What is BMI and How Is It Calculated?
Body Mass Index (BMI) is a numerical measure derived from a person's weight and height. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet as a simple way to classify body weight relative to height at a population level. It's widely used by healthcare providers as an initial screening tool, though it has important limitations.
BMI = weight (kg) / height² (m²) | or | BMI = 703 × weight (lbs) / height² (inches²)
BMI Categories (WHO Classification)
Below 18.5 — Underweight: May indicate nutritional deficiency, malabsorption, or other health concerns. Consult a healthcare provider.
18.5–24.9 — Normal weight: Associated with lowest risk of weight-related health conditions for most adults.
25.0–29.9 — Overweight: Moderate increased risk of cardiovascular disease, type 2 diabetes, and hypertension.
30.0–34.9 — Obese Class I: High risk of obesity-related health conditions. Lifestyle changes typically recommended.
35.0–39.9 — Obese Class II: Very high risk. Medical intervention often appropriate.
40.0+ — Obese Class III (Severe): Extremely high risk. May qualify for bariatric surgery.
BMI Limitations
Doesn't account for body composition: BMI can't distinguish between muscle mass and fat. Athletes and bodybuilders often have high BMI despite low body fat percentages. Conversely, someone with normal BMI can have high body fat ("skinny fat").
Age and sex differences: Body fat percentage changes with age. Women naturally have higher body fat than men at the same BMI. The same BMI cutoffs apply to all adults, which can be misleading.
Ethnic variation: Research suggests that Asian populations have higher health risks at lower BMI values. Some countries use adjusted cutoffs (e.g., overweight >23 for Asian adults).
Not for children: Children use BMI-for-age percentiles (comparing to peers), not adult BMI categories. A child at the 85th–95th percentile is considered overweight; above 95th is obese.
Better alternatives: Waist circumference, waist-to-hip ratio, DEXA scans (body fat %), and waist-to-height ratio are often more accurate predictors of metabolic health than BMI alone.
Healthy Weight Management
- Sustainable deficit: A calorie deficit of 500–750 calories/day typically results in 1–1.5 lbs/week weight loss — considered a safe, sustainable rate
- Protein priority: Higher protein intake (1.6–2.2g per kg body weight) preserves muscle mass during weight loss
- Strength training: Preserves muscle mass and increases metabolic rate, improving body composition even without large weight changes
- Avoid extreme restriction: Very low calorie diets (<1,200 cal/day) are counterproductive — they reduce metabolic rate and cause muscle loss
- Consult professionals: BMI is a screening tool, not a diagnosis. Work with a physician and dietitian for personalized guidance
Frequently Asked Questions
Is BMI accurate for everyone?
No. BMI is a population-level statistical tool that provides a rough estimate. It's most useful as a quick screening metric, not a precise health assessment. Athletes, older adults, pregnant women, and people with different body compositions may get misleading results. Always combine BMI with other health indicators and consult a healthcare provider for a full assessment.
What's a healthy BMI for older adults?
Research suggests that slightly higher BMI (25–27) may be protective for adults over 65. As people age, some additional weight may help maintain muscle and bone density. Underweight in older adults carries higher mortality risk than being slightly overweight. Consult a geriatric specialist for personalized guidance.
How often should I calculate my BMI?
Monthly tracking is usually sufficient for most people. Daily weigh-ins aren't needed and can be discouraging due to normal daily weight fluctuations (1–3 lbs from water, food, and time of day). Focus on trend over time rather than day-to-day numbers.