Health
BMR Mifflin
BMR Mifflin estimates a health-related number from standard formulas used in fitness and medicine.
What this does
BMR Mifflin estimates a health-related number from standard formulas used in fitness and medicine.
Health calculators translate your measurements into numbers that describe your body, but every one of them is a model, not a diagnosis. BMI relates your weight to your height. BMR estimates the energy your body burns at complete rest. Body fat equations estimate composition from circumferences or skinfolds. Heart rate zones estimate exercise intensity from your pulse. Each formula was built from population averages, which means it describes people like you on average, not you specifically.
The formula, explained plainly
BMI is weight in kilograms divided by height in meters squared. It was designed as a population screening tool, not a personal health verdict, and that origin explains its famous flaw: it cannot tell muscle from fat. A muscular athlete and a sedentary person can share the same BMI with very different health profiles. Use it as a rough screening flag, and pair it with waist circumference or body fat estimates before drawing conclusions.
BMR is the energy your body needs to keep you alive at rest: breathing, circulation, cell repair. The Mifflin-St Jeor equation (using weight, height, age, and sex) is the most validated estimator for most adults. TDEE, total daily energy expenditure, is BMR multiplied by an activity factor, and confusing the two is the most common calorie-planning error: BMR is what you burn lying still, TDEE is what you burn living your actual day, and eating at BMR while active creates a bigger deficit than intended.
Body fat estimation from tape measurements (like the U.S. Navy method) uses neck, waist, and hip circumferences in equations fitted to real body-composition data. It is more informative than BMI because it separates fat from lean mass, but it is still an estimate with a typical error of a few percentage points. Consistent technique matters more than the specific formula: measure the same way each time and the trend will be trustworthy even if the absolute number is approximate.
Heart rate math starts with maximum heart rate, commonly estimated as 220 minus age, though this formula has a wide individual scatter of 10 to 15 beats. Training zones are percentages of that maximum: roughly 50 to 60 percent for easy recovery, 60 to 70 for fat-burning aerobic base, 70 to 80 for cardio fitness, and above 80 for high intensity. A chest strap or watch gives your real numbers; the formulas give you the map for interpreting them.
How to use it
- Enter weight (kg).
- Enter height (cm).
- Enter age.
- Choose sex.
- Read the instant result and the breakdown below it.
- Adjust any input to compare scenarios.
Worked example
With weight (kg) = 70, height (cm) = 175, age = 30, sex = Male, the result is 1,649 cal/day At rest, your body burns is 1,649 calories.. Defaults describe an average adult so the output looks familiar.
Common mistakes
- Picking an activity multiplier that describes the person you wish you were ('very active') instead of your real week, which inflates TDEE by 300 to 500 calories.
- Comparing body fat percentages across different methods (Navy tape vs calipers vs DEXA) as if they were interchangeable, when each has its own bias and error band.
- Measuring waist at a different spot each time (belly button vs narrowest point vs over clothes), which creates phantom trends in body fat estimates.
- Using 220 minus age as a personal max heart rate for precise zone training, when individual maxes scatter widely and zones built on a wrong max mislabel every workout.
- Chasing a single 'ideal weight' from a BMI table instead of a healthy range, ignoring frame size, muscle mass, and where your body naturally settles.
- Counting exercise calories from a machine display as exact, when cardio machines commonly overestimate burn by 15 to 30 percent.
- Weighing once after a salty meal or hard workout and reading water fluctuation as fat gain or loss, when daily weight swings of 1 to 2 kg are normal.
- Applying adult BMI cutoffs to children and teens, who must be assessed on age-and-sex percentile charts because healthy BMI changes as they grow.
Limitations
- Tape-measure body fat methods carry a typical error of 3 to 4 percentage points versus lab methods like DEXA; trends over time are more reliable than single readings.
- Maximum heart rate formulas are population averages with wide individual variation; for precise training, determine your max from a supervised test.
- Calorie and macro targets do not account for medical conditions (thyroid disorders, diabetes, PCOS), medications, or disordered eating history; professional guidance matters here.
- These calculators provide estimates for information only and are not medical advice; talk to a doctor or registered dietitian before starting a diet, exercise program, or acting on any result that concerns you.
- BMI does not apply well to athletes, pregnant people, the elderly, or those with high muscle mass; it is a screening tool, not a diagnosis.
Expected accuracy
The equations themselves (Mifflin-St Jeor, Navy body fat, standard BMI) are computed exactly; their accuracy as descriptions of you depends on input quality and how average your body is for the formula's reference population. Expect BMR within about 10 percent for most adults, body fat within 3 to 4 points of lab methods, and heart rate zones as useful guides rather than precise thresholds.
