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BMR Calculator

Calculate your Basal Metabolic Rate with Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle. See all three formulas side by side and pick the one that fits your data.

BMR inputs

This calculator is for informational purposes only and is not a substitute for professional medical advice. Consult a qualified healthcare provider before making changes to your diet, exercise, or health regimen.

What is BMR and why does it matter?

Your Basal Metabolic Rate (BMR) is the number of calories your body burns doing nothing at all. Lying completely still in a temperature-neutral room, fasted and awake, you are still spending energy on the essential jobs of staying alive: breathing, pumping blood, maintaining body temperature, running your brain, and repairing cells. BMR captures exactly that baseline. For most adults it represents between 60% and 70% of the total daily calorie burn, which makes it the most important single number in any structured diet or training plan.

Knowing BMR lets you work out how much food your body needs before you even move. Everything on top — walking to the kitchen, typing at a keyboard, going to the gym — is extra. If you plan a calorie intake without knowing your BMR you are essentially guessing, and small errors compound quickly over weeks and months.

How this calculator works

We offer three of the most widely used predictive equations and show all of them side by side so you can see how they compare:

  • Mifflin-St Jeor (1990):
    • Men: BMR = 10×kg + 6.25×cm − 5×age + 5
    • Women: BMR = 10×kg + 6.25×cm − 5×age − 161
  • Revised Harris-Benedict (Roza & Shizgal, 1984):
    • Men: BMR = 88.362 + 13.397×kg + 4.799×cm − 5.677×age
    • Women: BMR = 447.593 + 9.247×kg + 3.098×cm − 4.330×age
  • Katch-McArdle (1981): BMR = 370 + 21.6 × lean body mass (kg), where lean body mass = weight × (1 − body-fat%/100).

Mifflin-St Jeor is the default because it is the most accurate for a typical healthy adult and has been formally recommended by the Academy of Nutrition and Dietetics. If you know your body fat percentage with good accuracy (e.g. from a DEXA scan), Katch-McArdle becomes the best choice because it works from lean mass rather than total weight. Harris-Benedict is included for reference and comparison.

Worked example

Consider a 35-year-old man who weighs 82 kg, stands 180 cm tall, and has an accurately measured 18% body fat.

  • Mifflin-St Jeor: 10×82 + 6.25×180 − 5×35 + 5 = 820 + 1,125 − 175 + 5 = 1,775 kcal/day.
  • Harris-Benedict: 88.362 + 13.397×82 + 4.799×180 − 5.677×35 = 88.4 + 1,098.6 + 863.8 − 198.7 ≈ 1,852 kcal/day.
  • Katch-McArdle: lean mass = 82 × 0.82 = 67.24 kg. BMR = 370 + 21.6 × 67.24 ≈ 1,822 kcal/day.

All three agree to within about 80 kcal — well inside the margin of error of any predictive equation. For day-to-day planning, picking any one of them and sticking with it is fine; the consistency matters more than the absolute number.

How to interpret your BMR

BMR on its own is not a calorie target. It is the floor, the energy your body needs before any movement. To turn it into a useful daily intake number you multiply by an activity factor (1.2 for sedentary to 1.9 for very heavy labour), producing your Total Daily Energy Expenditure (TDEE). From TDEE you then subtract or add calories depending on whether you are trying to lose, maintain, or gain weight. If you want the full chain — BMR → TDEE → calorie targets → macros — try our TDEE calculator.

Common mistakes

  • Using a bad body-fat number with Katch-McArdle. Cheap bathroom-scale bioimpedance readings can be off by 5–10%. A bad BF% gives a worse BMR than Mifflin, not a better one.
  • Eating at BMR. BMR is not a diet plan. Eating at BMR for weeks is typically unsustainable and can cost you lean mass.
  • Ignoring weight change. Lose 5 kg and your BMR drops by around 60 kcal/day. Recalculate periodically.
  • Averaging all three formulas. Pick one and track it. Averaging washes out the consistency you need to spot trends.
  • Treating BMR as a hard fact. Predictive equations have a ±10% error band. Use it as a starting point, not a ground truth.

Accuracy and limitations

Predictive equations assume a typical body composition for your height, weight, age, and sex. The further you are from that typical body the less accurate they become. Ultra-lean athletes, very obese individuals, the elderly, and people with thyroid disease or other metabolic conditions all sit in zones where equation error grows. The gold-standard measurement is indirect calorimetry — a metabolic cart measures the oxygen you consume and the carbon dioxide you produce and converts that directly into calories burned. It is available at some sports medicine clinics and hospitals and is worth doing once if you have reason to think your metabolism is unusual.

