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Learn exactly how to calculate Body Mass Index (BMI), understand what your result means, see the BMI charts by age and gender, and discover the limitations of BMI as a health metric.
Everything you need to know
Body Mass Index (BMI) is a numerical value calculated from your height and weight. It's the most widely used screening tool for weight categories that may lead to health problems. Doctors, insurance companies, and researchers all use BMI — so understanding yours is important.
But BMI is a starting point, not a complete picture. It has real limitations (which we'll cover). First, let's understand how it works.
BMI uses a simple formula:
Metric units:
BMI = weight (kg) ÷ height² (m²)
Imperial units:
BMI = [weight (lbs) ÷ height² (inches²)] × 703
For a person who is 5'9" (175 cm) and weighs 165 lbs (75 kg):
Metric: BMI = 75 ÷ (1.75)² = 75 ÷ 3.0625 = 24.5
Imperial: BMI = [165 ÷ (69)²] × 703 = [165 ÷ 4,761] × 703 = 0.03466 × 703 = 24.4
| BMI Range | Category | Health Implications |
|---|---|---|
| Below 18.5 | Underweight | Nutritional deficiency risk, weakened immunity |
| 18.5–24.9 | Normal weight | Lowest disease risk |
| 25.0–29.9 | Overweight | Increased risk of heart disease, diabetes |
| 30.0–34.9 | Obese (Class I) | High risk — medical evaluation recommended |
| 35.0–39.9 | Obese (Class II) | Very high risk |
| 40.0+ | Obese (Class III) | Extremely high risk — intervention recommended |
BMI healthy ranges are the same for adults 20+. For children and teenagers (ages 2–19), BMI is interpreted as a percentile compared to peers of the same age and sex, not absolute values.
The standard WHO categories above apply.
| Percentile | Weight Status |
|---|---|
| Below 5th | Underweight |
| 5th–84th | Healthy weight |
| 85th–94th | Overweight |
| 95th and above | Obese |
Important: Never use adult BMI categories for children. Always use age- and sex-specific percentile charts.
Men and women with the same BMI may have different health profiles. Women naturally carry more body fat for reproductive function. The same BMI in men typically represents higher muscle mass. Some researchers suggest lower BMI cutoffs for women (healthy range 18.5–22) and higher for men (18.5–26), but the WHO standard categories remain widely used.
Standard BMI cutoffs were developed from primarily European populations. Research shows different thresholds for:
Asian populations: Health risks appear at lower BMI levels
This is why the American Diabetes Association now recommends diabetes screening for Asian Americans starting at BMI 23.
BMI is a useful screening tool but has well-documented limitations:
1. Doesn't distinguish muscle from fat A 200 lb athlete with 8% body fat and a sedentary person with 35% body fat can have the same BMI. Professional athletes are routinely "overweight" by BMI.
2. Doesn't measure fat distribution Visceral fat (around organs) is far more dangerous than subcutaneous fat (under skin). BMI doesn't differentiate. Waist circumference is a better predictor of metabolic risk than BMI for this reason.
3. Doesn't account for age-related changes Older adults naturally lose muscle and gain fat, meaning their BMI may look "normal" while their body composition is actually unhealthy.
4. Gender bias in the formula The formula was derived from Adolphe Quetelet's studies of primarily white European men in the 1830s. Modern researchers have documented its limitations across diverse populations.
5. Doesn't capture cardiovascular fitness A "normal" BMI person who is sedentary may be at higher risk than an "overweight" person who exercises regularly.
For a more complete picture of health:
Use our free BMI Calculator to get your BMI instantly, see your category, and understand what it means for your health goals. The calculator also shows your ideal weight range and how your BMI compares to population norms.