BMI Calculator
BMI (Body Mass Index) is your weight in kilograms divided by the square of your height in meters. It began as a population statistic devised by the Belgian statistician Adolphe Quetelet in the 1830s, and it is still the most widely used screening indicator for obesity today. Its appeal is practical: a tape measure and a scale are enough, everyone computes it the same way, and across large cohort studies it tracks consistently with mortality, type 2 diabetes, and cardiovascular risk. One important caveat for international readers is that this calculator uses the Asia-Pacific criteria published by the Korean Society for the Study of Obesity, where obesity begins at a BMI of 25 rather than the WHO's global threshold of 30 — so your result here may sit one category higher than on a calculator built for Western populations. Enter your height and weight below to see your BMI, its six-level category, and the healthy weight range for your height (BMI 18.5–22.9); the result is a reference point, not a diagnosis, so speak with a healthcare professional for a proper assessment.
Examples
Height 170 cm, weight 65 kg — near the top of the normal range
Converting 170 cm to meters gives 1.7 m, and squaring it gives 2.89 m². Dividing weight by that value: BMI = 65 ÷ 2.89 ≈ 22.5, which sits inside the normal range (18.5–22.9) but close to its upper edge. At this height the upper bound of the normal range is 22.9 × 2.89 ≈ 66.2 kg, and the overweight (pre-obese) category starts at 23 × 2.89 ≈ 66.5 kg — only about 1.5 kg away from the current weight. The position within a category is often more informative than the label itself, so when you are near the top of a band it helps to also track your weight trend over the past few years and your waist circumference.
Height 160 cm, weight 70 kg — where the two standards disagree
160 cm is 1.6 m, which squares to 2.56 m², so BMI = 70 ÷ 2.56 ≈ 27.3. Under the Korean and Asia-Pacific criteria this is Class 1 obesity (25–29.9); under the WHO global criteria it would only be classified as overweight, since obesity there begins at 30. This is a textbook example of the same number producing different labels. At this height, obesity by the Asia-Pacific criteria begins at 25 × 2.56 = 64.0 kg, and the top of the normal range is 22.9 × 2.56 ≈ 58.6 kg — roughly 6 kg and 11.4 kg away respectively. Those figures describe the categories, not a prescription: a realistic weight target depends on body composition, age, and existing conditions, and is best set with a clinician.
Working backwards to the healthy weight range for 175 cm
Rearranging the formula gives weight = BMI × height (m)², so you can multiply each threshold by your squared height to get your own target band. For 175 cm, height in meters is 1.75 and its square is 3.0625 m². The lower bound is 18.5 × 3.0625 ≈ 56.7 kg, the upper bound is 22.9 × 3.0625 ≈ 70.1 kg, and obesity begins at 25 × 3.0625 ≈ 76.6 kg. Notice that the healthy band spans more than 13 kg. That width exists because BMI counts muscle, bone, and fat as the same kilogram. Two people who are both 175 cm and 70 kg can carry very different health risks, which is why BMI works best as a starting point to be refined with waist circumference or a body composition measurement.
Obesity criteria of the Korean Society for the Study of Obesity (Asia-Pacific)
| Category | BMI | Risk with waist circumference |
|---|---|---|
| Underweight | Below 18.5 | Low / Moderate |
| Normal | 18.5 – 22.9 | Moderate / Slightly increased |
| Overweight (pre-obese) | 23 – 24.9 | Slightly increased / Increased |
| Class 1 obesity | 25 – 29.9 | Increased / High |
| Class 2 obesity | 30 – 34.9 | High / Very high |
| Class 3 obesity (severe) | 35 and above | Very high / Very high |
Comorbidity risk is shown as 'normal waist circumference / abdominal obesity (90 cm or more for men, 85 cm or more for women)'. Underweight carries a low risk of obesity-related disease but its own risks, such as nutritional deficiency and osteoporosis. Source: KSSO clinical practice guidelines for obesity.
