BMI
Calculator

What is BMI?

Body mass index is a simple ratio of your weight to your height, used worldwide as a quick screening tool for whether someone is a healthy weight. It doesn't measure body fat directly — it just compares your mass to your height — but across large populations it correlates well enough with health risk to be a useful first check. Because it needs only two numbers you already know, it's the most widely used weight measure on the planet.

How to calculate BMI

The formula is straightforward:

BMI = weight(kg) ÷ height(m)²

So a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9. If you think in pounds and inches, the equivalent is weight(lb) ÷ height(in)² × 703. The calculator above handles it instantly and also tells you which category you fall into and your healthy weight range.

BMI categories

The World Health Organization classifies adult BMI as follows:

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Normal weight
25.0 – 29.9	Overweight
30.0 and above	Obese

Healthy weight by height

The weight range that corresponds to a "normal" BMI (18.5–24.9) for a range of heights:

Height	Healthy weight range
155 cm	44–60 kg
160 cm	47–64 kg
165 cm	50–68 kg
170 cm	53–72 kg
175 cm	57–76 kg
180 cm	60–81 kg
185 cm	63–85 kg
190 cm	67–90 kg

What BMI tells you — and what it doesn't

BMI's strength is also its weakness: it's simple. It can't distinguish muscle from fat, so a muscular athlete may register as "overweight" or even "obese" while carrying very little fat. It says nothing about where you store fat, even though abdominal fat carries more health risk than fat on the hips. And it can mislead for older adults, who may have a "normal" BMI but low muscle mass. Treat BMI as a starting flag, not a diagnosis — a reason to look closer, not a verdict.

BMI for different groups

The standard adult categories don't apply universally. Children and teens use age- and sex-specific BMI percentiles rather than fixed cut-offs, because healthy ranges change as they grow. Some ethnic groups face elevated health risks at lower BMIs — for instance, lower thresholds are often recommended for people of South Asian descent. And pregnant people shouldn't use standard BMI at all. If you're in one of these groups, interpret your number with that context and ideally with a healthcare professional.

Going beyond BMI

Because BMI is blunt, it's best paired with measures that capture body composition and fat distribution. Waist circumference and waist-to-height ratio flag abdominal fat that BMI misses. Body fat percentage directly estimates how much of you is fat versus lean — try our body fat calculator. And if you want a target weight for your height, the ideal weight calculator builds on the same healthy-range logic. Together these give a far richer picture than BMI alone.

What BMI is linked to

The reason BMI endures despite its bluntness is that, across large groups, it tracks health risk reasonably well. A BMI in the obese range is statistically associated with higher rates of type 2 diabetes, high blood pressure, heart disease, certain cancers and joint problems. At the other end, being underweight carries its own risks — weakened immunity, bone loss and nutritional deficiencies. These are population-level associations, not individual destinies: plenty of people sit outside the "normal" band and are healthy. But they explain why BMI remains a useful first screen for clinicians and public-health researchers.

Where BMI came from

BMI isn't a modern invention. It was devised in the 1830s by the Belgian mathematician Adolphe Quetelet as a way to describe the "average man" in population statistics — which is exactly what it's good at, and exactly why it falls short for individuals. It was never designed as a personal health diagnostic. The term "body mass index" was coined much later, in the 1970s, when researchers confirmed it correlated with body fat across populations better than rival indices. Knowing its statistical origins helps explain both its usefulness and its limits.

How to move toward a healthy BMI

If your goal is to shift your BMI into the healthy range, the levers are the familiar ones: a sustainable calorie balance and regular activity. To lose weight, a moderate calorie deficit — see our TDEE calculator — paired with strength training preserves muscle so you lose mostly fat (which, helpfully, also improves your body composition beyond what BMI shows). To gain healthily if you're underweight, a modest surplus plus resistance training adds muscle. In both cases, slow and steady changes you can maintain beat dramatic ones you can't.

BMI vs waist-to-height ratio

One increasingly recommended companion to BMI is the waist-to-height ratio: your waist circumference divided by your height, ideally kept under 0.5. Its advantage is that it captures abdominal fat — the kind most strongly tied to metabolic risk — which BMI ignores entirely. Two people with the same BMI can have very different waist-to-height ratios, and the one carrying more weight around the middle generally faces higher risk. A simple rule of thumb is to "keep your waist to less than half your height." Checked alongside BMI, it adds the fat-distribution dimension that BMI alone can't see.

Using BMI sensibly

For most people, BMI is a reasonable, free, instant check that's worth knowing — just don't over-interpret a single number. If yours sits outside the healthy range, it's a useful prompt to consider your habits and, if needed, speak to a professional. If you're very muscular or very active, lean more on body fat and waist measures. The smartest use of BMI is as one data point among several, not the final word on your health.