How to Calculate BMI: Formula, Charts & Tips

Body Mass Index, or BMI, is one of the fastest ways to estimate whether body weight is low, average, high, or very high relative to height. Doctors, researchers, and public health agencies have relied on it for decades precisely because it requires no blood test, no special equipment, and no clinical visit — just a scale and a tape measure. More importantly, it gives a consistent, reproducible number that can be tracked over time and compared across populations.

That matters because unchecked weight gain is one of the leading contributors to preventable disease. Obesity is associated with type 2 diabetes, hypertension, sleep apnea, certain cancers, and cardiovascular disease. Catching a trend early — even before symptoms appear — gives people a chance to act. BMI is not a diagnosis, but it is a reliable first signal worth paying attention to.

It is also worth knowing what BMI does not measure: it cannot distinguish between fat and muscle, it does not account for fat distribution, and it does not reflect metabolic health directly. Used alone, BMI tells an incomplete story. Used as one data point alongside body fat percentage, waist circumference, and routine bloodwork, it becomes genuinely useful. If you want to run the numbers immediately, use the BMI Calculator.

The BMI Formula

The metric formula is:

BMI = weight (kg) / height (m)^2

If you use pounds and inches, the imperial version is:

BMI = (weight in lb x 703) / height (in)^2

That means BMI is a ratio. It compares your body mass with your height squared, which helps normalize for the fact that taller people naturally weigh more than shorter people.

You might wonder why the formula squares height instead of just dividing weight by height. If you simply divided kilograms by meters, taller people would almost always look underweight and shorter people would look overweight, because mass scales roughly with the cube of height. Belgian mathematician Adolphe Quetelet figured this out in the 1830s. Squaring height turned out to be the best simple exponent for producing a number that stays consistent across a wide range of adult heights. Some researchers have argued that an exponent of 2.5 would be more accurate, but height squared strikes a practical balance between simplicity and usefulness.

Step-by-Step Example (Imperial)

Suppose someone weighs 170 pounds and is 5 feet 10 inches tall.

1. Convert height to inches: 5 x 12 + 10 = 70
2. Square the height: 70 x 70 = 4,900
3. Multiply weight by 703: 170 x 703 = 119,510
4. Divide: 119,510 / 4,900 = 24.39

BMI is about 24.4.

That falls in the normal-weight range for adults.

Step-by-Step Example (Metric)

Now let us try a metric example. Imagine someone who is 1.65 meters tall and weighs 72 kilograms.

1. Square the height: 1.65 x 1.65 = 2.7225
2. Divide weight by height squared: 72 / 2.7225 = 26.45

BMI is about 26.4.

That puts this person in the overweight category. Notice how the metric version is more straightforward — no conversion factor needed, just divide and you are done.

How to Read BMI Categories

For most adults, the standard categories are:

• Underweight: below 18.5
• Normal weight: 18.5 to 24.9
• Overweight: 25.0 to 29.9
• Obesity: 30.0 and above

These ranges are useful for broad screening, but they should not be treated as the whole story. Waist size, body-fat distribution, exercise habits, and blood work can matter just as much.

BMI Across Different Populations

BMI does not mean exactly the same thing for everyone. Age, sex, and ethnicity all influence how the number should be interpreted.

Women generally carry more body fat than men at the same BMI. A woman and a man who both land at 25 may have quite different body compositions, thanks to hormonal differences and fat distribution patterns. That does not make BMI useless, but it does mean the categories are rougher than they look.

Age matters too. Older adults tend to lose muscle and gain fat gradually, so a BMI of 24 at age 70 might correspond to a higher body-fat percentage than the same number at age 30. Some research even suggests that a slightly higher BMI in older adults is linked to better health outcomes.

Ethnicity is another important factor. People of South Asian and East Asian descent tend to develop metabolic complications like type 2 diabetes at lower BMI values than people of European descent. The WHO has acknowledged this by noting that some Asian populations may benefit from lower cutoffs — around 23 for overweight rather than 25. Meanwhile, some Pacific Islander populations tend toward higher bone density and muscle mass, which shifts the picture in the opposite direction.

For children and teens, adult BMI categories do not apply at all. Kids need age-and-sex-specific percentile charts, which is why a BMI Calculator for Kids or BMI Calculator for Teens is more appropriate for younger users.

When BMI Can Be Misleading

BMI is practical, but it is not perfect.

Athletes and muscular people can have a high BMI without having excess body fat. A rugby player could easily register as “obese” by BMI alone, which clearly misses the mark. Conversely, someone sedentary with little muscle might have a normal BMI while carrying a worrying amount of visceral fat around their organs. BMI simply cannot distinguish between fat and muscle.

Pregnant women, people with conditions that cause fluid retention, and individuals with unusual body proportions can also get misleading results.

BMI vs. Healthy Weight Goals

BMI is most useful when you combine it with real context:

• How active are you each week?
• Is your weight stable or changing quickly?
• Do you have a large waist measurement?
• Are your cholesterol, glucose, and blood pressure healthy?

For many people, the best use of BMI is not obsessing over a single number. It is a quick check that tells you whether it may be worth looking deeper with your doctor.

Additional Worked Example: Finding Your Healthy Weight Range

BMI categories work in reverse too. If you know your height, you can calculate the weight range that falls within “normal” BMI (18.5 to 24.9).

