BMI Explained Simply — What It Does and Doesn't Tell You

Body Mass Index is the most widely used weight-health metric — and the most misunderstood.

The formula

BMI = weight (kg) ÷ height (m)²

For a 70 kg, 175 cm person: 70 ÷ 1.75² = 22.9.

The categories (WHO, adults)

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Normal weight
25 – 29.9	Overweight
30 and above	Obese

A BMI of 22.9 lands comfortably in “normal”. For a 175 cm person, that range corresponds to roughly 56.7–76.3 kg.

What BMI is good at

It’s a fast, cheap, widely comparable screening tool. Population studies show strong links between high BMI and conditions like type-2 diabetes, heart disease and joint problems — which is why insurers and health systems rely on it.

What BMI misses

• Muscle vs fat — BMI can’t tell them apart. A muscular athlete can be “overweight” with very low body fat.
• Where the fat is — belly fat is riskier than fat elsewhere, but BMI ignores distribution. Waist circumference is a better predictor of metabolic risk.
• Age and ethnicity — muscle mass declines with age, and healthy BMI ranges vary by ethnicity (many Asian populations have elevated risk at lower BMIs).

How to use it sensibly

Treat BMI as one data point, not a verdict. Combine it with waist circumference and, ideally, a body-fat measurement. For children and teens, use age- and sex-specific growth charts — the adult categories don’t apply.

Calculate your BMI and healthy weight range with the BMI Calculator, and pair it with the Calorie Calculator if you’re planning a change.