The complete BMI guide
Body Mass Index is a screening number β not a diagnosis. It uses your height and weight to estimate whether you're carrying a healthy amount of mass for your frame. It can't see muscle versus fat, can't see where the fat sits, and can't see what your bones, blood pressure, or family history are doing. But it's free, takes ten seconds, and on populations of millions it correlates well enough with health outcomes that doctors still use it as a starting point. The trick is knowing what BMI does and doesn't tell you.
How BMI is calculated
The formula is simple: weight in kilograms divided by height in metres, squared. So a 70 kg person who is 1.75 metres tall has a BMI of 70 Γ· (1.75 Γ 1.75), which works out to 22.9. In imperial units, multiply pounds by 703 and divide by inches squared. The number itself doesn't have units that mean much β it's just a ratio designed to roughly normalize weight against height.
Why Asian populations need a different scale
One of the most important things about BMI that the standard CDC and NHS calculators don't surface: BMI cutoffs are not the same worldwide. The World Health Organization has formally recommended different thresholds for adults of Asian descent β including people of Indian, Chinese, Japanese, Korean, Vietnamese, Filipino, and other Southeast Asian backgrounds β because their bodies typically carry more fat and less muscle at the same BMI value as white European populations.
The numbers matter. At a BMI of 23β25, an Asian adult faces roughly the same metabolic risk that a white European faces at 25β30. Studies have shown that Asians with a BMI similar to whites are more than twice as likely to develop type 2 diabetes. Singapore officially revised its national cutoffs in 2005 for this reason; India, China, and Japan use similar adjusted thresholds in clinical settings.
| Category | WHO standard | Asian standard |
|---|
| Underweight | < 18.5 | < 18.5 |
| Healthy weight | 18.5 β 24.9 | 18.5 β 22.9 |
| Overweight | 25.0 β 29.9 | 23.0 β 27.4 |
| Obese | β₯ 30.0 | β₯ 27.5 |
If you're of Asian descent, switch the standard at the top of this page to "Asian" to see categories that better reflect your metabolic risk. If you're mixed-race, most metabolic specialists recommend using the stricter (Asian) cutoffs as a precaution.
Healthy BMI for older adults
BMI guidelines were originally calibrated on younger adults. For people over about 65, research now suggests a slightly higher BMI β between 25 and 27 β may be associated with better outcomes. This is partly because older adults who lose weight tend to lose muscle along with fat, which weakens them, and partly because a small reserve of mass helps recovery from illness. If you're older and your BMI is in the "overweight" range, that doesn't automatically mean you should try to lose weight. Talk to your doctor; muscle preservation usually matters more than chasing a number.
BMI for children and teens
Children and teenagers (ages 2β19) are not assessed against adult cutoffs. Their BMI is plotted against age- and sex-specific percentile charts because growing bodies have different normal ranges at every age. Switch this page's standard to "Child / Teen" mode to see your child's percentile interpretation:
- Below 5th percentile: underweight
- 5th β 85th percentile: healthy weight
- 85th β 95th percentile: overweight
- At or above 95th percentile: obese
The CDC publishes the full percentile growth charts for boys and girls. If you have concerns about your child's weight, those charts and a paediatrician are the right next step β not a single BMI reading.
Waist-to-hip ratio: a better predictor for some people
BMI tells you how much you weigh for your height. Waist-to-hip ratio (WHR) tells you where the weight sits β and where matters. Belly fat (an "apple" shape, more common in men) carries substantially higher health risk than fat around the hips and thighs (a "pear" shape, more common in women). For people over 75, WHR is actually a better predictor of mortality than BMI alone.
The thresholds:
- Men: WHR above 0.90 = elevated risk
- Women: WHR above 0.85 = elevated risk
Use the optional waist + hip fields above to add WHR to your result. You only need a tape measure.
Body fat percentage estimate
This calculator uses the Deurenberg formula to estimate body fat percentage from BMI, age, and sex: Body Fat % = 1.20 Γ BMI + 0.23 Γ age β 10.8 Γ sex β 5.4 (sex = 1 for male, 0 for female). It's a rough estimate β not as accurate as a DEXA scan or callipers β but useful for context. Healthy ranges: men 8β25%, women 21β33%. Athletes can be lower; people who appear fit but have lost muscle may be higher.
When BMI gets it wrong
BMI gives wrong answers for several groups:
- Muscular athletes. Muscle is denser than fat, so heavy lifters often score "overweight" on BMI without any excess fat. If you can see your abs, ignore BMI.
- Pregnant women. Don't use BMI during pregnancy. Talk to your obstetrician about appropriate weight gain.
- Older adults losing muscle. Sarcopenia (age-related muscle loss) can mean your weight stays the same while body composition gets worse, so your BMI looks fine but health risk is rising. Track waist circumference instead.
- Very tall or very short people. The squared-height term over- and under-estimates body fat at extreme heights. The Ponderal Index (which cubes height instead of squaring it) is more accurate at the extremes.
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