How do I calculate my BMI?

What Is BMI?

Body Mass Index (BMI) is a screening tool that estimates body fat based on your height and weight. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it was originally designed as a population-level statistical measure — not a diagnostic tool for individuals. Despite its limitations, BMI remains the most widely used method for classifying weight status because it is simple, free, and requires no special equipment.

BMI is calculated by dividing your weight by the square of your height. In the imperial system used in the United States, the formula is: BMI = (weight in pounds × 703) / (height in inches)². The factor of 703 converts from imperial to metric units. In the metric system, the formula is even simpler: BMI = weight in kg / (height in m)².

BMI Categories and What They Mean

The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) define four primary BMI categories for adults:

• Underweight: BMI below 18.5 — May indicate malnutrition, eating disorders, or other health conditions. Associated with increased risk of osteoporosis, weakened immune function, and fertility issues.
• Normal weight: BMI 18.5–24.9 — Generally associated with the lowest health risks. This is the range most health professionals recommend maintaining.
• Overweight: BMI 25.0–29.9 — Indicates excess weight that may increase the risk of heart disease, type 2 diabetes, and high blood pressure. Lifestyle changes such as diet and exercise are typically recommended.
• Obese: BMI 30.0 and above — Subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (40+, sometimes called “severe” or “morbid” obesity). Associated with significantly elevated health risks including heart disease, stroke, certain cancers, and reduced life expectancy.

Limitations of BMI — When It Gets It Wrong

BMI is a useful starting point, but it has significant limitations that everyone should understand:

• Muscle vs. fat: BMI cannot distinguish between muscle mass and fat mass. A muscular athlete with low body fat may be classified as “overweight” or even “obese” by BMI alone. For example, many NFL running backs and professional bodybuilders have BMIs above 30 despite being in excellent physical condition.
• Age: Older adults tend to have more body fat and less muscle mass than younger adults at the same BMI. A BMI of 25 may represent very different body compositions in a 25-year-old versus a 70-year-old.
• Sex: Women naturally carry more body fat than men. Two people with the same BMI can have different body fat percentages depending on sex.
• Ethnicity: Research shows that health risks associated with BMI vary by ethnic group. Asian populations may face elevated health risks at lower BMIs (23–24), while some Pacific Islander and Black populations may have lower health risks at higher BMIs.
• Fat distribution: BMI tells you nothing about where fat is stored. Visceral fat (around the organs, measured by waist circumference) is far more dangerous than subcutaneous fat (under the skin). A person with a normal BMI but a large waist circumference may be at higher risk than their BMI suggests.

Healthy Weight Ranges by Height

Based on the “normal” BMI range of 18.5–24.9, here are approximate healthy weight ranges for common heights:

• 5′2″ (62 in): 104–136 lbs
• 5′4″ (64 in): 110–145 lbs
• 5′6″ (66 in): 118–155 lbs
• 5′8″ (68 in): 125–164 lbs
• 5′10″ (70 in): 132–174 lbs
• 6′0″ (72 in): 140–184 lbs
• 6′2″ (74 in): 148–194 lbs

These ranges are guidelines, not rigid cutoffs. Your ideal weight depends on many factors including muscle mass, bone density, age, and overall health.

Better Alternatives and Complementary Measures

If you want a more complete picture of your body composition and health risk, consider these measures in addition to BMI:

• Waist circumference: A waist measurement over 40 inches for men or 35 inches for women indicates higher health risk from visceral fat, regardless of BMI.
• Waist-to-hip ratio: Dividing waist circumference by hip circumference provides insight into fat distribution. Ratios above 0.90 for men or 0.85 for women suggest elevated risk.
• Body fat percentage: Measured via DEXA scan, bioelectrical impedance, or skinfold calipers, this tells you exactly how much of your weight is fat versus lean mass. Healthy ranges are typically 10–20% for men and 18–28% for women.
• Blood work: Cholesterol, blood sugar, blood pressure, and inflammatory markers give a direct picture of metabolic health that BMI can only estimate.

What to Do About Your BMI Result

If your BMI falls outside the normal range, here is practical guidance:

• BMI under 18.5: Speak with a healthcare provider. Being underweight can be a sign of nutritional deficiencies or underlying health conditions. Focus on nutrient-dense foods and consider working with a registered dietitian.
• BMI 25–29.9: Small changes can make a big difference. A modest weight loss of 5–10% of body weight has been shown to significantly improve blood pressure, cholesterol, and blood sugar. Focus on increasing physical activity and reducing calorie-dense, nutrient-poor foods.
• BMI 30+: Consult your doctor. At this level, the evidence for increased health risk is strong. Your provider can help you develop a comprehensive plan that may include dietary changes, exercise, behavioral counseling, medication, or in some cases surgical options.

Remember: BMI is a screening tool, not a diagnosis. A high or low BMI is a reason to investigate further, not a definitive statement about your health.