BMI Calculator
Your BMI in US or metric units — plus an honest explanation of what the number does and doesn’t mean.
Calculate your body mass index
How BMI is calculated
Body mass index is your weight divided by the square of your height, in metric units:
Example: someone 5’10″ (70 inches) weighing 175 pounds has a BMI of 703 × 175 ÷ 70² = 25.1. Standard adult categories: under 18.5 underweight, 18.5–24.9 healthy weight, 25–29.9 overweight, 30+ obesity.
What BMI is actually for
BMI was designed as a quick screening tool for populations. It correlates with body fat across large groups and costs nothing to calculate. Public-health researchers and doctors use it as a first-pass flag — a reason to look closer, not a verdict.
What BMI can’t tell you
BMI cannot distinguish muscle from fat, doesn’t know where you carry weight, and ignores age, sex, and frame. A muscular athlete can score “oveweight” while carrying very little body fat. Waist circumference, body-fat percentage, blood pressure, and lab work all say more about health than BMI alone.
Waist circumference matters more than BMI
Visceral fat around your organs is metabolically active. A waist above 35 inches for women and 40 inches for men adds cardiovascular risk at any BMI level. A simple rule: keep your waist below half your height (waist-to-height ratio under 0.5).
BMI and ethnicity
People of Asian descent carry more metabolic risk at lower BMI values. Several countries use lower thresholds (23 for overweight, 27.5 for obese) for Asian populations. Standard US cutoffs underestimate risk for this group.
Is BMI accurate for athletes?
Often not. Muscle is denser than fat, so muscular people frequently score “oveweight” or higher while being lean. Body-fat percentage is far more informative for athletes.
Does this calculator work for children?
No — these are adult categories. Children and teens use BMI-for-age percentiles that account for growth. A pediatrician can interpret those properly.
What’s a healthy weight for my height?
The calculator shows the weight range for a BMI of 18.5–24.9 at your height. Research shows lowest mortality risk in the 22–25 range for most adults, with older adults doing well at 25–27.
Should I worry about a BMI of 25–27?
Context matters. BMI slightly over 25 with good waist measurement, blood pressure, and labs is very different from the same number with poor markers. Discuss the full picture with a doctor.
The BMI formula — what the math actually does
Body mass index divides your weight in kilograms by the square of your height in meters. In US units the constant 703 handles the conversion: BMI = 703 × weight(lb) ÷ height²(in²). The squaring of height makes BMI a ratio of weight relative to body surface area — not just raw weight. A 6-foot person needs proportionally more mass than a 5-foot person to reach the same BMI, which is why the formula has survived 150 years as a population screening tool.
Adolphe Quetelet invented the formula in the 1830s — not to assess individual health, but to describe average body proportions of European men for statistical research. It was not adopted as a clinical measure until the 1970s, when physiologist Ancel Keys proposed it for large-scale epidemiological studies. BMI was always a population tool, never a diagnostic one.
What the four BMI categories mean — and where they fall short
The WHO defines four adult categories: Under 18.5 (underweight), 18.5–24.9 (normal weight), 25–29.9 (overweight), and 30+ (obese). At the extremes the mortality relationship is strong. In the middle ranges it gets complicated.
The overweight category (25–29.9) is contested. A 2013 JAMA meta-analysis covering nearly 3 million people found individuals in the "overweight" range had lower all-cause mortality than those in the normal range — the so-called "obesity paradox." The reason: BMI cannot distinguish metabolically healthy weight-bearing mass from fat tissue that drives inflammation. Context — where weight is carried, fitness level, age, blood work — matters far more than the category alone.
Worked examples with real numbers
Example 1 — 5'10", 175 lb: 703 × 175 ÷ 70² = 25.1. Just above "normal weight." The healthy range at 5'10" runs roughly 129–174 lb — this person is one pound over the ceiling, a clinically meaningless distinction.
