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Free BMI calculator. Calculate your Body Mass Index using height and weight. Supports imperial and metric units.
Body Mass Index is a 1830s-era statistical proxy invented by Belgian astronomer Adolphe Quetelet to describe the average body across a population — not to diagnose any individual. It compares your total mass to your height squared. The squaring is what makes BMI roughly height-independent: a healthy 5'2" adult and a healthy 6'4" adult can both land near 22, even though their absolute weights differ by 80 lb. That property is why BMI became the default screening number in clinics, insurance underwriting, and public-health surveillance — not because it captures body composition (it does not).
The number is a screening signal: it tells you the population most people with your BMI sit in. It cannot tell you whether you personally are carrying excess fat, low muscle, high bone density, or just an unusual frame. Treat the result as the start of a conversation, not a verdict.
Metric: BMI = weight (kg) ÷ height (m)². A person who is 178 cm and 77 kg: 77 ÷ (1.78 × 1.78) = 77 ÷ 3.1684 ≈ 24.3. That sits just under the upper edge of the WHO healthy-weight range (18.5–24.9).
Imperial: BMI = 703 × weight (lb) ÷ height (in)². A person at 5'10" (70 in) and 170 lb: 703 × 170 ÷ (70 × 70) = 119,510 ÷ 4900 ≈ 24.4. The 703 factor folds together the pound-to-kilogram (÷ 2.2046) and inch-to-meter (÷ 39.37) conversions so the imperial result lands on the same scale as the metric one.
The two formulas are mathematically identical — you should never get a different BMI by switching unit systems, only a rounding difference in the last decimal.
The standard four buckets — underweight, normal, overweight, obese — flatten out a finer clinical breakdown that doctors actually use:
| Category | BMI range | Clinical note |
|---|---|---|
| Severe underweight | < 16.0 | Significant mortality risk; usually warrants medical evaluation |
| Moderate underweight | 16.0 – 16.9 | Often nutritional or disordered-eating screening territory |
| Mild underweight | 17.0 – 18.4 | Lower edge of the population norm |
| Normal / healthy weight | 18.5 – 24.9 | Reference range for adult populations of European descent |
| Overweight (pre-obese) | 25.0 – 29.9 | Elevated cardiometabolic risk; often lifestyle-modifiable |
| Obesity, Class I | 30.0 – 34.9 | GLP-1 medication eligibility under FDA labeling |
| Obesity, Class II | 35.0 – 39.9 | Bariatric surgery threshold with comorbidities |
| Obesity, Class III (severe) | ≥ 40.0 | Bariatric surgery threshold without comorbidities |
For people of South Asian, East Asian, and Southeast Asian descent, the World Health Organization's Asia-Pacific guidance — and many national health authorities in the region — use overweight = BMI ≥23 and obesity = BMI ≥27.5. The reason is body composition. At any given BMI, populations of Asian descent tend to carry more visceral (abdominal) fat and less skeletal muscle than European-descent populations, and the cardiometabolic risk — type 2 diabetes, hypertension, atherogenic dyslipidemia — kicks in at a lower absolute BMI. A BMI of 24 is "healthy" by WHO global standards and "overweight" by WHO Asia-Pacific standards; both are correct for their reference populations.
Pediatric BMI uses the same arithmetic but a completely different interpretation. A growing child's healthy BMI changes dramatically with age and sex, so absolute thresholds don't work — a BMI of 19 is overweight at age 6 and underweight at age 16. Instead, clinicians plot the child's BMI on age-and-sex-specific CDC or WHO growth charts and express the result as a percentile. The standard CDC cutoffs for ages 2–19:
The calculator on this page is built for adults. Do not use it for children under 18; ask a pediatrician for a BMI-for-age plot.
There are five well-known populations where BMI mis-classifies:
Waist-to-height ratio (WHtR). Waist circumference ÷ height. A WHtR above 0.5 is the simple rule-of-thumb threshold for elevated cardiometabolic risk — "keep your waist to less than half your height." Better than BMI at flagging visceral fat specifically. Costs you a tape measure.
Body fat percentage. Measured directly by DEXA scan (gold standard, ~$50–150 at a sports-medicine clinic), hydrostatic weighing, BIA scales (cheap, noisy), or skinfold calipers (cheap, technique-dependent). Healthy adult ranges: roughly 10–20% for men, 18–28% for women, climbing slightly with age.
Waist-to-hip ratio (WHR). Waist circumference ÷ hip circumference. WHO defines elevated risk as >0.90 for men and >0.85 for women. Captures fat distribution, which BMI completely ignores.
Ponderal Index. Mass ÷ height cubed (kg/m³). Designed by Rohrer in 1921 to be less biased by height than BMI. Used in pediatrics and in some research contexts but never broke into clinical practice the way BMI did.
BMI is a gatekeeper for two big categories of treatment in US medicine:
GLP-1 weight-loss medications. Wegovy (semaglutide), Zepbound (tirzepatide), and Saxenda (liraglutide) are FDA-approved for chronic weight management at BMI ≥30, or BMI ≥27 with at least one weight-related comorbidity (type 2 diabetes, hypertension, dyslipidemia, sleep apnea). Insurance coverage typically follows the same threshold.
