BMI Categories by Age for Adults (2026)

BMI (Body Mass Index) is calculated the same way for all adults — weight in kilograms divided by height in metres squared — but what the number means shifts slightly with age. Standard WHO cutoffs apply across the full adult range, yet research consistently shows that older adults carry more body fat at a given BMI than younger adults, and that the health risks associated with lower BMI increase after 65. Understanding these nuances helps you interpret your result more accurately.

Standard WHO BMI Categories for Adults

The four core categories apply to all adults aged 18 and over. They are defined by the World Health Organisation and used globally in clinical practice.

How Interpretation Changes with Age

For adults under 65, standard cutoffs track metabolic risk reasonably well. Above 65, the picture is more nuanced. Muscle mass declines with age (sarcopenia), which means an older adult can have a "normal" BMI while carrying a higher proportion of body fat than the number implies. Conversely, a BMI slightly above 25 in someone over 65 is not automatically a concern — some studies associate a modest overweight range with better outcomes in older populations, a pattern known as the "obesity paradox." Clinicians often use a working target of 22–27 for adults over 65 rather than the standard 18.5–24.9.

BMI as One Signal Among Several

BMI does not distinguish muscle from fat, nor does it capture where fat is distributed. Waist circumference — above 88 cm for women or 102 cm for men — is a stronger predictor of visceral fat and metabolic risk than BMI alone. Use BMI as an initial screen, not a diagnosis. If your result sits near a category boundary, or if you are over 65, discuss the result with a clinician who can factor in your full health picture. Use the BMI Category Reference to look up your category and see the full breakdown instantly.