Last updated: September 2026
Body mass index (BMI) is one of the most widely used health screening measurements in the world. It's a simple calculation from your height and weight that places you in a broad category — and it's also one of the most misunderstood. This guide explains the formula, what the categories mean, what BMI can and can't tell you, and why doctors still use it despite its well-known limits.
Metric: BMI = weight in kilograms ÷ (height in meters)²
US units: BMI = (weight in pounds ÷ (height in inches)²) × 703
Worked example: A person who weighs 70 kg and is 1.75 m tall. Square the height: 1.75 × 1.75 = 3.0625. Divide weight by that: 70 ÷ 3.0625 ≈ 22.9. Their BMI is about 22.9.
In US units, the same person weighs about 154 lb and is about 69 inches tall: (154 ÷ (69 × 69)) × 703 = (154 ÷ 4761) × 703 ≈ 22.7. Small rounding differences between unit systems are normal.
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
These cutoffs were set by the World Health Organization and are used internationally for adults. Note that they apply to adults — children's BMI is interpreted differently, using age- and sex-specific percentiles, because kids are still growing.
BMI measures the relationship between your weight and your height. That's genuinely useful: across large populations, higher BMI correlates with higher rates of conditions like type 2 diabetes and heart disease, which is why insurers, researchers, and public-health agencies rely on it. It's cheap, fast, and consistent.
But BMI cannot see what your weight is made of, and that creates real blind spots:
This is why clinicians treat BMI as a screening tool — a first flag that says "worth a closer look" — not a diagnosis. A doctor pairs it with waist measurement, blood pressure, blood work, family history, and lifestyle before drawing any conclusions.
Despite its flaws, nothing else combines BMI's virtues: it takes ten seconds, costs nothing, requires no equipment beyond a scale and a tape measure, and produces a number that means the same thing in every clinic on earth. Better measurements exist — waist-to-height ratio, body-fat percentage via calipers or scans — but each adds cost, time, or equipment. So BMI remains the universal starting point, with its limits openly acknowledged.
Knowing your BMI is only useful if it leads somewhere sensible. If your BMI falls in the healthy range and you feel well, there's nothing to act on — just keep doing what you're doing. If it's outside that range, treat it as a prompt for a conversation, not a verdict: a clinician can check the things BMI can't see, like waist circumference, blood pressure, cholesterol, blood sugar, and your personal and family history. Many people also find it helpful to track trends rather than single readings — a BMI drifting steadily upward over a few years says more than any one measurement. And if the number causes you distress, say so to your clinician; health includes your relationship with your body, not just the arithmetic.
BMI is an informational screening tool, not medical advice. It cannot diagnose any condition, and a BMI outside the "healthy" range does not by itself mean you are unhealthy — just as a BMI inside it does not guarantee health. If you have questions about your weight or health, talk to a qualified clinician who can look at the full picture. Never start or stop a diet, exercise program, or medication based on a BMI number alone.