Healthy weight range by height (BMI 18.5–22.9)
| Height | Healthy weight range | Obesity starts at (BMI 25) |
|---|---|---|
| 150 cm | 41.6 – 51.5 kg | 56.3 kg |
| 155 cm | 44.4 – 55.0 kg | 60.1 kg |
| 160 cm | 47.4 – 58.6 kg | 64.0 kg |
| 165 cm | 50.4 – 62.3 kg | 68.1 kg |
| 170 cm | 53.5 – 66.2 kg | 72.3 kg |
| 175 cm | 56.7 – 70.1 kg | 76.6 kg |
| 180 cm | 59.9 – 74.2 kg | 81.0 kg |
| 185 cm | 63.3 – 78.4 kg | 85.6 kg |
Calculated as weight = BMI × height (m)² and rounded to one decimal place. The obesity threshold shown follows the Asia-Pacific criteria; under the WHO global standard obesity would begin at BMI 30. Suitable weight varies with sex, age, and muscle mass, so treat these as reference values.
FAQ
Why does Korea use a lower BMI threshold than the WHO?
The WHO global standard defines overweight as a BMI of 25 or more and obesity as 30 or more. The Korean Society for the Study of Obesity instead treats 23 or more as pre-obese and 25 or more as obese. The reason is epidemiological: at the same BMI, people of Asian descent tend to carry a higher proportion of body fat, and in particular more visceral fat, so the risk of type 2 diabetes, hypertension, and dyslipidemia rises at a lower BMI than in European-descent populations. A WHO expert consultation in 2004 acknowledged this, proposing BMI 23 and 27.5 as additional public-health action points for Asian populations. The lower cut-off is not a claim that Asians are heavier; it marks the point where risk begins to climb.
Why is BMI misleading for people with a lot of muscle?
BMI uses total body weight and does not care what that weight is made of. Muscle is denser than fat, so it weighs more for the same volume. As a result, someone who trains regularly, or a competitive athlete with a body fat percentage around 10%, can easily land above 25 and be labelled obese. The reverse also happens: a person with little muscle and a lot of fat can weigh little enough to fall in the normal range. BMI is closer to a population screening tool than a personal diagnostic instrument, so pair it with a waist measurement or a body composition test before drawing conclusions.
How does body fat percentage differ from BMI?
BMI is a ratio of weight to height; it says nothing directly about how much fat you carry. Body fat percentage is the share of total weight that is fat, measured by bioelectrical impedance analysis, skinfold calipers, or DEXA (dual-energy X-ray absorptiometry). A commonly cited threshold treats roughly 25% or more in men and 30–35% or more in women as obesity, though cut-offs and readings vary by device and by the institution reporting them. BMI is crude but perfectly reproducible; body fat percentage is far more informative but sensitive to measurement conditions such as hydration and recent meals. The two are complements, not substitutes.
How do I measure waist circumference, and what is the threshold?
Measure at the midpoint between the lowest rib and the top of the hip bone (iliac crest), with the tape horizontal, against bare skin, at the end of a normal exhalation. In Korea, abdominal obesity is defined as 90 cm (about 35.4 inches) or more for men and 85 cm (about 33.5 inches) or more for women; other countries use somewhat different cut-offs for their own populations. Waist circumference matters because visceral fat is more closely tied to metabolic problems than subcutaneous fat, and the waist measurement is a practical proxy for it. In the guideline tables, a normal BMI combined with abdominal obesity is rated one step higher in comorbidity risk, which is why the tape measure deserves as much attention as the scale.
Do the same criteria apply to children, older adults, and pregnant women?
No — none of these groups should be assessed with the adult cut-offs. For growing children and adolescents, sex- and age-specific BMI percentiles are used: at or above the 85th percentile is overweight, and at or above the 95th percentile is obese. In older adults, muscle mass declines and height often decreases slightly, so BMI can overstate body fat, and being too thin may carry its own risks, which calls for individual assessment. During pregnancy, BMI is not used to judge obesity at all, because the weight of the fetus, placenta, and amniotic fluid is included; instead, recommended weight gain is guided by the pre-pregnancy BMI. In all three cases, follow the judgement of the treating clinician.
Can someone with a normal BMI still be unhealthy?
Yes. A body weight within the normal range combined with low muscle mass and a high proportion of body fat is often called 'skinny fat', and appears in the literature as normal weight obesity. Studies have reported that people in this state can accumulate visceral fat and show insulin resistance, dyslipidemia, and an elevated risk of metabolic syndrome despite an unremarkable BMI. Aggressive crash dieting makes this more likely, because muscle is lost alongside fat. If your BMI is normal but your waist exceeds the threshold, or a check-up shows borderline fasting glucose, triglycerides, or blood pressure, do not let the BMI number reassure you — discuss the full picture with a healthcare professional.
How should I set a weight loss target?
