The Body Fat Calculator estimates body-fat percentage from tape measurements using the US Navy circumference method, then reports your category and, with your weight, how many kilograms (or pounds) of you are fat mass and lean mass. Enter sex, height, neck, waist, and hip (required for women), in metric or imperial units.
How It Works
The Navy method estimates body density from circumferences and height using logarithmic formulas fitted against hydrostatic weighing, then converts density to body-fat percentage with the Siri equation, BF% = (495 / density) - 450. Men use waist minus neck; women use waist plus hip minus neck. The calculator validates the logic of the measurements (a man's waist must exceed his neck, for instance) and floors the result at 2%.
Parameters
| Input | Required | Meaning | |---|---|---| | Sex | Yes | Selects the male or female formula | | Height | Yes | Metric or imperial | | Neck | Yes | Circumference just below the larynx | | Waist | Yes | Men at navel level; women at the narrowest point | | Hip | Women | At the widest point | | Weight | No | Enables fat-mass and lean-mass results |
Worked examples
A man, height 178 cm, neck 38 cm, waist 86 cm: the formula gives a density of about 1.0595 and a body-fat estimate of 17.2%, which at 80 kg is about 13.8 kg of fat mass and 66.2 kg of lean mass. A woman, height 165 cm, neck 32 cm, waist 78 cm, hip 95 cm: the estimate is about 28.9%. Both figures sit in the ranges these methods were designed for, and both would be quoted as estimates with a few points of uncertainty by any honest practitioner, which is the right way to hold them.
How to measure so the number means something
Measure on bare skin, tape snug but not compressing, at the same time of day, and record to the half centimetre. The waist landmark differs by sex in the Navy protocol (men at the navel, women at the natural narrowest waist), and swapping them is the most common reason a result jumps oddly between calculators. Take each measurement twice and average. Because the formulas use differences (waist minus neck), a half-centimetre error in both tape positions can move the result by a point or more, which is why tracking the trend of the same person's same-protocol measurements is far more reliable than any single reading.
Categories and what to do with them
The calculator assigns a broad category (the standard bands run from essential fat, through athlete and fitness ranges, to average and obese). Treat the category as orientation: a result near a boundary is the same body on either side of it. For training decisions the useful outputs are the trend and the lean-mass figure: in a fat-loss phase, lean mass holding steady while fat mass falls is success, whatever the category label says; in a gaining phase, lean mass rising faster than fat mass is the goal. Compare like with like: Navy estimates, skinfold calipers, bioimpedance scales, and DEXA each carry their own bias, and switching methods manufactures fake progress and fake regressions.
Accuracy and limitations
Against DEXA, the Navy method typically lands within about 3 to 4 percentage points for most people, worse at the extremes of leanness and obesity and for bodies whose fat distribution is unusual (very central or very peripheral). It cannot see visceral fat specifically, and it is an estimate for healthy adults, not a medical measurement: it is not validated for children, pregnancy, or clinical use. If a precise figure drives a medical or competitive decision, use DEXA or hydrostatic testing and keep this as the between-tests tracker.
Frequently Asked Questions
Why does the formula need my neck?
The neck is used as a lean-tissue reference: it grows with muscle and frame but accumulates relatively little fat, so waist relative to neck separates fat gain from frame size better than waist alone.
My scale's body-fat reading differs by 5 points. Which is right?
Possibly neither in absolute terms: bioimpedance varies with hydration through the day, and the Navy method varies with tape placement. Pick one method, one protocol, and follow the trend; reserve absolute judgments for DEXA-class measurements.
Can body fat be too low?
Yes. Essential fat is roughly 2 to 5% for men and 10 to 13% for women, below which hormonal and immune problems follow. The calculator floors its output at 2% and any reading near the floor should be treated as a measurement problem first.