Example 2 — 5'4", 140 lb: 703 × 140 ÷ 64² = 24.0. Solidly healthy weight. Normal range at 5'4" runs about 108–145 lb.
Example 3 — Muscular athlete, 5'9", 215 lb: 703 × 215 ÷ 69² = 31.8. Classified "obese" despite carrying under 8% body fat. This is BMI's most well-known failure mode.
Better metrics to pair with BMI
Waist circumference
Visceral fat — packed around internal organs — drives insulin resistance, inflammation, and cardiovascular risk regardless of BMI. Elevated risk: men over 40 inches, women over 35 inches. Simpler rule: waist below half your height.
Body fat percentage
Far more informative than BMI. Healthy ranges: men roughly 10–20%, women 18–28%. DEXA scan is the gold standard; the Navy method (height, waist, neck) gives a reasonable estimate. See the Body Fat % Calculator.
Waist-to-hip ratio
WHO considers WHR above 0.90 (men) or 0.85 (women) elevated metabolic risk — capturing whether weight is carried at the waist (higher cardiovascular risk) versus hips and thighs (lower risk).
When to talk to a doctor about your BMI
Use this as a starting point, not an endpoint. BMI below 17 or above 35, or significant unexplained change, warrants a physician conversation. Pair your BMI with waist circumference, blood pressure, and fasting glucose for a genuinely useful metabolic snapshot.
Frequently asked questions
Is BMI accurate for athletes?
Often not. Muscle is roughly 18% denser than fat by volume. A muscular 200-lb person at 5'11" has a BMI of 27.9 — technically overweight — but if body fat is 8%, the classification is meaningless. Body fat percentage and waist circumference are far more informative for athletes.
Does BMI work for children and teens?
No. Children use BMI-for-age percentiles that account for growth and puberty — 85th–94th percentile is overweight, 95th+ is obese. Never apply adult BMI cutoffs to anyone under 18. A pediatrician interprets these in growth-chart context.
What is a healthy weight for my height?
The calculator shows the weight range for BMI 18.5–24.9 at your height. Research suggests lowest all-cause mortality in the 22–25 range for most adults, and slightly higher (25–27) for adults over 65. Your ideal weight is ultimately a physician conversation using your full health picture.
Should I worry about a BMI of 25–27?
Context is everything. A BMI of 26 with normal waist circumference, blood pressure, and blood glucose is very different from the same number with abdominal obesity and elevated triglycerides. The "overweight" label between 25–29.9 is one of the most misapplied in medicine. Focus on metabolic markers, not the single number.
What BMI cutoffs apply to Asian adults?
Standard WHO cutoffs underestimate cardiometabolic risk in people of Asian descent, who carry more visceral fat at lower BMI values. Adjusted thresholds used in many Asian countries: 23 for overweight, 27.5 for obesity. If you are of Asian descent, ask your physician about ethnicity-adjusted thresholds.
How often should I check my BMI?
Once or twice a year for most healthy adults. Daily weight fluctuations from hydration and food volume can move BMI by 0.5–1 point without any real body composition change. Track trends over months, not days.
Can I improve my BMI through exercise alone?
Yes, but the BMI change may be smaller than the health benefit. Gaining 5 lb of muscle while losing 5 lb of fat leaves BMI unchanged but significantly improves metabolic health. Waist circumference often reflects body composition improvement better than BMI when resistance training is involved.
What does Class III obesity (BMI 40+) mean?
Class I obesity = 30–34.9, Class II = 35–39.9, Class III (formerly "morbid obesity") = BMI 40+. Class III is associated with substantially elevated risk for type 2 diabetes, cardiovascular disease, sleep apnea, and several cancers. A physician-led conversation, not a calculator, is the right starting point.
Disclaimer: BMI is a screening tool, not a diagnostic measure. This calculator provides general educational information only and does not constitute medical advice. Consult a licensed healthcare provider for health decisions.