Suppose someone is 5 feet 6 inches tall (66 inches).

For BMI = 18.5 (lower bound of normal): weight = (18.5 x 66^2) / 703 = (18.5 x 4,356) / 703 = 114.6 lb

For BMI = 24.9 (upper bound of normal): weight = (24.9 x 4,356) / 703 = 154.3 lb

So a person who is 5’6” falls in the normal BMI range between roughly 115 and 154 pounds. This gives a concrete target range rather than an abstract number. The BMI Calculator shows this range automatically when you enter your height.

Common Mistakes When Using BMI

Using the wrong formula for your unit system. The metric formula divides kilograms by meters squared. The imperial formula multiplies pounds by 703 and divides by inches squared. Mixing units — for example, using pounds with the metric formula — produces nonsensical results. Always double-check which system you are using.

Applying adult categories to children. Adult BMI cutoffs (18.5, 25, 30) do not apply to anyone under 20. Children and teens grow at different rates, so their BMI must be plotted on age-and-sex-specific growth charts from the CDC. A 10-year-old boy with a BMI of 22 might be at the 90th percentile (overweight), while a 17-year-old boy with the same BMI might be perfectly average.

Treating BMI as a measure of health. BMI measures body mass relative to height. It says nothing about blood pressure, cholesterol, blood sugar, cardiovascular fitness, or mental health. A person with a BMI of 27 who exercises regularly, eats well, and has normal lab values may be healthier than someone with a BMI of 22 who is sedentary and has prediabetes. BMI is a screening tool that tells you when to investigate further, not a verdict.

Ignoring the limitations for muscular individuals. If you strength train seriously, BMI will almost certainly overestimate your risk category. In this case, body fat percentage is a far better metric. Use the Body Fat Calculator for a more nuanced picture, or pair BMI with waist circumference — the CDC suggests that a waist measurement over 40 inches for men or 35 inches for women indicates elevated health risk regardless of BMI.

Beyond BMI: Complementary Health Metrics

No single number captures the full picture of metabolic health. Here are the most useful metrics to track alongside BMI:

Metric	What It Measures	Tool
Body fat percentage	Fat vs lean tissue ratio	Body Fat Calculator
BMR	Resting calorie burn	BMR Calculator
Waist-to-hip ratio	Fat distribution pattern	Tape measure
TDEE	Total daily calorie needs	Calorie Calculator
Fasting glucose	Blood sugar regulation	Lab test
Blood pressure	Cardiovascular strain	Cuff measurement

The combination of BMI, body fat percentage, and waist circumference gives you a much more reliable snapshot than any one of those numbers alone. When all three are in healthy ranges, the odds of metabolic disease are significantly lower.

Frequently Asked Questions

Q: Is BMI accurate for older adults?

BMI becomes less reliable as a standalone measure in people over 65. Aging is typically associated with gradual muscle loss (sarcopenia) and an increase in body fat, even when total body weight stays stable. A 70-year-old and a 30-year-old can share the same BMI of 24 while having very different body compositions — the older person may carry significantly more fat relative to lean mass. Some research suggests that a slightly higher BMI (in the 25-27 range) may actually be associated with better survival outcomes in older adults, possibly because a modest fat reserve provides energy during illness or recovery. For older adults, waist circumference and functional fitness tests are often more useful signals than BMI alone.

Q: What is a healthy BMI for a teenager?

Adult BMI categories (18.5, 25, 30) do not apply to children or adolescents. For anyone under 20, BMI must be interpreted using age-and-sex-specific growth percentile charts from the CDC. A BMI that is perfectly average for a 17-year-old boy might be in the overweight range for a 10-year-old girl. The CDC defines overweight as BMI at or above the 85th percentile and obesity as BMI at or above the 95th percentile for age and sex. The BMI Calculator for Teens applies the appropriate percentile charts automatically, so you do not need to look up the reference data manually.

Q: Can I have a normal BMI but still be metabolically unhealthy?

Yes, and this is more common than most people realize. The condition is sometimes called normal weight obesity or metabolically obese normal weight (MONW). People in this category have a BMI below 25 but carry a high proportion of visceral fat — the fat stored around internal organs — which is the type most strongly linked to insulin resistance, high triglycerides, and cardiovascular risk. Conversely, a BMI of 27 in a lean, muscular athlete may come with excellent metabolic markers. This is precisely why BMI should not be the only measure you track. Waist circumference above 40 inches (men) or 35 inches (women), elevated fasting glucose, or abnormal lipid panels signal metabolic risk regardless of what the BMI says.

Related Reading

• Body Fat Percentage: What is Healthy, How to Measure It explains why body composition matters more than scale weight.
• What is BMR? Basal Metabolic Rate Explained shows how your resting metabolism connects to weight management.

The Bottom Line

BMI is easy to calculate, easy to compare, and useful as a starting point. The key is to treat it as a screening tool, not a diagnosis. Your age, sex, ethnicity, and fitness level all shape what your number actually means. If you want an instant result with category labels and healthy-weight estimates, try the BMI Calculator.

Sources

• World Health Organization — Body Mass Index (BMI)
• Centers for Disease Control and Prevention — About Adult BMI
• National Institutes of Health — Calculate Your BMI