When to consult a professional

If you have a medical condition, are pregnant or breastfeeding, are under 18 or over 65, are recovering from an eating disorder, or are preparing for a weight-class sport, talk to a registered dietitian or physician before acting on any calculator output. This tool is general information and is not a substitute for individualised medical advice.

Frequently Asked Questions

What is BMR?
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — just to keep you alive. It powers breathing, heartbeat, temperature regulation, cell repair, brain activity, and kidney function. For a typical sedentary adult BMR accounts for 60–70% of total daily calorie burn, which is why it is the foundation of any calorie plan.
BMR vs RMR vs TDEE — what is the difference?
BMR is measured under strict laboratory conditions after an overnight fast and complete rest. RMR (Resting Metabolic Rate) is a looser version measured without fasting or full rest and is typically 5–10% higher. TDEE (Total Daily Energy Expenditure) is BMR plus digestion, daily movement, and exercise — the number you actually eat to. Most online "BMR calculators", including this one, use predictive equations that approximate either BMR or RMR.
Which formula should I use?
For most healthy adults, Mifflin-St Jeor (1990) is the accuracy leader — the Academy of Nutrition and Dietetics recommends it as the default. Harris-Benedict (revised 1984) is a classic and is still widely cited; it tends to read slightly higher than Mifflin. Katch-McArdle is the best choice if you have an accurate body fat percentage, because it calculates from lean body mass rather than total weight and is therefore more accurate for lean athletes and very obese people.
Why do the three formulas give different numbers?
They were derived from different populations using different methods. Mifflin-St Jeor pooled data from 498 subjects in the early 1990s, Harris-Benedict comes from much older data (original 1919, revised 1984), and Katch-McArdle is structurally different — it uses lean body mass instead of total weight plus age and sex. Differences of 50–150 kcal are normal. For planning purposes, pick one and stick with it rather than averaging.
How accurate is BMR estimation?
Mifflin-St Jeor is within ±10% of measured BMR for about 80% of non-obese adults. Accuracy drops for extreme body compositions (very lean or very obese), the elderly, and people with thyroid or other metabolic conditions. The only way to measure BMR precisely is indirect calorimetry, which requires a metabolic cart in a clinic and costs real money. For everyday planning an equation is good enough.
Should I eat my BMR?
No. BMR is only the portion of energy you need at rest. You also need calories for walking, working, and exercising — together with BMR that adds up to TDEE. Eating at (or below) BMR for sustained periods leaves you in a large deficit that is difficult to maintain, often leads to fatigue, irritability, poor training, and rebound eating, and increases the risk of losing lean mass. Use BMR as an input to a TDEE calculator, not as a calorie target.
Does BMR change over time?
Yes. BMR drops with age (roughly 1–2% per decade after age 20), drops when you lose weight (less tissue to maintain), and rises when you gain weight or add muscle. Thyroid activity, hormones, sleep, and certain medications can also shift it. For this reason we recommend recalculating every 2–4 kg of weight change or once a year, whichever comes first.
Does muscle really burn that many more calories than fat?
Less than Internet folklore claims. At rest, muscle burns roughly 13 kcal per kg per day and fat burns about 4.5 kcal per kg per day. So adding 5 kg of muscle raises BMR by about 45 kcal/day — meaningful over a year but not dramatic. The bigger benefit of strength training is during and after exercise, not from resting metabolism.
Do I really need body fat % for Katch-McArdle?
Yes — the formula is literally built on lean body mass. If you do not know your body fat percentage with reasonable accuracy (within 3–4%), leave the field blank and use Mifflin-St Jeor. A bad body-fat estimate (say, a cheap bioimpedance scale reading that is 10% off) will give a worse BMR than Mifflin, not a better one. DEXA scans, hydrostatic weighing, and Bod Pod are the reliable options.
Is this a substitute for medical advice?
No. This calculator is for general informational purposes. If you have thyroid disease, diabetes, an eating disorder, are pregnant, are under 18 or over 65, or are preparing for a competitive event, consult a qualified healthcare professional or a registered dietitian. They can order lab work, assess body composition, and set calorie targets that fit your specific situation.