Updated September 2026 Estimate body fat % using the US Navy circumference method — the most accessible non-lab measurement.
Measure neck at narrowest point; waist at navel; hips at widest point.
How the Navy method works
The US Navy circumference method was developed to estimate body fat percentage using simple tape measurements. It uses height, neck, waist (and hip for females) to produce a body density estimate, which is converted to body fat percentage.
While not as accurate as DEXA scans or hydrostatic weighing, the Navy method has a standard error of ~3–4% and is useful for tracking changes over time. Take measurements in the morning before eating, relaxed (not flexed), and use the same tape for consistency.
How the US Navy method works
Developed by Hodgdon and Beckett in 1984 for US Navy body composition screening, this method estimates body fat from circumference measurements rather than from weight. The logic is that where fat is deposited is measurable with a tape, and waist girth relative to neck girth is a reasonable proxy for total adiposity in most bodies.
Women: %BF = 495 ÷ [1.29579 − 0.35004 × log(waist + hip − neck) + 0.22100 × log(height)] − 450
All measurements are in centimetres and the logarithms are base 10. Women’s equation includes hip circumference because female fat distribution is more gynoid, so waist alone is a poorer signal.
Measuring properly — this matters more than the equation
A centimetre of error at the waist shifts the estimate by roughly a full percentage point, so technique dominates accuracy here.
- Time: first thing in the morning, after the bathroom, before eating or drinking.
- Waist: at the navel for men, at the narrowest point for women. Relax the abdomen — do not suck in, and do not push the tape into the skin.
- Neck: just below the larynx, with the tape sloping slightly downward at the front.
- Hips (women): at the widest point of the buttocks, feet together.
- Repeat: take each measurement three times and average. If two readings differ by more than half a centimetre, take a fourth.
How accurate is it?
Typically within three to five percentage points of a DEXA scan — which makes it the best free option available, and not dramatically worse than skinfold calipers in untrained hands. Its predictable failure modes follow from what it measures: a thick neck from heavy training reads as leaner than reality, and a naturally narrow waist with high total fat also reads low. Because the equation was regression-fitted to a military population, it drifts at the extremes of body composition.
Worth putting in perspective: even DEXA, which everyone treats as truth, carries one to three points of error and varies between machines. Our guide to body fat measurement methods compares all of them honestly.
Why consistency beats accuracy
Suppose this method reads three points low for you. Over four months it returns 22, 21, 19.5 and 18 percent, while your real values were 25, 24, 22.5 and 21. Every absolute number was wrong — and the trend, the rate of change and every decision you would make from them were all correct. A consistent error cancels when you compare two readings, which is the whole reason a free tape measure is a legitimate tool.
So: pick one method, standardise the conditions, measure monthly rather than daily, and read the direction rather than the digit.
Reference ranges
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletic | 6–13% | 14–20% |
| Fit | 14–17% | 21–24% |
| Acceptable | 18–24% | 25–31% |
| Above healthy range | 25%+ | 32%+ |
Women carry more essential fat for reproductive and hormonal reasons; comparing across the columns is meaningless. Sustained intake below the essential range is associated with hormonal disruption, menstrual dysfunction and impaired immune function in both sexes.
Two things to do with the number
First, convert it to lean body mass — weight multiplied by (1 minus body fat percentage) — which lets you set protein from lean mass instead of total weight, useful at higher body fat. See the protein target table. Second, use it to decide whether to cut, bulk or recomp, which is the one decision where being three points off does not change the answer.
Body fat calculator — FAQ
For men, 10–20% is considered fitness range, with 6–13% being athletic and 21–24% acceptable. For women, 18–28% is fitness range, 14–20% athletic, and 25–31% acceptable. Essential fat (the minimum for health) is 2–5% for men and 10–13% for women. Body fat below essential fat levels is dangerous and associated with organ damage and hormonal disruption.
Monthly is the recommended frequency for most people. Daily measurements are too noisy — body fat estimates fluctuate based on hydration, food volume, and measurement error. Measuring monthly under consistent conditions (same time of day, same tape measure) gives you reliable trend data without obsessing over day-to-day noise.
Typically within three to five percentage points of a DEXA scan, which makes it the best free option available. It reads low for people with thick necks or naturally narrow waists, and drifts at the extremes of body composition because it was fitted to a military population.
Because the two infer fat from completely different physical properties. Bioimpedance scales are dominated by hydration status and carry four to eight points of error; a five-point disagreement between the two methods is entirely expected. Pick one and stay with it.
Monthly. Real body composition change is slower than the measurement noise of every available method, so weekly readings mostly tell you about your hydration and recent meals.
Lean body mass is everything that is not fat — muscle, bone, organs, water. It is what drives your resting metabolic rate, and it is the better basis for setting protein intake if you carry a lot of body fat, since fat tissue has little protein requirement.