of lean mass (sarcopenia) rather than the passage of time itself. Adults who lift weights three times a week and hit 1 gram of protein per pound of body weight keep their BMR within 5% of their 30-year-old baseline into their 60s. Age is a proxy — lean mass is the actual variable.
Can I change my BMR?
Yes, but more slowly than fitness marketing suggests. Adding 10 pounds of muscle raises BMR by roughly 50–70 kcal per day — useful over a year, not enough to out-eat a bad diet. Chronic under-eating below 80% of BMR for months suppresses BMR via T3 and leptin downregulation, and restoring it requires weeks of maintenance or reverse dieting. Short-term boosters (caffeine, spicy food, cold exposure) raise BMR 3–8% for a few hours at most.
Disclaimer: This tool provides estimates for educational purposes and is not medical or nutritional advice. Individual results vary. Always consult a licensed physician or registered dietitian before starting a new diet, fasting protocol, or exercise program — especially if you have a medical condition, are pregnant or nursing, or are under 18.