How to Calculate BMI: Formula, Chart, and What It Means

Body Mass Index (BMI) is a simple screening tool that uses height and weight to estimate whether a person is in a healthy weight range. Here is how to calculate it and what the numbers mean.

BMI Formula

Metric: BMI = weight (kg) ÷ height (m)²

Imperial: BMI = (weight (lbs) × 703) ÷ height (inches)²

Example Calculation (Metric)

Person: 75 kg, 1.75 m tall
BMI = 75 ÷ (1.75)² = 75 ÷ 3.0625 = 24.5 — Normal weight

Example Calculation (Imperial)

Person: 165 lbs, 5 ft 9 in (69 inches) tall
BMI = (165 × 703) ÷ (69)² = 116,000 ÷ 4,761 = 24.3 — Normal weight

BMI Categories (WHO)

BMI Range	Category	Health Risk
Below 18.5	Underweight	Moderate
18.5 – 24.9	Normal weight	Low
25.0 – 29.9	Overweight	Increased
30.0 – 34.9	Obese Class I	High
35.0 – 39.9	Obese Class II	Very High
40 and above	Obese Class III	Extremely High

BMI for Children and Teens

For people under 20, BMI is calculated the same way but interpreted differently using age- and sex-specific percentile charts. Overweight is defined as BMI at or above the 85th percentile; obese is above the 95th percentile for age and sex.

Limitations of BMI

BMI is a useful population-level screening tool, but it has significant limitations for individuals:

• Muscle vs fat: BMI cannot distinguish muscle mass from fat. A muscular athlete may be classified as "overweight" or even "obese" with very low body fat.
• Fat distribution: BMI does not capture where fat is stored. Abdominal fat (visceral fat) is more dangerous than fat stored in hips and thighs. Waist circumference adds important context.
• Ethnic variation: Asian populations show higher metabolic risk at lower BMI thresholds. Many health organizations recommend lower cut-offs for Asian adults: 23 for overweight, 27.5 for obese.
• Age and sex: Older adults and women typically have higher body fat percentages at the same BMI compared to younger adults and men.

For a more complete picture, combine BMI with waist circumference, body fat percentage (measured by Navy formula, DEXA, or bioimpedance), and blood metabolic markers (fasting glucose, triglycerides, HDL cholesterol).