BMI Calculator

Metric formula:

• BMI = weight (kg) / height (m)²

So if you weigh 70 kilograms and you're 1.75 meters tall, you square your height (1.75 × 1.75 = 3.0625) and divide your weight by that number.

Imperial formula:

• BMI = [weight (lbs) / height (in)²] × 703

That 703 is a conversion factor. It's what brings the imperial result in line with the metric scale. Leave it out and you'd get a completely different number that doesn't map to any of the standard BMI categories.

Let's run through a concrete example in both systems.

Metric example: A person weighs 80 kg and is 1.80 m tall.

1. Square the height: 1.80 × 1.80 = 3.24
2. Divide weight by that result: 80 ÷ 3.24 = 24.7
3. BMI = 24.7 (falls in the healthy weight range)

Imperial example: A person weighs 176 lbs and is 71 inches tall (5 feet 11 inches).

1. Square the height: 71 × 71 = 5,041
2. Divide weight by that result: 176 ÷ 5,041 = 0.03491
3. Multiply by 703: 0.03491 × 703 = 24.5
4. BMI = 24.5 (also in the healthy weight range)

The tiny difference between the two comes down to rounding in the unit conversions. In practice they land at essentially the same score.

These classifications come directly from the World Health Organization and are used as reference points in clinical settings around the world. Some countries and health organizations use slightly adjusted cutoffs for specific populations, particularly people of Asian descent, where health risks tend to show up at lower BMI values.

A BMI under 18.5 suggests the body may not be getting enough nutrition to support normal function. That can come from a lot of different places: not eating enough, an underlying illness, an eating disorder, or in some cases just a naturally slight build with a fast metabolism.

The health risks are real. Nutrient deficiencies, a weaker immune system, bone density loss, fertility problems. It's not a harmless category just because it doesn't involve excess weight. If you're consistently in this range, it's worth talking to a doctor or registered dietitian.

People in this range tend to have lower risks of weight-related chronic diseases like type 2 diabetes, heart disease, and certain cancers. That's a statistical tendency across large populations, not a guarantee for any one person. Plenty of people outside this range are healthy, and being in it doesn't automatically mean everything is fine.

For most adults, maintaining a BMI in this range through balanced eating and regular movement is a reasonable goal. But it's one data point, not the whole story.

This range suggests someone is carrying more weight than what's considered optimal for their height. Health risks start climbing here: elevated blood pressure, higher cholesterol, more strain on the joints.

That said, some people in this category are metabolically healthy, especially those who are physically fit or carry significant muscle. A BMI of 26 on a recreational athlete is a very different situation from a BMI of 26 on someone who's mostly sedentary. Context matters a lot, and honestly this is one of the ranges where BMI's limitations become hardest to ignore.

Obesity is associated with substantially higher risks for a long list of serious conditions: type 2 diabetes, cardiovascular disease, sleep apnea, certain cancers, osteoarthritis, and more. Those risks generally increase as BMI climbs through the three classes.

Class III obesity, BMI 40 and above, is sometimes called severe or morbid obesity. It carries the greatest health risks and is associated with reduced life expectancy. At this level, medical intervention is often considered alongside lifestyle changes.

It's worth saying clearly: obesity is a complex condition. Genetics, environment, hormones, psychology, they all play a role. A BMI score doesn't explain why someone is in this range, and it says nothing about a person's worth or willpower.

Muscle is denser than fat. Someone carrying a lot of muscle will weigh more for their height than someone with a similar frame who has more fat. BMI only sees total weight relative to height, so it can't tell the difference.

This is why some professional athletes and bodybuilders technically land in the "overweight" or even "obese" categories despite having very low body fat. An NFL linebacker at 6'2" and 250 pounds of mostly muscle would register a BMI around 32. Classifying that as obese isn't meaningful.

For regular people who lift weights consistently, the distortion is less dramatic but still real. If you're active and carry solid muscle, your BMI may be overstating your actual health risk.

A few other measurements give a more useful picture of body composition and health risk.

• Waist circumference: Abdominal fat is closely tied to metabolic disease. A waist above 40 inches for men or 35 inches for women is considered a risk factor regardless of BMI.
• Waist-to-hip ratio: Dividing waist circumference by hip circumference helps assess where fat is distributed. A higher ratio points to more central, visceral fat, which carries greater health risk.
• Body fat percentage: Methods like DEXA scans, hydrostatic weighing, and bioelectrical impedance analysis can estimate how much of your weight is actually fat versus lean mass. More precise, but also more expensive or harder to access.
• Waist-to-height ratio: Some researchers argue this is a better predictor of cardiovascular risk than BMI. The general guideline: your waist should be less than half your height.

None of these is perfect either. But using them alongside BMI gives a much fuller picture than any single number on its own.