Body Mass Index (BMI) is the quickest way to get a rough picture of whether your weight is in a healthy range for your height. It takes two numbers — weight and height — and produces one figure that health organisations use as a first screening tool. But it is often misunderstood, so let us look at what it does and does not tell you.

What BMI is and how it is calculated

The BMI formula is simple:

BMI = weight (kg) ÷ height (m)²

For example, someone who weighs 70 kg and is 1.75 m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9. Skip the manual maths with our free BMI Calculator.

The standard BMI ranges

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

These thresholds come from the World Health Organization and are meant for population screening. Some countries, particularly in Asia, use slightly lower cut-offs because health risks appear at a lower BMI.

What BMI does well

  • It is instant and free — no equipment beyond a scale and a tape measure.
  • It gives a consistent baseline you can track over months.
  • It is useful for spotting broad trends across large groups.

What BMI does not tell you

This is the important part. BMI cannot distinguish muscle from fat. A muscular athlete can register as "overweight" while being extremely fit, and someone with low muscle mass can sit in the "healthy" range while carrying too much fat. BMI also ignores:

  • Where body fat is stored (belly fat is riskier than hip fat)
  • Age and sex differences in body composition
  • Bone density and natural body frame
  • Pregnancy

That is why BMI should be one input, not the whole verdict. Pair it with a waist measurement, and talk to a healthcare professional for anything serious.

Frequently asked questions

Is BMI accurate for athletes?

No. Because muscle weighs more than fat, athletes often score higher than their true body-fat level would suggest.

What is a healthy waist measurement?

As a common guideline, men are advised to keep waist circumference below about 94 cm and women below about 80 cm, though guidance varies.

Should children use the same BMI ranges?

No. For children and teens, BMI is compared against age- and sex-specific growth percentiles, not the adult table.

Final thoughts

Use BMI as a fast, free directional signal — then look at the bigger picture. Calculate yours in seconds with the BMI Calculator, and explore our other free calculators for body fat, calories and more.