BMI's Formula and Its Limits: Why Muscle Mass and Age Matter
The BMI formula, WHO's weight categories, and why BMI can't distinguish muscle from fat or account for age. General guidance, not medical advice.
What is BMI and how is it calculated?
Body Mass Index (BMI) divides weight in kilograms by height in meters, squared. For 170 cm and 70 kg, that's 70 / 1.70^2 = 70 / 2.89, or roughly 24.2. Height has to be in meters for this to work -- squaring a centimeter figure directly would throw the result off by orders of magnitude.
What are the WHO weight categories?
The World Health Organization's international classification treats a BMI under 18.5 as underweight, 18.5-24.9 as normal weight, 25.0-29.9 as overweight (pre-obese), and 30 or above as obese. Japan's own reference, set by the Japan Society for the Study of Obesity, instead marks 25 and above as "obese" domestically and cites roughly 22 as the value with the lowest statistical disease risk -- a different cutoff from the WHO scale.
Limitation 1: BMI can't see muscle mass
Because BMI only uses total weight and height, it can't tell whether that weight comes from muscle or fat. Athletes and people who strength-train regularly can register as "overweight" or "obese" on the BMI scale despite having low body fat. The reverse also happens: someone with an average-looking BMI can carry a high body-fat percentage and low muscle mass, a pattern BMI alone can't detect.
Limitation 2: BMI ignores age and sex
Muscle mass tends to decline and body fat tends to rise with age, even if weight -- and therefore BMI -- stays the same. The formula itself makes no distinction for age or sex, so the same BMI number can represent quite different body compositions in different people. For growing children and teenagers, a separate BMI-for-age approach uses age- and sex-specific percentiles instead of fixed adult cutoffs.
Why BMI is paired with other measurements
Because of these gaps, BMI is commonly used alongside other measures rather than on its own: waist circumference, which reflects abdominal fat more directly, waist-to-height ratio (WHtR), and body-fat percentage from a body composition scale. In practice, BMI works well as a quick first-pass reference, with other measurements filling in the detail it misses.
A note on medical guidance
BMI is a population-level statistical reference, not a diagnosis for any individual. The formula and categories here are general guidance, not medical advice. Anyone with concerns about their weight or health should talk to a doctor or registered dietitian rather than relying on a BMI number alone.
Frequently asked questions
What is the formula for BMI?
Weight in kg divided by height in meters, squared. For 170 cm and 70 kg: 70 / 1.70^2, roughly 24.2.
Can someone with a high BMI still be healthy?
Yes. People with substantial muscle mass, like athletes, can register as overweight or obese on the BMI scale despite low body fat, since BMI can't distinguish muscle from fat.
Can BMI alone determine someone's health status?
No. BMI is general guidance, not medical advice. It's best read alongside other measures like waist circumference or body-fat percentage, or discussed with a doctor.