A worked example: two men at 10% and 20% body fat
Running the calculator's formula for a man at 10% body fat returns the "Athletes" category, because the men's Athletes band runs 6-13%. Running the same formula for a man at 20% body fat returns "Acceptable" instead, because the men's Acceptable band runs 18-24%. Both men are evaluated against the exact same men's reference table — only the input percentage changed, and it moved the result across two category boundaries.
The formula: sex-specific ACE reference bands
The calculator works by picking a full set of ACE (American Council on Exercise) reference bands based on the sex input, then checking which band the current-body-fat-percent input falls into: essential fat, athletes, fitness, acceptable, or above acceptable ("Obese" in the calculator's output). For men, those bands are essential 2-5%, athletes 6-13%, fitness 14-17%, and acceptable 18-24%. Anything below 2% is flagged "Below essential fat," and anything above 24% falls outside the acceptable band entirely.
Same percentage, different category: women at 17% and 28%
Swap in the women's reference table and the boundaries shift. A woman at 17% body fat lands in "Athletes," because the women's Athletes band runs 14-20% — a full band position higher than where 17% would sit on the men's table. A woman at 28% body fat lands in "Acceptable," since the women's Acceptable band runs 25-31%. The same raw percentage that would read as "Acceptable" or higher for a man can read as "Athletes" for a woman, purely because the calculator selects a different reference table for each sex.
Why sex-specific ranges matter
The calculator's own reference material explains this shift: women naturally carry more essential fat than men, needed for hormonal and reproductive function, so every category threshold in the women's table is shifted higher than the corresponding threshold in the men's table. That's also why the calculator flags very low percentages as "Below essential fat" rather than treating lower as unconditionally better — dropping under the essential-fat floor for either sex can impair hormonal, immune, and reproductive function.