Body fat calculator (U.S. Navy)

The Navy method estimates body fat from circumference measurements instead of equipment like DEXA scans or hydrostatic weighing. It uses a different formula for each sex:

All circumferences and height go in as centimetres in this version of the formula.

1. Measure neck (38 cm), waist (85 cm), plus height (170 cm).
2. Compute waist − neck = 85 − 38 = 47.0 cm.
3. Take the base-10 logarithm of that value and of the height, and drop both into the formula.
4. That comes out to approximately 17.8% body fat — the “Fitness” range for a man.

Commonly cited ranges (American Council on Exercise) split meaningfully by sex:

Women naturally carry more essential fat than men — it's needed for hormone regulation and reproductive health — so the same label lands at a different number for each: “Athletes” is 6-13% for men but 14-20% for women.

Waist circumference dominates the formula. With neck and height held fixed, a few centimetres either way shifts the estimate noticeably:

That sensitivity is exactly why measuring technique matters here more than for most calculators — the wrong spot on the torso, a tape pulled too tight, or measuring right after a meal can move the reading by a few centimetres and swing the result by a full percentage point or two.

• Neck: just below the larynx (Adam's apple), tape sloping slightly downward at the front.
• Waist: at the narrowest point, usually at or just above the navel — don't pull the tape tight.
• Hip (women): at the widest point around the buttocks.
• Stand normally, breathe naturally, and avoid measuring right after a large meal.
• Use a flexible cloth or vinyl tape, not a rigid ruler or metal tape.
• Measure at the same time of day — morning tends to be most consistent — so readings stay comparable over time.

Skinfold calipers can beat the Navy method on accuracy — pinching subcutaneous fat at a few standard sites and running it through a population-specific equation. The catch is technique: the same person measured by two different testers can get noticeably different numbers, while a tape measure is far more forgiving of who's holding it. That trade-off — a little accuracy for a lot more consistency at home — is basically the case for using this method at all.

For monthly self-tracking, that's a fair deal. If you need a precise number — for a medical work-up, a physique competition, or a research study — a DEXA scan is worth the trip.

• Pregnancy. Waist circumference changes for reasons that have nothing to do with fat, so the formula doesn't apply.
• Very muscular or very lean people. The formula was calibrated on a general population and gets less reliable at either extreme.
• Older adults with significant muscle loss. Body composition shifts with age in ways a circumference formula alone can't capture.
• Unusual fat distribution from certain medical conditions or medications, where circumference estimates tend to diverge more from a clinical scan.

• Measuring waist too high (near the ribs) or too low (near the hips) instead of at the narrowest point.
• Pulling the tape tight enough to compress the skin, which understates the true circumference.
• Comparing a Navy-method result directly against an impedance scale reading and expecting the same number.
• Re-measuring daily and reacting to fluctuations that are mostly noise, not real change.
• Using the male formula for a female body or vice versa — it produces a systematically biased result.

Research consistently links excess visceral fat — the kind stored around the abdominal organs, not just under the skin — with higher risk of type 2 diabetes, cardiovascular disease, and metabolic syndrome, independent of overall body weight. That's part of why waist circumference alone is used as a screening signal in many health guidelines. Commonly cited risk thresholds are above roughly 94 cm for men and 80 cm for women, rising to 102 cm and 88 cm at higher risk in many Western guidelines — but several health bodies recommend lower cut-offs for South Asian populations, reflecting research showing higher relative risk at a given waist size in this group. These are population-level screening numbers, not an individual diagnosis; talk to a doctor if you have concerns about metabolic risk.

Since waist circumference drives most of the estimate, you can work backward and ask roughly how much waist reduction would move you a category, holding neck and height fixed. This is illustrative only — real fat loss changes neck and hip measurements too, so the relationship isn't perfectly linear.

Waist size typically shifts slowly even with a consistent calorie deficit and resistance training, so weigh month-to-month change more than week-to-week — the latter is mostly water retention and sodium intake.

At 170 cm with a 85 cm waist, that's about 17.8% body fat — useful for tracking direction over months, not a verdict from one reading. Measure the same way each time, re-check every 4-6 weeks, and let the trend speak rather than any single number. Strength, energy, sleep, and how your clothes fit all carry information a tape measure can't — treat this as one input among several.

Figures use the standard U.S. Navy circumference formula with the measurements you entered, or illustrative defaults. The underlying coefficients were fitted on a reference military population decades ago; later research has generally replicated the method's accuracy across broader civilian groups, but individual results can still vary by body shape and fat distribution more than the formula alone accounts for. This is general educational information, not a medical or clinical assessment — prioritise consistent long-term tracking over precision in any single reading.