BMI — Body Mass Index — is one of the most searched health terms online, and for good reason: it's the quickest way to get a snapshot of whether your weight is in a healthy range for your height. But it's also widely misunderstood and frequently misapplied.
This guide explains what BMI actually measures, what the number ranges mean, how to calculate it accurately, and — critically — what BMI doesn't tell you about your health.
BMI is a number calculated from your height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s as a population-level statistical tool — not as a diagnostic tool for individuals. Despite this, it became the standard screening tool used by healthcare professionals worldwide to identify potential weight-related health risks.
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Nutritional deficiency, bone density loss, anaemia |
| 18.5 – 24.9 | Normal weight | Lowest risk of weight-related conditions |
| 25.0 – 29.9 | Overweight | Increased risk of type 2 diabetes, hypertension |
| 30.0 – 34.9 | Obese (Class I) | High risk — cardiovascular disease, sleep apnoea |
| 35.0 – 39.9 | Obese (Class II) | Very high risk |
| 40 and above | Morbidly Obese | Extreme risk — often requires clinical intervention |
Rather than a single "target" weight, it's more useful to know the weight range that keeps you in the Normal BMI zone (18.5 – 24.9). The formula for this is:
The standard BMI ranges are designed for adults (18–65). For children and teenagers, BMI is interpreted differently — using age- and gender-specific percentile charts rather than fixed ranges. For adults over 65, some research suggests a slightly higher BMI (25–27) may actually be associated with better outcomes, as modest body reserves can be protective in illness.
There are also modest gender differences: women tend to have slightly higher body fat at the same BMI as men due to differences in body composition, which is why some health providers use gender-adjusted interpretations.
Metric or imperial. BMI gauge, category, and ideal weight range — instantly in your browser.
Check My BMI →BMI is useful as a population screening tool, but it has significant limitations when applied to individuals:
Muscle is denser than fat. Athletes and bodybuilders often have "overweight" BMIs with very low body fat.
People with higher bone density register higher BMI without corresponding fat increase.
Abdominal (visceral) fat is far more dangerous than subcutaneous fat — BMI doesn't distinguish between the two.
South Asians and East Asians tend to have higher health risks at lower BMI values. WHO has separate cutoffs for Asian populations (23 = overweight).
A better picture of your health comes from combining BMI with waist circumference, waist-to-hip ratio, body fat percentage (via DEXA scan or bioimpedance), blood pressure, blood glucose, and lipid panels.
Body mass index is weight in kilograms divided by height in metres squared. The Belgian statistician Adolphe Quetelet devised it in the 1830s, and it is worth knowing why: he was describing the distribution of body measurements across populations, not assessing individuals. The index was adopted for individual screening more than a century later mainly because it needs only two easily measured numbers.
A worked example: someone 1.72 m tall weighing 78 kg has an index of 78 ÷ (1.72 × 1.72) = 78 ÷ 2.958 = 26.4. In imperial units the same calculation is weight in pounds divided by height in inches squared, multiplied by 703.
| Range | WHO international category | Asian-Indian reference |
|---|---|---|
| Below 18.5 | Underweight | Underweight |
| 18.5 – 22.9 | Normal | Normal |
| 23.0 – 24.9 | Normal | Overweight |
| 25.0 – 29.9 | Overweight | Obese |
| 30.0 and above | Obese | Obese |
The lower thresholds used in Indian and several other Asian clinical guidelines are not arbitrary. Research consistently finds a higher proportion of body fat, and more of it stored around the abdomen, at any given index in South and East Asian populations compared with European reference groups. A reading of 24 sits in the healthy band under one set of guidelines and in the overweight band under another. Which applies to you is a conversation for a clinician, not a calculator.
The index takes no account of what the weight is made of or where it sits. Several well-documented consequences follow.
Small errors matter more than people expect, because height is squared. Being 2 cm out on height shifts the result by around 0.6 points, enough to move someone across a category boundary. Measure height without shoes, standing straight against a wall, first thing in the day when the spine is least compressed. Weigh yourself at a consistent time, ideally in the morning after using the bathroom and before eating, in similar clothing. Day-to-day swings of one to two kilograms come mostly from water, food in transit and glycogen, and mean very little on their own. A trend measured over several weeks is informative; a single reading is noise.
Treat the index as one screening signal among several, in the same way a single blood pressure reading is a prompt rather than a diagnosis. If your number falls outside the healthy band, useful next questions include what your waist measurement is, how your weight has trended over the past year or two, how active you are, what your blood pressure and blood sugar look like, and whether there is relevant family history. Those answers together form a picture. The index alone does not.
It is also worth saying plainly that the number is not a verdict on you. It is a ratio of two measurements, useful for spotting patterns across large groups and moderately useful as a personal prompt to look further. People at every point on the scale can be healthy or unhealthy, and fixating on a single figure is more likely to cause anxiety than to improve anything.
Height and weight are personal health information. The calculation here happens entirely within the page: nothing is transmitted, stored or logged anywhere. You can confirm this by opening developer tools and watching the Network panel while you calculate, or by disconnecting from the internet after the page has loaded and using it anyway.
This page provides general educational information only. It is not medical advice, cannot account for your individual circumstances, and is not a substitute for assessment by a qualified healthcare professional. If you have questions or concerns about your weight, health or a result you have calculated, please speak with a doctor.
Indian and several other Asian clinical guidelines use lower cut-offs because body fat tends to be higher, and more abdominally distributed, at any given index. Which set of thresholds is appropriate for you is best confirmed with a doctor.
Not really. Muscle is denser than fat, so a very muscular person can register as overweight or obese while carrying a low body-fat percentage. Body composition measurement is more meaningful in that case.
Waist circumference, or waist divided by height. Keeping that ratio below about 0.5 is a widely used guideline, and it captures abdominal fat, which the index cannot see at all.
No. Children and adolescents are assessed against age-and-sex-specific percentile charts, because normal body composition changes substantially throughout growth.
More than you would think, because height is squared in the formula. A two-centimetre error changes the result by roughly 0.6 points, which can move you across a category line.
There is little value in it. Normal daily fluctuation of one to two kilograms comes from water and food, so weekly or fortnightly readings viewed as a trend are far more informative.
No. The calculation runs entirely inside your browser and nothing is transmitted. You can verify this in the Network panel of developer tools or by disconnecting from the internet first.