What it means
Body fat percentage tells you what scale weight cannot — how much of your mass is fat versus muscle, bone and water. Tape-based methods estimate it to within about ±3–5 points, so read your result as a zone, not a score, and watch the trend over weeks.
The ACE fitness scale — a different lens
| Category | Women | Men |
|---|---|---|
| Essential fat | 10–13% | 2–5% |
| Athletes | 14–20% | 6–13% |
| Fitness | 21–24% | 14–17% |
| Acceptable | 25–31% | 18–24% |
| Above acceptable | 32%+ | 25%+ |
This is the American Council on Exercise's athletic classification — a training-oriented scale, not the same thing as a health range. The calculator itself judges your result against age- and sex-adjusted health ranges (Gallagher 2000), which is why its zones and this table won't line up: they measure different things.
Which method to trust
This calculator runs four tape/BMI methods and shows their range:
- US Navy (tape) — circumference-based; best all-round without equipment.
- RFM (tape) — waist and height only; validated against DXA, often the most accurate of the four.
- BMI-based (Deurenberg) — needs no tape; overestimates for muscular builds.
- Covert Bailey (tape) — circumferences, age-adjusted.
Beyond this calculator: skinfold (Jackson–Pollock) is more accurate at home if you have calipers, and DEXA is the practical reference standard (home BIA scales drift with hydration).
For women
Healthy ranges sit 8–10 points above men's. Cycle water shifts move a tape estimate by a point — measure on the same week.
For weight loss
If weight drops but body fat holds, you're likely losing muscle — usually too aggressive a deficit or too little protein.
For athletes
Tape and BMI overestimate fat in muscular people. Use skinfolds or DEXA and treat the tape as a floor.
FAQ
How accurate is a body fat calculator?
Tape-based estimates land within 3–5 percentage points of a DEXA scan — enough to track direction over months, not to compare against someone else's number.
What is a healthy body fat percentage?
It depends on your age and sex, so there is no single number — the calculator applies age-adjusted health ranges (Gallagher 2000) for you. Across adulthood that is broadly 21–36% for women and 8–25% for men, drifting higher with age. The ACE table above is a separate fitness scale, not a health cut-off. Below about 13% (women) or 5% (men), essential functions are affected.
Why does my smart scale give a different number?
Bioimpedance scales read hydration as much as fat, so they swing several points within a day. Stick to one method.
How often should I measure?
Every two to four weeks. More often and you're reading noise.
Can I lower body fat without losing weight?
Yes — building muscle at steady weight lowers the percentage, common early in resistance training.
Does the calculator store my data?
The calculator runs in your browser — your measurements aren't sent anywhere to be calculated. If you save your history, it's kept in this browser. When accounts and the diary launch, saving to your account will be your choice, and you'll be able to export or delete everything at any time.