BMI Calculator

Body Mass Index (BMI) is the most widely used screening tool in public health for assessing whether an adult’s weight falls within a healthy range relative to their height.

Reviewed by: CalcMojo Editorial Team

This BMI calculator computes your BMI using the standard formula endorsed by the World Health Organization (WHO) and the National Institutes of Health (NIH), then classifies your result into one of the internationally recognized WHO weight categories: underweight, normal weight, overweight, or obese.

The calculation itself is straightforward: your weight in kilograms divided by your height in meters squared. A 5-foot-10 person weighing 170 pounds, for example, has a BMI of approximately 24.4, which falls within the normal weight range. The tool also displays your healthy weight range for your specific height, so you can see exactly where you stand and how far you are from category boundaries.

BMI is a useful population-level screening metric, but it is not a diagnostic tool. It does not distinguish between muscle mass and fat mass, and it does not account for fat distribution, bone density, age, or sex differences in body composition. Athletes, older adults, and individuals with high muscle mass may receive misleading results. For a more complete picture, pair this tool with our Body Fat Calculator or consult your healthcare provider for individualized assessment.

How BMI Is Calculated

The Body Mass Index formula was developed by Belgian mathematician Adolphe Quetelet in the 1830s and was formally adopted as a public health screening tool by the WHO in the 1990s. The formula is:

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

For imperial units, the equivalent formula is:

BMI = [weight (lbs) / height (in)^2] x 703

The result is a dimensionless number that provides a rough index of body fatness for most adults. The formula is identical for men and women and does not change with age for adults over 20.

For example, a person who weighs 75 kg and stands 1.75 m tall has a BMI of 75 / (1.75 x 1.75) = 24.5. In imperial units, a person weighing 165 lbs at 5 feet 9 inches (69 inches) has a BMI of (165 / 4,761) x 703 = 24.4.

WHO BMI Classification Categories

The World Health Organization defines the following BMI categories for adults:

  • Underweight: BMI below 18.5
  • Normal weight: BMI 18.5 to 24.9
  • Overweight: BMI 25.0 to 29.9
  • Obese Class I: BMI 30.0 to 34.9
  • Obese Class II: BMI 35.0 to 39.9
  • Obese Class III: BMI 40.0 and above

These thresholds are based on epidemiological data linking BMI ranges to health outcomes across large populations. Research consistently shows that BMI values above 25 are associated with increased risk of type 2 diabetes, cardiovascular disease, certain cancers, and all-cause mortality, with risk escalating as BMI increases above 30.

It is important to note that the WHO has also suggested lower BMI thresholds for certain Asian populations, where health risks may begin to increase at a BMI of 23 rather than 25. Some countries in the Asia-Pacific region have adopted adjusted categories accordingly.

What BMI Does and Does Not Tell You

BMI is valuable as a quick, inexpensive screening tool that correlates reasonably well with body fat percentage at the population level. For public health surveillance, clinical triage, and large-scale research, it remains the standard metric. However, BMI has well-documented limitations that every user should understand.

BMI does not measure body fat directly. It uses total body weight, which includes muscle, bone, water, and fat. A person with significant muscle mass, such as a competitive athlete or regular weight trainer, may have a BMI in the overweight or obese range despite having a low body fat percentage and excellent metabolic health.

BMI does not account for fat distribution. Where you carry your fat matters as much as how much you carry. Visceral fat (fat stored around the abdominal organs) is far more metabolically dangerous than subcutaneous fat (fat stored under the skin). Two people with identical BMI values can have vastly different health risk profiles depending on their waist circumference and visceral fat levels. A waist circumference above 40 inches for men or 35 inches for women is an independent risk factor for metabolic disease, regardless of BMI.

BMI may underestimate risk in older adults. As people age, they tend to lose muscle mass and gain fat mass even if their weight stays the same. An older adult with a normal BMI may actually have a high body fat percentage, a condition sometimes called sarcopenic obesity.

BMI may overestimate risk in muscular individuals. Athletes, bodybuilders, and anyone with above-average muscle mass will have an inflated BMI that does not reflect their actual health risk.

For a more complete assessment, consider using our Body Fat Calculator which uses the US Navy circumference method, or our Ideal Weight Calculator to see formulas that account for frame size and sex.

BMI and Health Risks

Despite its limitations, BMI remains a useful predictor of health outcomes when applied appropriately. Here is what the research shows about each category.

Normal weight (BMI 18.5-24.9). This range is associated with the lowest overall risk of chronic disease and premature mortality in large population studies. However, being at the low end of normal is not necessarily healthier than the middle. Some research suggests that a BMI of 22-25 is the optimal range for longevity.

Overweight (BMI 25-29.9). Moderately increased risk of type 2 diabetes, hypertension, dyslipidemia, and cardiovascular disease. However, the "overweight" category is controversial. Some large meta-analyses have found that people in this range have similar or even slightly lower all-cause mortality than those in the normal range, a phenomenon sometimes called the "obesity paradox." This may be partly explained by the protective effect of muscle mass in physically active individuals whose BMI falls in this range.

Obese (BMI 30+). Significantly increased risk of type 2 diabetes, cardiovascular disease, stroke, sleep apnea, certain cancers (breast, colon, endometrial), osteoarthritis, and reduced life expectancy. Risk increases progressively through the obesity classes. Class III obesity (BMI 40+) is associated with the highest rates of comorbidity and the greatest reduction in life expectancy.

