BMI Explained: What Your Number Means and Where It Falls Short

Body Mass Index is the number doctors, insurers and fitness apps reach for first. It's quick and useful, but it has real blind spots. Here's how to read your BMI properly.

How BMI is calculated

BMI = weight in kg ÷ (height in metres)²

Example: 70 kg and 1.70 m tall: 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 = 24.2.

In pounds and inches, multiply by 703: BMI = weight (lb) ÷ height (in)² × 703.

The standard ranges

The World Health Organization's adult categories are:

BMI	Category
Below 18.5	Underweight
18.5–24.9	Healthy weight
25.0–29.9	Overweight
30.0 and above	Obese

Lower cut-offs for South Asians

People of South Asian descent tend to develop type 2 diabetes and heart disease at lower BMIs than people of European descent. For this reason, health bodies including the WHO expert consultation and the UK's NICE suggest acting at lower thresholds for Asian populations: increased risk from about 23 and high risk from about 27.5. If you're from Pakistan, India, Bangladesh or Sri Lanka, keep this in mind when reading your result.

Where BMI falls short

• Muscle vs fat. BMI only sees weight. A muscular athlete can score as "overweight" with very little body fat.
• Where fat is stored. Fat around the waist is more strongly linked to health problems than fat on the hips, and BMI can't tell the difference.
• Age and sex. Older adults lose muscle, so the same BMI can mean more body fat. Women typically carry more body fat than men at the same BMI.
• Children and teenagers. BMI for under-18s must be compared with age-and-sex growth charts, not the adult table.

Better together: waist and body fat

Use BMI alongside one or two other measures:

• Waist-to-hip ratio. Measure your waist at the narrowest point and your hips at the widest. The WHO considers a ratio above 0.90 for men and 0.85 for women a sign of increased risk. Try the waist-to-hip ratio calculator.
• Waist-to-height ratio. A simple rule of thumb: keep your waist under half your height.
• Body fat percentage. The body fat calculator estimates it from tape-measure readings using the US Navy method.

What to do with your number

A BMI outside the healthy range is a reason to look closer, not a diagnosis. If you're concerned, talk to a doctor, who can check blood pressure, blood sugar and cholesterol, the things that actually matter for your health.

Check yours

The BMI calculator works in metric or imperial units, shows your category, and tells you the healthy weight range for your height.