Aiming straight for the bottom of the normal BMI range usually ends in failure or rebound weight gain. A common clinical approach is to set an initial goal of losing 5–10% of current body weight over roughly six months, because even that much has been shown to produce meaningful improvements in blood pressure, blood glucose, and triglycerides. For someone weighing 80 kg, that is 4–8 kg, or a gentle pace of about 0.5 kg per week. Pushing faster with severe calorie restriction tends to cost muscle and lower resting energy expenditure, which works against you over the long run. Combine at least 150 minutes of aerobic activity per week with strength training twice a week, and if you have an underlying condition or are considering medication, plan it with a clinician.
Where BMI came from, and what it cannot see
The measure now called BMI began as the Quetelet index, proposed in the 1830s by the Belgian mathematician and statistician Adolphe Quetelet. He was not trying to diagnose individuals; he was describing the average build of populations, and observed that body weight scales roughly with the square of height, which gives the familiar kg/m² form. The modern name arrived in 1972, when the American physiologist Ancel Keys compared several obesity indices and concluded that this one offered the best correlation with body fat for the least measurement effort, christening it the Body Mass Index.
That statistical origin explains the tool's character. Across thousands of people it separates risk groups well; applied to one person it leaves a great deal out. It cannot distinguish muscle, fat, bone, and water; it says nothing about where fat is stored; and it ignores differences in frame and limb proportions. The relationship between BMI and actual body fat also shifts with age and ancestry, which is precisely why regional criteria exist.
Modern guidelines therefore position BMI as a screening step rather than a diagnosis. The usual sequence is to flag a risk group by BMI, then judge actual health risk by combining waist circumference, body composition, blood work, and existing conditions. Treat the result from this calculator the same way: a first signal worth following up, not a verdict.
The medical case for lower thresholds in Asian populations
From the late 1990s onward, cohort studies across several Asian countries kept finding the same pattern. Compare people of Asian and European descent at the same BMI, and the Asian group tends to have a higher body fat percentage, with a larger share of it stored as visceral fat inside the abdominal cavity. Unlike subcutaneous fat, visceral fat drains free fatty acids and inflammatory mediators directly to the liver, and it is more strongly linked to insulin resistance — the mechanism usually invoked to explain why type 2 diabetes and metabolic syndrome appear at lower BMI values in these populations.
On that basis, the WHO Western Pacific Regional Office, the International Association for the Study of Obesity, and the International Obesity Task Force published Asia-Pacific criteria in 2000, and a WHO expert consultation reported in 2004 that BMI 23 and 27.5 could serve as additional public-health action points for Asian populations. The Korean Society for the Study of Obesity, drawing on this work and on domestic cohort data, adopted 23 as the pre-obese threshold and 25 as the obesity threshold, and these are the values used in Korean national health screening and clinical practice.
So if this calculator disagrees with one you used abroad, it is not an error but a difference in the reference standard. A BMI of 26, for instance, is 'overweight' under the WHO global criteria and 'Class 1 obesity' under the Korean criteria. Neither is wrong; they simply draw the line on different population risk curves.
Measures that complement BMI: waist circumference, WHtR, and body fat
Waist circumference is the easiest addition. It needs nothing but a tape measure, and because it reflects visceral fat it supplies the one thing BMI never provides: where the fat is. Korean criteria set abdominal obesity at 90 cm for men and 85 cm for women, and guideline tables combine that with the BMI category to grade comorbidity risk. For the numbers to be comparable over time, measure against bare skin, at the end of an exhalation, and at a consistent time of day.
Waist-to-height ratio (WHtR) — waist circumference divided by height — is another common companion measure. It reduces the distortion that comes from comparing tall and short people using absolute centimetres, and it comes with an unusually memorable target: keep WHtR below 0.5. At 170 cm, that means a waist under 85 cm. Bear in mind that 0.5 is a practical boundary suggested by a body of research rather than a diagnostic criterion validated for every population.
For a more direct look at composition, body composition analysis by bioelectrical impedance reports both body fat percentage and skeletal muscle mass, while DEXA is the closer-to-reference method used in research and clinical settings. Impedance readings drift with hydration and with recent meals or exercise, so measure under consistent conditions — first thing in the morning, fasted, for example — and read the direction of change rather than the absolute number. In short, a practical stack is BMI for the broad picture, waist circumference for the location of fat, and body composition for the muscle-to-fat mix. No single number settles the question of health, and if something looks off, have your check-up results interpreted by a healthcare professional.
Sources
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