How this estimate is built
True body-fat percentage is measured with methods such as DEXA, air-displacement plethysmography, underwater weighing, or a well-done skinfold protocol. Those are not available in a browser.
What this page does is apply a published BMI-based prediction associated with Deurenberg and colleagues (adult form):
Body fat % ≈ 1.2 × BMI + 0.23 × age − 10.8 × sex − 5.4
Sex is 1 for men and 0 for women in that notation. BMI itself is weight ÷ height², which you can generate on the BMI calculator.
Why the number can be wrong
- High muscle raises BMI and therefore this fat estimate, even when fat mass is modest.
- Older adults may lose muscle and keep the same BMI, so fat can be under-read.
- The equation does not know waist size. Central fat is the risk signal many clinics care about.
That is the same reason we keep a full note on where BMI falls short. If you want energy context rather than composition, use the calorie calculator or BMR tool.
What to do with a result
Treat it as one line next to waist, strength, walking tolerance, and bloodwork. Changing how you eat still starts with plates you can repeat — see the meal framework and anti-inflammatory foods — not with chasing a percentage from a web form.
Frequently asked
Is this the US Navy tape method?
No. The Navy method uses waist, neck, and (for women) hip. This page only uses BMI, age, and sex.
What is a “healthy” body-fat range?
Charts differ by age and sex, and athletic norms are lower. A clinician or accredited physiologist should interpret your number, especially if it sits at an extreme.
Can I improve the estimate without a clinic?
A waist-to-height ratio and a strength test add information this formula lacks. We may add a waist tool later; until then, measure waist at home and take the tape figure to your clinician.
See Deurenberg et al. on BMI-to-fat prediction; Wikipedia, “Body fat percentage” and “Body mass index.”
Worked example
A 45-year-old woman with BMI 24: 1.2×24 + 0.23×45 − 10.8×0 − 5.4 = 28.8 + 10.35 − 5.4 ≈ 33.8%. A 45-year-old man with the same BMI: subtract an extra 10.8 and land near 23%. Same index, different estimate, because the equation includes sex. Both figures remain screens.
Compare that with what you would learn from a tape at the waist. Central fat can be high while this percentage looks moderate, and the reverse can be true in a muscular adult. That is why BMI limitations should be read with this tool, not after you have already treated the percentage as a fact.
What to do next
If the question was energy, leave this page for the calorie calculator. If the question was dinner, use the meal framework. If the question was a joint, use the mobility articles. Body-fat percentages from a browser do not rehabilitate a knee.
Home scales that claim to measure fat with a small current through the feet have their own error, especially when you are dehydrated. They are not a reason to ignore this page, and this page is not a reason to ignore them. Both are screens. Clinic methods remain the reference when a decision depends on the number.
Enter BMI from our calculator rather than from memory if you can. A remembered “about 27” and a measured 25.4 will move this estimate by more than most people expect, because the equation multiplies BMI by 1.2.