What is BMI and How to Calculate It — Formula, Chart & Limitations

Body Mass Index (BMI) is a simple number derived from your weight and height. It is widely used in clinics and public health as a quick screening tool for weight categories — not as a diagnosis on its own, but as a starting point for conversation about nutrition, activity, and overall health risks tied to body weight.

What BMI measures (and what it does not)

BMI estimates whether your weight is high or low relative to your height. It does not directly measure body fat, bone density, or where fat sits on your body. Two people with the same BMI can look very different: one might carry more muscle, the other more fat. That is why doctors pair BMI with other checks — waist circumference, blood pressure, blood tests, and your history — when assessing health.

The BMI formula: metric and imperial

In metric units (kilograms and metres), BMI is weight divided by height squared:

BMI = weight (kg) ÷ height (m)²

If you use pounds and inches, the standard conversion is:

BMI = (weight (lb) ÷ height (in)²) × 703

Always use consistent units: convert height to metres (or inches) and weight to kg (or lb) before plugging in. A BMI calculator avoids rounding mistakes when you switch between systems.

WHO adult categories: underweight to obese

For adults, the World Health Organization groups BMI into broad bands used in many countries (exact cutoffs can vary slightly by guideline):

• Underweight: BMI below 18.5
• Normal range: 18.5 to 24.9
• Overweight: 25 to 29.9
• Obese (Class I): 30 to 34.9
• Higher classes continue above 35 and 40 with increasing health risk discussions in clinical settings.

These thresholds are population-level guides. Your doctor may interpret them differently if you are pregnant, very muscular, or managing a chronic condition.

Reading a BMI chart by height and weight

A BMI chart (or table) lists heights along one axis and weights along the other; the cell where they meet gives an approximate BMI band. Charts are handy for quick lookups but inherit the same limits as the formula: they assume average body composition. If your measured weight puts you near a category boundary, small measurement errors (clothes, time of day, scale calibration) can shift the label — so treat the edge of a band as a signal to discuss trends over time, not a single fixed verdict.

Important limitations of BMI

• Muscle mass: Athletes and strength trainers often have high BMI with low body fat; the index cannot tell muscle from fat.
• Age: Older adults may lose muscle; BMI might sit in the "normal" range while fat percentage rises — another reason to use more than one metric.
• Ethnicity and body type: Some health organizations suggest different action thresholds for certain populations because disease risk can rise at lower BMI values; follow local clinical guidance.
• Fat distribution: Visceral fat around organs carries different risk than fat stored elsewhere; BMI does not capture that pattern.

When body fat percentage helps more than BMI

If you train regularly, are evaluating a fitness program, or want a clearer picture of composition, body fat percentage (from calipers, bioimpedance scales, DEXA, or professional assessment) often complements BMI. Body fat targets depend on sex, age, and sport; they are still estimates, but they speak more directly to adiposity than weight alone.

BMI for children and teens

For anyone under 18, BMI is not interpreted with the same adult cutoffs. Pediatric BMI is expressed as a percentile for age and sex on growth charts: the same number means something different at age 8 than at age 15. Parents should rely on paediatricians or growth-chart tools designed for youth rather than adult WHO bands. If you are tracking family health goals, pair professional advice with consistent habits — sleep, balanced meals, and activity — rather than fixating on a single index.