Bariatric surgery. Gastric sleeve, gastric bypass, and gastric band procedures are typically covered at BMI ≥40, or BMI ≥35 with comorbidities. The American Society for Metabolic and Bariatric Surgery has more recently endorsed lowering the threshold to BMI ≥35 (no comorbidities required) and BMI ≥30 with metabolic disease, but insurance hasn't fully followed.
Whatever you think of BMI as a metric, the practical reality is that crossing the 27 / 30 / 35 / 40 thresholds opens or closes treatment doors in the US healthcare system. That alone makes the number worth knowing.
No. Skeletal muscle is denser than fat, so heavily muscled people often land in the overweight or obese BMI categories despite having low body fat. NFL linemen, Olympic weightlifters, and competitive bodybuilders routinely post BMIs above 30 with body-fat percentages in the single digits. For athletes, body fat percentage measured by DEXA, hydrostatic weighing, or skinfold calipers is a far better health indicator than BMI.
The World Health Organization's Asia-Pacific guidelines and many Asian national health authorities use lower BMI thresholds — overweight starts at 23 and obesity at 27.5 — because cardiometabolic risk (type 2 diabetes, hypertension, cardiovascular disease) appears at a lower BMI in people of South Asian, East Asian, and Southeast Asian descent. Body composition at the same BMI tends to include more visceral fat and less skeletal muscle than in European-descent populations.
The BMI formula is identical, but the interpretation is completely different. Children's BMI is plotted against age- and sex-specific CDC or WHO growth charts and expressed as a percentile, not a category. The CDC defines underweight as below the 5th percentile, healthy weight as the 5th to 85th, overweight as the 85th to 95th, and obesity as at or above the 95th percentile for age and sex.
BMI = 703 × weight in pounds ÷ (height in inches)². The 703 conversion factor folds the pound-to-kilogram and inch-to-meter conversions into a single multiplier so the result is on the same scale as the metric formula (kg/m²). Example: 170 lb at 5'10" (70 inches) = 703 × 170 ÷ 4900 ≈ 24.4.
Clinically, BMI ≥40 is Class III obesity, often labeled "severe" or "morbid" obesity in older literature. The term "morbid" is being phased out in favor of "Class III" or "severe" because the older word carries stigma and over-emphasizes mortality without nuance. Class III obesity is the threshold for bariatric surgery eligibility under most US insurance plans, or BMI ≥35 with weight-related comorbidities.
No. BMI is a two-variable calculation using only total weight and height. It cannot distinguish between fat mass, muscle mass, bone density, or fluid retention. This is BMI's single largest limitation: a muscular 200-pound athlete and a sedentary 200-pound office worker of the same height have identical BMIs but very different metabolic health profiles.
Yes. GLP-1 weight-loss medications such as Wegovy, Zepbound, and Saxenda are FDA-approved for chronic weight management at BMI ≥30, or BMI ≥27 with at least one weight-related comorbidity (type 2 diabetes, hypertension, dyslipidemia, sleep apnea). Bariatric surgery is typically covered at BMI ≥40, or BMI ≥35 with comorbidities. These thresholds drive a lot of clinical use of BMI — even when its limitations are obvious.
Self-reported BMI is systematically biased. People tend to over-report their height (especially men) and under-report their weight (especially women), which pulls calculated BMI roughly 0.5 to 1.5 units lower than a measured value. This is large enough to bump people across category boundaries — a self-reported 24.8 may be a measured 26.0 — which is why epidemiology studies built on phone surveys understate obesity prevalence.
BMI is not used to assess weight during pregnancy. Pre-pregnancy BMI is used as a baseline that determines the recommended gestational weight-gain range, but a calculator like the one above is not informative for a pregnant person. The Institute of Medicine recommends 28–40 lb of gestational gain for underweight pre-pregnancy BMI, 25–35 lb for normal, 15–25 lb for overweight, and 11–20 lb for obese.
BMI is a proxy: it estimates total mass relative to height and assumes most variation comes from fat. Body fat percentage measures composition directly using DEXA scans, hydrostatic weighing, bioelectrical impedance scales, or skinfold calipers. For population screening, BMI is cheap and good enough. For individual decisions about training, nutrition, or metabolic risk in athletic or elderly people, body fat percentage is more useful.
Adult BMI below 18.5 is the WHO underweight threshold. The category is further subdivided into mild (17.0–18.4), moderate (16.0–16.9), and severe (below 16.0) thinness. Severe underweight in adults is a clinical concern in its own right — linked to malnutrition, eating disorders, and increased all-cause mortality — not just "the opposite of obesity."
BMI's mortality curve shifts upward in older adults. Several large cohort studies have found the lowest mortality in people over 65 sits around BMI 25–27, not 22, partly because sarcopenia (age-related muscle loss) means that for the same BMI an older person has less protective lean mass and is closer to frailty. A 70-year-old at the bottom of "normal weight" may actually be at higher risk than one in the lower overweight band.