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

What Is BMI?

Body Mass Index (BMI) is a number calculated from your height and weight that is used as a screening tool to categorize whether a person is underweight, normal weight, overweight, or obese. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and remains widely used today despite its limitations.

The BMI Formula

Metric: BMI = weight (kg) ÷ height² (m²)
Imperial: BMI = (weight (lbs) ÷ height² (in²)) × 703

Metric Example

Weight: 70 kg | Height: 1.75 m
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9

Imperial Example

Weight: 154 lbs | Height: 69 inches (5'9")
BMI = (154 ÷ 69²) × 703 = (154 ÷ 4761) × 703 = 0.03235 × 703 = 22.7

BMI Categories (Adults)

BMI Range	Category
Below 18.5	Underweight
18.5 – 24.9	Normal weight
25.0 – 29.9	Overweight
30.0 – 34.9	Obese (Class I)
35.0 – 39.9	Obese (Class II)
40.0 and above	Severely obese (Class III)

BMI for Children and Teens

For people under 20, BMI is calculated the same way but interpreted differently using age- and sex-specific growth charts. Rather than fixed cutoff numbers, a child's BMI is compared to percentiles for their age group.

Limitations of BMI

BMI has significant limitations that health professionals acknowledge:

• It doesn't measure body fat directly. A muscular athlete may have a high BMI but low body fat. A sedentary person may have a normal BMI but unhealthy body composition.
• It doesn't account for fat distribution. Visceral fat (around the organs) is more dangerous than subcutaneous fat, but BMI doesn't distinguish between them.
• It varies by ethnicity. Asian populations are at higher health risk at lower BMI values; some health organizations use 23 as the overweight threshold for Asian adults.
• Age and sex affect accuracy. Older adults tend to have more body fat at the same BMI; women generally have more body fat than men at the same BMI.

Better Indicators of Health

For a more complete picture, consider also measuring waist circumference (over 35 inches for women, 40 inches for men indicates increased risk), waist-to-hip ratio, body fat percentage via DEXA scan or calipers, and blood pressure, blood sugar, and cholesterol levels.