Privacy
Everything you type stays on your device. The calculation runs in your browser with JavaScript; no input is sent to a server, stored in an account, or shared with anyone.
Sources and standards
- BMI uses the Quetelet index (kg/m squared) with WHO category cutoffs; BMR uses the Mifflin-St Jeor equation, the most validated predictor for healthy adults; body fat estimation follows the U.S. Navy circumference method; heart rate zones use the standard percentage-of-max framework built on the 220-minus-age estimate.
Bottom line
BMR Mifflin turns your measurements into a clear, honest number with the context to interpret it. Remember what each formula can and cannot see, track trends rather than single readings, and bring a qualified professional into the picture for anything medical.
Key insight
Trends beat snapshots. One weigh-in or one heart-rate reading means little; the direction over weeks is what tells you whether a change is working.
Frequently asked questions
How much protein do I need?
General guidance is about 0.8 g per kg of body weight for sedentary adults, rising to 1.2 to 2.0 g per kg for active people and those building muscle or dieting. Spread intake across the day. People with kidney conditions should get medical advice before raising protein intake.
Can I target fat loss from one body area?
No, spot reduction is a myth: exercise burns fat systemically, and genetics decide where it comes off first. Abdominal exercises strengthen the muscles underneath but do not selectively burn belly fat. Overall energy balance plus full-body training changes composition everywhere.
When should I see a doctor about these numbers?
See a professional if your BMI is very high or very low, if you have symptoms (fatigue, chest pain, dizziness, unexplained weight change), if you have conditions like diabetes, thyroid disorders, or heart disease, or before starting any aggressive diet or exercise program. Calculators inform; clinicians diagnose.
How is BMI calculated?
Divide weight in kilograms by height in meters squared. A 70 kg person at 1.75 m has a BMI of 70 / (1.75 x 1.75) = 22.9. WHO categories are under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, and 30+ obese, but these are screening bands, not personal diagnoses.
Why is BMI criticized?
Because it measures weight relative to height, not fat relative to lean mass. Muscular athletes score as overweight, while people with low muscle and high fat ('skinny fat') can score as healthy. It remains useful for population screening, but individuals should pair it with waist measurement or body fat estimates.
What is the difference between BMR and TDEE?
BMR is the calories your body burns at complete rest; TDEE is BMR multiplied by an activity factor covering everything you do all day. A moderately active person might have a BMR of 1,600 and a TDEE of 2,400. Diet plans should be built from TDEE, not BMR, since eating at BMR creates an unintentionally aggressive deficit.
Which BMR formula is most accurate?
Mifflin-St Jeor is generally the most validated for typical adults. The older Harris-Benedict and Katch-McArdle (which uses lean body mass) are alternatives; Katch-McArdle can be better for very muscular or very lean people. All of them estimate within roughly 10 percent for most people.
How do I estimate my body fat percentage at home?
The U.S. Navy method uses neck and waist circumferences (plus hips for women) in a validated equation, needing only a tape measure. It is typically within 3 to 4 percentage points of lab methods. Measure consistently, same time of day and same technique, and trust the trend over weeks more than any single reading.
What is a healthy body fat percentage?
General athletic ranges are often cited as roughly 10 to 20 percent for men and 18 to 28 percent for women, with essential fat around 3 to 5 percent for men and 10 to 13 percent for women. Healthy ranges shift with age, and where you carry fat (visceral versus subcutaneous) matters for health risk too.
How do I calculate my maximum heart rate?
The classic estimate is 220 minus age; newer formulas like 208 minus (0.7 x age) fit population data slightly better. Both have wide individual scatter, so treat the result as a starting point. For precise zone training, a supervised max test or a hard-observed peak from training data beats any formula.
What are heart rate training zones?
Zones are intensity bands as percentages of max heart rate: about 50-60% easy recovery, 60-70% aerobic base building, 70-80% cardio fitness, 80-90% threshold work, 90%+ maximum efforts. Most weekly training time belongs in the lower zones; the common mistake is spending all training time in the middle, too hard to recover and too easy to adapt.
How many calories should I eat per day?
Start from your TDEE (BMR times activity factor), then adjust toward your goal: roughly 300 to 500 below TDEE for gradual fat loss, near TDEE for maintenance, slightly above for muscle gain. These are starting estimates to refine over 2 to 4 weeks of tracking weight trends, ideally with a dietitian's guidance.
Why does my weight fluctuate day to day?
Daily weight swings of 1 to 2 kg are normal and come from water, glycogen, food volume, and sodium, not fat changes. A hard workout, salty meal, or hormonal cycle can mask a week's fat loss overnight. Weigh under consistent conditions (morning, after bathroom, before food) and compare weekly averages.