How to Calculate BMI: Formula, Ranges, and Limitations

What Is BMI?

Body Mass Index (BMI) is a numerical value calculated from a person's height and weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and became widely adopted by health organizations in the 1970s as a simple screening tool for weight-related health risks.

BMI is not a direct measure of body fat — it's a ratio that correlates with body fat measurements on a population level, which makes it useful for public health statistics but limited for individual assessment.

The BMI Formula

Metric (SI) formula:

BMI = weight (kg) ÷ height² (m²)

Example: A person who is 1.75 m tall and weighs 70 kg: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9

Imperial formula:

BMI = 703 × weight (lbs) ÷ height² (inches²)

Example: 155 lbs, 5'9" (69 inches): BMI = 703 × 155 ÷ (69 × 69) = 108,965 ÷ 4,761 = 22.9

WHO BMI Ranges for Adults

BMI Range	Classification
Below 18.5	Underweight
18.5 – 24.9	Normal weight
25.0 – 29.9	Overweight
30.0 – 34.9	Obesity Class I
35.0 – 39.9	Obesity Class II
40.0 and above	Obesity Class III (Severe)

These ranges apply to adults aged 20 and older. Children use age- and sex-specific BMI percentile charts instead.

BMI for Children and Teens

For people aged 2–19, BMI is calculated the same way but interpreted differently using percentiles compared to others of the same age and sex:

• Below 5th percentile: Underweight
• 5th to 85th percentile: Healthy weight
• 85th to 95th percentile: Overweight
• 95th percentile and above: Obese

What BMI Tells You — and What It Doesn't

BMI is useful because it's simple, free, and a reasonable population-level indicator. Studies consistently show that high BMI correlates with increased risk of type 2 diabetes, cardiovascular disease, hypertension, and certain cancers.

However, BMI has significant limitations for individual assessment:

It doesn't account for muscle mass. Athletes and bodybuilders often have BMIs in the "overweight" range despite having very low body fat percentages. A 5'10" NFL linebacker at 220 lbs has a BMI of 31.6 (Obese Class I) — which is clearly misleading.

It doesn't distinguish fat distribution. Visceral fat (belly fat around organs) is far more dangerous than subcutaneous fat (under the skin). Two people with identical BMIs can have very different health profiles depending on where they carry weight.

It doesn't account for age. Older adults naturally lose muscle mass and gain fat, so a "healthy" BMI in an elderly person may still indicate elevated metabolic risk.

Ethnicity matters. Asian populations tend to have higher disease risk at lower BMI values. Many health organizations recommend lower BMI cutoffs for Asian adults:

• Overweight threshold: 23.0 (vs 25.0)
• Obesity threshold: 27.5 (vs 30.0)

Sex differences. Women naturally have a higher body fat percentage than men at the same BMI. A BMI of 25 means something different metabolically for a 25-year-old woman vs a 25-year-old man.

Better Alternatives to BMI

• Waist circumference — A waist measurement above 35 inches (88 cm) for women or 40 inches (102 cm) for men indicates elevated cardiovascular risk regardless of BMI
• Waist-to-hip ratio (WHR) — Divides waist circumference by hip circumference; above 0.85 (women) or 0.90 (men) indicates abdominal obesity
• Body fat percentage — Measured by DEXA scan, hydrostatic weighing, or bioelectrical impedance; the most direct measure
• Waist-to-height ratio — Keep your waist circumference below half your height; research suggests this is a better predictor of metabolic risk than BMI

Bottom Line

BMI is a useful first-pass screening tool, not a diagnosis. If your BMI falls outside the healthy range, it's a signal to discuss further evaluation with a healthcare provider — not a definitive statement about your health.

Use our free BMI Calculator to instantly calculate your BMI and see how it falls on the WHO ranges.