Underweight (BMI below 18.5). Associated with increased risk of nutritional deficiency, osteoporosis, weakened immune function, fertility problems, and increased mortality. Being underweight carries its own serious health risks that are often overlooked in a culture focused primarily on overweight and obesity.

BMI for Children and Teens

BMI is calculated the same way for children and teens, but the results are interpreted differently. Because body composition changes with age and differs between boys and girls during growth, children’s BMI is expressed as a percentile relative to other children of the same age and sex, using growth charts developed by the CDC.

  • Underweight: Below 5th percentile
  • Healthy weight: 5th to 84th percentile
  • Overweight: 85th to 94th percentile
  • Obese: 95th percentile and above

This calculator is designed for adults aged 20 and older. For children and adolescents, consult your pediatrician who can plot BMI-for-age on the appropriate CDC growth chart.

Improving Your BMI

If your BMI falls outside the normal range and your healthcare provider has confirmed that weight management would benefit your health, the following evidence-based strategies can help.

For weight loss (high BMI). Create a moderate calorie deficit of 250-500 calories per day below your TDEE. Use our TDEE Calculator to find your maintenance calories. Combine dietary changes with regular physical activity, including both cardiovascular exercise and resistance training. Aim for 150-300 minutes of moderate-intensity aerobic activity per week, as recommended by the American College of Sports Medicine. Focus on sustainable changes rather than crash diets.

For weight gain (low BMI). Eat in a calorie surplus of 300-500 calories above your TDEE, emphasizing nutrient-dense foods rather than empty calories. Prioritize protein intake (0.7-1.0 g per pound of body weight) and incorporate progressive resistance training to ensure weight gained includes muscle mass rather than only fat. If unintentional weight loss has caused a low BMI, consult your healthcare provider to rule out underlying medical conditions.

For everyone. Regular physical activity improves metabolic health markers independent of weight change. Even if your BMI does not move significantly, exercise reduces visceral fat, improves insulin sensitivity, lowers blood pressure, and improves cardiovascular fitness.

This calculator provides general estimates based on published formulas. It is not medical advice and does not replace consultation with a qualified healthcare provider.

Frequently Asked Questions

What is a healthy BMI for adults?

According to the World Health Organization, a healthy BMI for adults is between 18.5 and 24.9. This range is associated with the lowest risk of chronic disease and premature mortality in large population studies. However, BMI is a screening tool and does not account for individual factors like muscle mass, bone density, and fat distribution.

How accurate is BMI as a health indicator?

BMI is a reasonably accurate population-level screening tool that correlates with body fat percentage for most adults. However, it can misclassify individuals with high muscle mass as overweight and may underestimate body fat in older adults who have lost muscle. For a more complete picture, combine BMI with waist circumference measurement and body fat percentage estimation.

Does BMI apply differently to athletes?

Yes. Athletes and individuals with significant muscle mass often have BMI values in the overweight or obese range despite having low body fat and excellent metabolic health. Muscle is denser than fat, so muscular individuals weigh more at any given height. If you are physically active and muscular, a body fat percentage measurement is more informative than BMI alone.

Is BMI different for men and women?

The BMI formula and category thresholds are identical for men and women. However, women naturally carry a higher percentage of body fat than men at any given BMI value. A woman and a man with the same BMI of 24 will have different body compositions. This is one reason BMI is a screening tool rather than a diagnostic measure.

What BMI thresholds apply to Asian populations?

The WHO has suggested that health risks may begin at lower BMI values in some Asian populations. Several Asia-Pacific health organizations use adjusted thresholds: overweight at BMI 23 and obese at BMI 27.5, compared to 25 and 30 in the standard WHO categories. This reflects differences in body fat distribution and metabolic risk at lower BMI values in these populations.

Can my BMI be normal but my body fat percentage high?

Yes, this is sometimes called "normal weight obesity" or "skinny fat." It occurs when someone has a normal BMI but a high ratio of fat to lean mass, often due to low muscle mass and a sedentary lifestyle. This condition carries metabolic risks similar to being overweight. Use our body fat calculator and consider measuring waist circumference for additional insight.

How is BMI different for children?

BMI is calculated the same way for children, but results are expressed as percentiles relative to other children of the same age and sex using CDC growth charts. This accounts for the normal changes in body composition during growth and development. A child at the 85th percentile or above is considered overweight; at the 95th percentile or above, obese.

Should I use BMI or body fat percentage?

Both metrics provide useful but different information. BMI is a quick screening tool that requires only height and weight. Body fat percentage is more informative because it directly estimates how much of your weight is fat versus lean mass. Ideally, use both. If your BMI and body fat percentage tell different stories, the body fat number is generally more reliable for assessing individual health risk.

Sources & Methodology

  • BMI calculated using the standard Quetelet Index formula: weight (kg) / height (m)^2.
  • WHO BMI classification categories from the World Health Organization Global Database on Body Mass Index.
  • Health risk associations from the National Institutes of Health (NIH) Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults (1998).
  • Asian BMI thresholds from the WHO Expert Consultation on BMI in Asian Populations (Lancet, 2004).
  • Pediatric BMI percentile categories from the CDC Growth Charts (2000).
  • Obesity paradox research referenced from Flegal KM et al., "Association of All-Cause Mortality With Overweight and Obesity Using Standard Body Mass Index Categories," JAMA, 2013.

Default values shown are illustrative. Always verify with your healthcare provider. Data accurate as of: March 2026