BMI (body mass index)
What BMI is, its limitations, and why body composition matters more than BMI alone.
A useful starting point, not the full story
BMI divides your weight in kilograms by your height in meters squared. A person of 178 cm and 80 kg has a BMI of 80 / (1.78 x 1.78) = 25.2. The World Health Organization sorts adults into four bands: underweight (below 18.5), normal (18.5 to 24.9), overweight (25 to 29.9) and obese (30 and above). Doctors, insurers and population studies use it because it takes ten seconds and needs nothing but a scale and a tape measure.
At the population level the number works. Across millions of people, mortality and the risk of type 2 diabetes, high blood pressure and cardiovascular disease rise as BMI climbs past 25, and the relationship is well documented (Prospective Studies Collaboration 2009). The trouble starts when a statistic built for populations is read as a verdict on one person.
Where BMI falls short
The problem? BMI can't tell muscle from fat. A muscular rock climber and a sedentary office worker can have the same BMI but wildly different health profiles. That's why body composition, meaning body fat percentage, muscle mass and visceral fat, tells a much richer story.
Three more blind spots are worth knowing:
- Where the fat sits. Fat around the organs is metabolically far more harmful than fat under the skin, and BMI has no idea which one you carry. Two people with a BMI of 27 can have a normal and a doubled cardiometabolic risk
- Age. Older adults lose muscle and gain fat at a stable weight, so an unchanged BMI at 65 can hide a body composition that has drifted a long way from what it was at 35
- Ancestry. Risk rises at lower BMI in people of South and East Asian descent, which is why the WHO suggests action thresholds of 23 and 27.5 for those groups instead of 25 and 30
What to read alongside it
If you want one extra number, take waist circumference. Measured at the navel after a normal breath out, values above 94 cm in men and 80 cm in women mark raised risk, and above 102 cm and 88 cm mark substantially raised risk in the WHO and NIH guidance. Waist divided by height is even simpler: keeping the ratio below 0.5 is the rule of thumb backed by a large meta-analysis (Ashwell 2012), and it applies regardless of sex and height.
If you own a smart scale, body fat percentage from bioimpedance adds the composition BMI cannot see. The absolute number carries a few percentage points of error; the trend over weeks is what to watch.
How to use the number sensibly
- Treat BMI as a screening flag, not a diagnosis. A value above 25 says "look closer", not "you are unhealthy"
- Combine it with waist circumference. High BMI with a normal waist is usually muscle; normal BMI with a large waist is the combination that gets missed most often
- Track direction, not the category boundary. Moving from 31 to 28 is a real change even though both are "overweight or above"
- Do not chase a BMI of 20 through muscle loss. Low lean mass is its own risk, especially after 50
How MyBodyAI handles it
BMI sits in your profile as a familiar reference point. The data that counts comes from a smart scale, such as the Withings Body models or the Garmin Index, that measure actual body composition. When you have real body fat and muscle mass data, your biological age calculation and health assessments become significantly more accurate, because the body composition domain is then built on measurement rather than on a height-and-weight proxy.
Frequently asked questions
What is a healthy BMI?
For adults the WHO calls 18.5 to 24.9 the normal range, 25 to 29.9 overweight and 30 or more obese. For people of South or East Asian descent the action thresholds are lower, 23 and 27.5. A value inside the normal range is not proof of good health and a value outside it is not a diagnosis; waist circumference and body composition settle what the number cannot.
Is BMI accurate for athletes?
Usually not. Muscle is denser than fat, so a lean, well-muscled person often lands at a BMI of 26 to 30 with a body fat percentage in the low teens. For anyone who trains with weights, body fat percentage and waist circumference are the numbers to trust; BMI will over-read.
Does MyBodyAI use BMI in its scores?
Only as a fallback. When a connected smart scale reports body fat and muscle mass, the body composition domain of Biological Age is built on those measurements against age- and sex-matched norms. Without them BMI stands in as a coarse proxy that moves the domain between three years younger and eight years older than the reference, and once scale data arrives it takes over.