What's a healthy body fat percentage for my age and sex?

There is no single healthy body fat percentage. The ranges in common use are built from population data, and Gallagher and colleagues showed early on that a reasonable percentage depends on age, sex and ethnicity: women carry several percentage points more fat than men for physiological reasons, and the share tends to rise with age. In their sample of 1,626 adults no universal number emerged; instead they tied body fat to the already-established BMI thresholds. Different expert bodies therefore publish somewhat different 'healthy' bands rather than one shared cutoff.

That the percentage captures something the scale misses is nonetheless well established. Okorodudu and colleagues pooled 25 studies of nearly 32,000 people and found that BMI misses roughly half of everyone with elevated body fat: high specificity but low sensitivity. Bodies differ at the same weight.

At the same time, lower is not always better. In a cohort of 49,476 women and 4,944 men followed by Padwal and colleagues, both low BMI and high body fat percentage were independently linked to higher mortality, a pattern that may help explain the otherwise counterintuitive relationship between BMI and mortality. One possible interpretation is that a low BMI at a given fat level reflects low muscle mass, but the study did not measure muscle mass itself. Both extremes carry risk, and the middle is wide.

The upshot is that a single figure from a smart scale or InBody reading says little on its own. It becomes meaningful only in context: age, sex, muscle mass, and how it moves over time.

A number feels exact, and exactness feels like truth. A smart scale or InBody prints '27.4%' to the decimal, and that precision hides a genuinely large margin of error in the method itself. We also want a verdict, a figure to measure ourselves against, rather than a wide band that crowns no winner. Then there are the incentives: scale makers, gyms and body-composition companies benefit when the number feels important and worth tracking, because then you come back to measure again. Fitness and social-media culture also rewards ever-lower figures, which drags what counts as 'normal' well below what the research calls healthy.

Read body fat as a trend, not a grade. A single reading from a scale or handheld device shifts with fluid, food and time of day, so one figure tells you less than the direction over months, measured the same way at the same time. Compare yourself with yourself, not with an ideal from your feed. Remember that muscle mass, strength and how you feel day to day weigh at least as much as the number on the screen. If the figure becomes a source of worry more than of use, that is a fair reason to measure it less often.