Free BMI Calculator

Calculate your Body Mass Index instantly. Supports Imperial (lbs/inches) and Metric (kg/cm) units. Shows your healthy weight range.

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Enter your height and weight to see your BMI.

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What Is BMI?

Body Mass Index (BMI) is a numerical value calculated from a person's height and weight that is used as a screening tool to categorise adults as underweight, normal weight, overweight, or obese. The formula is simple: BMI = weight in kilograms divided by the square of height in metres (kg/m²). In Imperial units: BMI = (weight in pounds × 703) divided by (height in inches)². It was devised by Belgian mathematician Adolphe Quetelet in the 1830s and adopted by the World Health Organisation as a population-level screening metric in the late 20th century.

BMI is used in clinical settings and public health research because it is fast, cheap, and non-invasive — all you need is a scale and a measuring tape. It correlates reasonably well with body fatness at the population level and is associated with health outcomes including cardiovascular disease, type 2 diabetes, and all-cause mortality risk.

BMI Categories

The World Health Organisation classifies BMI into four main categories for adults 20 years and older:

  • Underweight: BMI below 18.5. May indicate nutritional deficiency, malabsorption, or other health conditions. Associated with increased risk of bone density loss, immune deficiency, and anaemia.
  • Normal weight: BMI 18.5–24.9. Associated with the lowest risk of weight-related health problems in most population studies.
  • Overweight: BMI 25.0–29.9. Associated with increased risk of cardiovascular disease, hypertension, and type 2 diabetes compared to normal weight.
  • Obese: BMI 30.0 and above. Further classified as Class I (30–34.9), Class II (35–39.9), and Class III or "morbid obesity" (40+). Associated with significantly increased risk of multiple chronic conditions.

Some health organisations use different cut-offs for Asian populations, as research shows Asian adults have higher body fatness and related health risks at lower BMI values. The WHO suggests lower cut-offs of 23 for overweight and 27.5 for obese may be more appropriate for Asian populations.

Limitations of BMI

BMI is a useful screening tool but has well-documented limitations that clinicians and researchers are careful to account for:

  • Does not measure body fat directly. BMI cannot distinguish between fat mass and lean mass. An elite athlete with high muscle mass may have a BMI in the overweight range despite having low body fat. Conversely, a sedentary person with low muscle mass may have a "normal" BMI but carry excess fat (sometimes called "skinny fat" or normal-weight obesity).
  • Does not account for fat distribution. Where fat is stored matters as much as how much. Visceral fat (around the organs, measured by waist circumference) is more strongly associated with metabolic disease than subcutaneous fat. Two people with the same BMI may have very different health profiles depending on fat distribution.
  • Varies by age. Older adults tend to have more body fat and less lean mass at the same BMI compared to younger adults. BMI thresholds are not age-adjusted in standard clinical use.
  • Varies by sex. Women typically have higher body fat percentages than men at the same BMI due to physiological differences. Standard BMI thresholds are the same for men and women.
  • Varies by ethnicity. As noted above, standard BMI cut-offs may underestimate obesity risk in Asian populations and may overestimate it in some other populations.

Healthy Weight Ranges for US Adults

For a practical interpretation of your BMI, here are the weight ranges corresponding to a "normal" BMI (18.5–24.9) for common heights among US adults:

  • 5'4" (163 cm): 108–145 lbs (49–66 kg)
  • 5'6" (168 cm): 115–154 lbs (52–70 kg)
  • 5'8" (173 cm): 122–164 lbs (55–74 kg)
  • 5'10" (178 cm): 129–174 lbs (59–79 kg)
  • 6'0" (183 cm): 140–188 lbs (64–85 kg)
  • 6'2" (188 cm): 148–199 lbs (67–90 kg)

These ranges are for adults 20 years and older. For children and teenagers, BMI is interpreted differently using age- and sex-specific percentile charts (BMI-for-age), as body fatness changes substantially with age during growth and development.

Frequently Asked Questions

BMI is a useful population-level screening tool but is not a direct measure of health. It correlates with health outcomes in large studies but has significant limitations at the individual level. Clinicians use BMI alongside other measurements — waist circumference, blood pressure, blood glucose, cholesterol levels, and medical history — to assess overall health status. A BMI in the normal range does not guarantee good health, and a BMI slightly outside the normal range does not mean you have a health problem.

The BMI formula is the same for men and women — weight divided by height squared. However, the interpretation differs: women naturally have higher body fat percentages than men at the same BMI due to reproductive biology. Some researchers argue that sex-specific cut-offs would be more accurate, but the WHO and most clinical guidelines still use the same thresholds (18.5, 25, 30) for both sexes.

The WHO defines a BMI of 18.5 to 24.9 as normal weight for adults. Research consistently shows the lowest risk of weight-related mortality in this range, though some studies suggest the lowest mortality risk is at the high end of normal (23–24.9) or even slightly overweight (25–27.5) for older adults. The healthiest BMI for any individual depends on their age, sex, ethnicity, muscle mass, and overall health status.

Lowering BMI requires reducing body fat, which involves creating a caloric deficit through a combination of diet and physical activity. The evidence-based approach is: reduce caloric intake moderately (500–750 calories per day below maintenance), increase aerobic activity (150+ minutes per week of moderate intensity), and maintain lean muscle mass through resistance training. Consult a registered dietitian and healthcare provider before starting a weight loss programme, especially if you have any medical conditions.

BMI is calculated the same way for children and teenagers, but interpreted differently. For people under 20, BMI is assessed using BMI-for-age percentile charts that account for the normal changes in body composition as children grow. A child is considered underweight if their BMI is below the 5th percentile, healthy weight between the 5th and 85th percentile, overweight at the 85th to 95th percentile, and obese at or above the 95th percentile for their age and sex.

More accurate alternatives include: waist circumference (directly measures abdominal fat), waist-to-hip ratio, DEXA scan (gold standard for body composition, measures lean mass and fat mass precisely), hydrostatic (underwater) weighing, bioelectrical impedance analysis (BIA, used in consumer smart scales), and skinfold caliper measurements. These methods are more informative than BMI but require equipment or clinical testing. Waist circumference is recommended by the WHO alongside BMI as a simple additional risk indicator.

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