What BMI Can and Can’t Tell You: Limits, Ethnic Cut-offs and Waist-to-Height Ratio
Short answer
BMI (weight in kg ÷ height in metres squared) is a fast way to screen adults for weight-related health risk, but it cannot tell fat from muscle or show where fat is stored. Use it alongside your waist measurement: a waist less than half your height is a simple healthy target recommended by the UK’s NICE. Many health bodies use lower BMI cut-offs for people of South Asian, Chinese and other Asian backgrounds.
Body mass index is the most widely used weight measure in the world, from doctors’ surgeries to insurance forms. It is useful because it needs only height and weight, and it works well for comparing large groups of people. For one individual, though, BMI is a starting point, not a verdict. This guide explains what our BMI calculator result really means, where BMI falls short, and simple extra checks you can do at home.
The standard adult BMI categories
The World Health Organization (WHO) uses these categories for adults aged 18 and over:
| BMI (kg/m²) | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Healthy weight |
| 25.0–29.9 | Overweight |
| 30.0 and above | Obesity |
For example, a person 1.70 m tall weighing 72 kg has a BMI of 72 ÷ (1.70 × 1.70) = 24.9, right at the top of the healthy range. Our guide on how to calculate BMI shows the imperial formula too.
Five things BMI cannot tell you
- Muscle versus fat. Muscle is denser than fat. Athletes, rugby players and people who lift weights can have a BMI in the “overweight” range while carrying little body fat.
- Where fat is stored. Fat around the waist and organs (visceral fat) is more strongly linked to type 2 diabetes and heart disease than fat on the hips and thighs. Two people with the same BMI can have very different risk.
- Age changes. Older adults often lose muscle and gain fat while their weight stays the same, so a “normal” BMI can hide rising body fat.
- Ethnic differences. At the same BMI, people of South Asian, Chinese and some other backgrounds tend to have more body fat and a higher risk of type 2 diabetes than white Europeans (see below).
- Overall health. Blood pressure, blood sugar, cholesterol, fitness, sleep, smoking and diet all matter, and BMI measures none of them.
Lower cut-offs for Asian populations
A WHO expert consultation found that many Asian populations face higher health risks at lower BMIs, and suggested extra public-health “action points” at 23 (increased risk) and 27.5 (high risk). Following this, the UK’s National Institute for Health and Care Excellence (NICE) advises using lower thresholds for people with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background: overweight from a BMI of 23 and obesity from 27.5. India’s national guidelines similarly use 23 and 25 as cut-offs for overweight and obesity in adults. If this applies to you, read your BMI result with these lower lines in mind.
The waist-to-height ratio: a simple extra check
Because BMI ignores fat distribution, NICE recommends that adults also measure their waist. The rule is easy to remember: keep your waist to less than half your height. That is a waist-to-height ratio below 0.5.
- Find the point halfway between your lowest rib and the top of your hip bone, usually around the belly button.
- Breathe out normally and wrap a tape measure around your bare waist, keeping it level and snug but not tight.
- Divide your waist by your height, using the same units.
| Waist-to-height ratio | What it suggests (NICE) |
|---|---|
| 0.4 to 0.49 | No increased health risk |
| 0.5 to 0.59 | Increased health risk |
| 0.6 or more | High health risk |
Example: someone 170 cm tall with a 90 cm waist has a ratio of 90 ÷ 170 = 0.53, an increased risk even if their BMI is “healthy”. Our body fat calculator gives another estimate using simple body measurements.
BMI for children and teenagers
For anyone under 18, adult categories do not apply. Children’s body fat changes as they grow and differs between boys and girls, so BMI is compared with growth charts for the same age and sex and given as a percentile. In the US system used by the Centers for Disease Control and Prevention, a BMI at or above the 85th percentile is classed as overweight and at or above the 95th percentile as obesity. Always discuss a child’s weight with a doctor or health visitor rather than relying on an online number.
How to use your BMI result sensibly
- Treat it as a screening flag: a reason to look more closely, not a diagnosis.
- Check your waist-to-height ratio as well.
- Watch the trend over months rather than worrying about a single reading.
- If you want to change weight, small sustainable changes in eating, sleep and activity matter more than the exact number. Our guide on how many calories to lose weight explains safe rates.
- See a health professional if your BMI is below 18.5 or above 30, if your weight changes quickly without explanation, or if you have symptoms such as tiredness, thirst or breathlessness.
Curious about a healthy weight range for your height? Try how to calculate ideal body weight.
Not medical advice. This article is general health information. Speak to a doctor or registered health professional about your own weight and health.
Frequently asked questions
Is BMI accurate?
It is a useful screening tool for groups and a quick check for adults, but it cannot measure body fat directly or show where fat is stored.
What is a healthy waist-to-height ratio?
Below 0.5, meaning your waist is less than half your height, according to NICE guidance.
Why are BMI cut-offs lower for Asian people?
Research shows many Asian populations have higher body fat and diabetes risk at the same BMI, so lower thresholds of 23 and 27.5 are used.
Can I use adult BMI for my child?
No. Children’s BMI is compared with age- and sex-specific growth charts and given as a percentile.
