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BMI Calculator
Calculate your Body Mass Index using your height and weight. Instantly see which BMI category you fall into —underweight, normal, overweight, or obese.
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BMI
Healthy range
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How BMI works
BMI stands for Body Mass Index, a numerical value of your weight relative to your height. The formula is weight in kilograms divided by height in meters squared: BMI = kg ÷ m². If you are using pounds and inches instead, multiply your result by 703. For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. A person who is 5'10" (178 cm) and weighs 180 lbs (81.6 kg) has a BMI of (81.6 ÷ (1.78)²) × 703 = 25.8.
BMI was developed in 1830 by Belgian mathematician Adolphe Quetelet as a quick way to assess whether a population was underweight or overweight. It was never designed to measure individual body fat percentage or individual health — that distinction matters. Quetelet himself noted that the index described populations well but performed poorly as an individual diagnostic. The index was adopted globally in the 20th century for its simplicity: no special equipment, no calipers, no underwater weighing — just a scale and a measuring tape.
The World Health Organization adopted the current BMI categories in the 1990s based on epidemiological data linking BMI ranges to health outcomes across large populations. These categories are: underweight (below 18.5), normal weight (18.5–24.9), overweight (25–29.9), and obese (30 and above). However, these thresholds were derived from data in populations of European descent and do not account for differences in body composition across ethnicities, ages, and fitness levels.
What BMI categories mean for your health
A BMI in the normal range (18.5–24.9) is associated with the lowest risk of weight-related diseases including type 2 diabetes, cardiovascular disease, and certain cancers. However, a BMI in the overweight range (25–29.9) does not automatically mean poor health — many athletes and muscular individuals fall here simply because muscle is denser than fat. Conversely, a normal BMI can coexist with high body fat if someone is sedentary but not overeating.
A BMI above 30 (obese) is associated with increased risk of heart disease, stroke, type 2 diabetes, sleep apnea, and certain cancers. The risk rises progressively with higher BMI, not just above a fixed threshold. A BMI above 40 (severely obese) carries significantly elevated risks and is often considered for medical interventions including bariatric surgery.
A BMI below 18.5 (underweight) carries its own health risks: weakened immune function, osteoporosis, fertility issues, and increased mortality from respiratory and infectious diseases. Being underweight can sometimes indicate underlying conditions such as hyperthyroidism, gastrointestinal disorders, or eating disorders.
Limitations of BMI
BMI does not distinguish between fat mass and lean body mass (muscle, bone, water). This is its most important limitation. A professional athlete with 10% body fat might weigh enough to push their BMI into the overweight range despite having very little actual body fat. A frail elderly person with high body fat and low muscle mass might have a normal BMI despite being overfat.
Age matters: BMI thresholds shift upward with age. Studies show that the BMI associated with the lowest mortality in adults over 65 is actually 25–27, not 18.5–24.9. Some health organizations now recommend higher BMI cutoffs for older adults. Pregnancy changes body composition dramatically and BMI is not applicable during or shortly after pregnancy.
Ethnicity significantly affects BMI interpretation. People of South Asian, Southeast Asian, and some East Asian descent have higher body fat percentages at lower BMIs and develop type 2 diabetes at lower BMI thresholds than people of European descent. The WHO and many national health bodies have issued lower BMI cutoff guidelines for Asian populations: overweight at 23 instead of 25, and obese at 27.5 instead of 30. Pacific Islander populations show the opposite pattern, with higher muscle mass pushing BMI higher without the same health risks.
Frequently asked questions
What is a healthy BMI range?
For most adults, a BMI between 18.5 and 24.9 is considered a healthy or "normal" range. However, this range represents population-level risk, not individual health. A healthy BMI for you may differ based on age, muscle mass, ethnicity, and medical history. The most useful single number is not your exact BMI but where you fall relative to your own baseline over time — a stable weight within a reasonable range is healthier than a fluctuating weight that always falls within guidelines.
Does BMI measure body fat?
No, BMI is an indirect proxy for body fat, not a direct measurement. It cannot distinguish between fat mass and lean tissue. Two people with identical BMIs can have very different body compositions: one might carry 25% body fat while the other carries only 12%. For a more accurate assessment of body fat, consider using body fat percentage calculators, skinfold measurements, bioelectrical impedance analysis, or DEXA scans — all of which directly measure or more precisely estimate fat mass.
Why do athletes sometimes have high BMI?
Muscle is about 18% denser than fat by volume. Professional football linemen, rugby players, powerlifters, and bodybuilders can carry significant muscle mass that pushes their weight well above what their height would suggest as "normal." Their BMI might be 30 or higher, technically classifying them as obese, when in reality they have very low body fat percentages. This is why BMI is never used as the sole assessment in sports medicine or individual health diagnostics.
Should I consult a medical professional?
If your BMI is outside the normal range, especially if you have a family history of heart disease, diabetes, or other weight-related conditions, it is worth discussing with your primary care provider. Similarly, if you are considering significant dietary or exercise changes, a healthcare provider can help you set appropriate goals. If your BMI is normal but you have other health concerns (fatigue, blood pressure issues, mobility limitations), do not dismiss them based on a "normal" BMI — weight is only one component of overall health.
How often should I recalculate my BMI?
For most people, checking BMI once every 3–6 months is sufficient for tracking purposes. Your weight fluctuates by 1–3 kg throughout the day due to hydration, food intake, and bowel movements. Weekly weigh-ins averaged over a month give a much more accurate picture than any single measurement. If you are actively trying to change your weight, monthly measurements are sufficient — daily weighing can encourage unhealthy fixation on short-term fluctuations that are not meaningful.
Does BMI apply to children and teenagers?
BMI in children and teenagers is calculated the same way as in adults but interpreted differently. Because children grow at different rates and body composition changes with age differently for boys and girls, BMI in people under 20 is expressed as a percentile relative to other people of the same age and sex. A child in the 85th percentile for BMI is heavier than 85% of children the same age and sex. Pediatricians use these percentiles rather than adult BMI categories to assess growth and weight status.
What is the difference between BMI and body fat percentage?
BMI is a height-to-weight ratio that estimates whether you are in a healthy weight range for your height — it says nothing about what that weight is made of. Body fat percentage divides your total fat mass by your body weight and expresses it as a percentage. A healthy body fat percentage for adult men is typically 10–20%; for adult women it is 18–28%. A bodybuilder at 5% body fat and a sedentary person at 25% body fat might weigh exactly the same and have the same BMI — but their health profiles are entirely different.
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For general wellness information only, not medical advice — see our disclaimer.