BMI is weight in kilograms divided by height in metres squared: 70 kg at 1.75 m gives 70 ÷ 3.06 = 22.9. The WHO adult ranges are under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight and 30+ obesity. For South and East Asian people, health risks start at lower values, so cut-offs of about 23 (overweight) and 27.5 (obesity) are often used. BMI is a screening number: it doesn’t measure body fat, and muscle, age and where fat sits all change what it means.
Body mass index takes two numbers anyone can measure, height and weight, and turns them into one. That simplicity is exactly why doctors, insurers and public-health researchers use it, and why it’s also easy to misread.
This guide is general information, not medical advice. If your result worries you, talk to a doctor who can look at the full picture.
How BMI is calculated
BMI = weight (kg) ÷ height (m)²
Example: 70 kg and 1.75 m. Height squared is 1.75 × 1.75 = 3.0625. 70 ÷ 3.0625 = 22.9.
In pounds and inches: BMI = 703 × weight (lb) ÷ height (in)². 154 lb at 5 ft 9 in (69 in): 703 × 154 ÷ 4,761 ≈ 22.7.
The WHO categories for adults
| BMI | WHO category | Often-used Asian cut-offs |
|---|---|---|
| Below 18.5 | Underweight | Below 18.5 |
| 18.5 – 24.9 | Healthy weight | 18.5 – 22.9 |
| 25 – 29.9 | Overweight | 23 – 27.4 (increased risk) |
| 30 and above | Obesity | 27.5 and above (high risk) |
Why lower cut-offs for South Asians
Research has found that people of South Asian, Chinese and other Asian backgrounds tend to develop type 2 diabetes and heart disease at lower BMIs than people of European origin, partly because of a tendency to carry more fat around the abdomen. A WHO expert consultation in 2004 suggested extra action points at 23 and 27.5 for Asian populations, and guidance in countries such as India, Pakistan and Singapore, as well as the UK’s NICE for people of these backgrounds, uses lower thresholds. So a BMI of 24 may count as “healthy” on the standard chart but already suggest increased risk for someone of South Asian heritage.
Where BMI misleads
- Muscle: BMI can’t tell muscle from fat. A fit rugby player or bodybuilder can be “overweight” or even “obese” on BMI with little body fat.
- Older adults: people lose muscle with age, so a normal BMI can hide too much fat and too little muscle.
- Where fat sits: fat around the waist is more strongly linked to health risk than fat on the hips and thighs. BMI ignores it.
- Children and teenagers: adult categories don’t apply. Children are assessed with BMI-for-age percentile charts.
- Pregnancy: BMI isn’t meaningful during pregnancy; pre-pregnancy BMI is what doctors use.
Add your waist measurement
A tape measure adds a lot. Two simple checks:
- Waist-to-height ratio: UK NICE guidance suggests keeping your waist under half your height. 175 cm tall? Aim for under 87.5 cm.
- Waist size: commonly used risk thresholds for South Asian adults are about 90 cm for men and 80 cm for women.
Measure around the middle, halfway between the lowest rib and the top of the hip bone, after breathing out normally.
How to use your BMI sensibly
- Treat it as a starting point.
A screening number that tells you whether it’s worth looking further, not a diagnosis.
- Read it against the right chart.
Use the Asian cut-offs if they apply to you.
- Add your waist.
It tells you more about risk than BMI alone.
- Watch the trend.
A steady rise over a few years matters more than one reading.
- Talk to a doctor
if your BMI or waist is high, if you’ve lost weight without trying, or if you’re underweight.
Frequently asked questions
How is BMI calculated?
What is a healthy BMI?
Is a BMI of 24 healthy for an Asian person?
Is BMI accurate for muscular people?
Can I use the adult BMI chart for my child?
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