What Is BMI? What It Measures, Its Limits, and How to Read Your Number

The formula

BMI is your weight in kilograms divided by your height in meters squared: BMI = kg ÷ m². For a person who is 70 kg and 1.75 m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9.

In imperial units the formula is weight in pounds × 703, divided by height in inches squared. The calculator handles both, so you never need to convert by hand.

The standard categories

The World Health Organization categories for adults are:

• Below 18.5 — underweight
• 18.5 to 24.9 — healthy weight
• 25.0 to 29.9 — overweight
• 30.0 and above — obese

What BMI gets wrong

BMI doesn't measure body fat — it measures weight relative to height, nothing more. That creates well-known blind spots.

A muscular athlete can have a BMI in the "overweight" range with very low body fat, because muscle is denser than fat. Older adults can have a "healthy" BMI while having lost muscle and gained fat. BMI also doesn't account for where fat is stored, and fat around the abdomen carries more health risk than fat elsewhere.

How to actually use it

Think of BMI as a first-pass screening tool, not a diagnosis. It's useful for spotting trends in large populations and as a starting point for a conversation with a doctor — not as a verdict on your individual health.

For a fuller picture, pair it with waist circumference, body composition measurements, blood pressure, and blood work. And remember: BMI categories apply to adults; children and teens use age- and sex-specific percentiles instead.