BMI Calculator

What this calculator does & how it works

Body Mass Index (BMI) is a simple ratio of your weight to your height. The formula was popularised as a population screening tool and is defined by the World Health Organization as your weight in kilograms divided by the square of your height in metres: BMI = kg / m². If you enter imperial units, this tool converts pounds and feet/inches to metric first, so the result is identical either way.

Once it has your BMI, the calculator places it into the WHO adult categories and works backwards to show the weight range that would put you in the "normal" band (a BMI of 18.5 to 24.9) for your specific height. That range is often more useful than the single number, because it gives you a concrete target rather than an abstract score.

How to interpret your results

The WHO adult categories are: under 18.5 — underweight; 18.5 to 24.9 — normal weight; 25 to 29.9 — overweight; 30 and above — obese (often split into class I, II and III). These cut-offs describe statistical risk across large populations; they are not a verdict on any individual. A BMI in the "overweight" band, for instance, does not by itself mean you are unhealthy.

BMI is best read alongside other simple measures. Your waist-to-height ratio captures where you carry weight, which matters for metabolic risk, and your daily calorie needs help you turn a target weight into a daily plan.

Limitations and when to consult a professional

BMI cannot tell muscle from fat. A muscular athlete and someone carrying excess fat can share an identical BMI. It also does not account for sex, age, ethnicity, bone density, or fat distribution, and the standard adult cut-offs are not appropriate for children, pregnant people, or frail older adults. Some populations carry higher metabolic risk at lower BMI values. For these reasons, clinicians use BMI as one input among many — never on its own.

If you are trying to lose or gain weight, are concerned about your health, or have a condition that affects body composition, talk to a doctor or a registered dietitian. They can interpret your numbers in the context of your full health picture, which a calculator cannot do. Nothing here is medical advice.

A worked example: same BMI, different bodies

Take the calculator's default: 170 cm and 70 kg gives a BMI of about 24.2 — inside the normal band, near its top edge. For a height of 170 cm, the normal range (18.5 to 24.9) works out to roughly 53.5 kg at the low end and 72 kg at the high end, which is the "healthy weight range" the tool reports. Notice how wide that is: an 18-plus-kilogram span all counts as normal for the same height. That width is the point — there is no single correct weight, only a range, and where you sit inside it is far less important than the trend over time and how you feel and function.

Now consider two people who both read 27 — squarely in the "overweight" band. One is a recreational lifter carrying extra muscle; the other has been gradually gaining fat over several years. Their BMIs are identical and their health trajectories are opposite. BMI cannot tell them apart because it only knows total mass and height, not what that mass is made of. This is the single most important thing to understand about the number: it is a screening flag for a population, not a measurement of an individual's health.

Common mistakes when reading BMI

• Treating a category boundary as a cliff. A BMI of 24.9 and 25.1 are essentially the same body; one is labeled "normal" and the other "overweight." The categories are convenient bins, not biological thresholds where risk suddenly jumps.
• Using adult BMI for children or teens. Growing bodies are assessed with age-and-sex percentile charts instead. The adult cut-offs on this page do not apply to anyone under 18.
• Applying it during pregnancy. Pregnancy changes body mass in ways the formula was never designed for; pre-pregnancy BMI and clinician guidance are what matter there.
• Ignoring ethnicity-related differences. Some populations — several Asian groups in particular — face elevated metabolic risk at BMI values below the standard "overweight" threshold, which is why some health bodies use lower cut-offs for them.
• Chasing a lower number through unhealthy means. Losing muscle or becoming dehydrated will lower BMI without improving health. The number is a proxy; optimizing the proxy instead of the underlying health is a classic trap.

What to pair with BMI for a fuller picture

Because BMI is deliberately simple, it is most useful as one reading among several. Your waist-to-height ratio adds the dimension BMI misses entirely — fat distribution — and a ratio under about 0.5 is a widely used low-risk marker regardless of BMI. If you're acting on your result, the calorie/TDEE calculator converts a target weight into a daily energy budget, and our guide to understanding BMI goes deeper on the history and the evidence. No single tool decides anything; together they sketch a picture that a professional can then read in the context of your full health.

Frequently asked questions