Body Fat Calculator

How to Use the Body Fat Calculator

The US Navy circumference method estimates body fat from a tape measure and your height. It is not as accurate as a DEXA scan and it is free, repeatable at home, and considerably more informative than the scale alone.

1. Measure your neck just below the larynx, with the tape sloping very slightly downward at the front. Do not flex or pull the tape tight.
2. Measure your waist at the navel for men, and at the narrowest point for women. Stand relaxed, breathe out normally, and do not hold your stomach in — the measurement is only useful if it is honest.
3. Measure your hips at the widest point if using the female equation, which needs all three circumferences rather than two.
4. Add your weight to convert the percentage into kilograms of fat and lean mass. Lean mass is the figure worth tracking across a diet, because holding it steady is the whole objective.
5. Set a target percentage to see what you would weigh at that body fat if lean mass stayed constant — which is a far more useful goal than a scale number.

Take every measurement twice and use the average. A centimetre of difference on the waist moves the estimate by nearly a percentage point, so consistency of technique matters more than precision of the tape.

US Navy Body Fat Formulas

Two different published equations, both using base-10 logarithms.

The waist-minus-neck term is doing nearly all the work. Neck circumference changes very little with body composition, so it acts as a stable reference against which the waist is compared — which is why the method is so sensitive to waist measurement technique and so forgiving of everything else.

Example

A man, 178 cm, waist 92 cm, neck 39 cm

1. Waist minus neck: 92 − 39 = 53 cm.
2. log₁₀(53) = 1.724276, and log₁₀(178) = 2.250420.
3. Denominator: 1.0324 − (0.19077 × 1.724276) + (0.15456 × 2.250420) = 1.051285.
4. 495 ÷ 1.051285 = 470.8524.
5. Subtract 450: body fat is 20.85%.
6. At 82 kg that is 17.10 kg of fat and 64.90 kg of lean mass.

What reaching 15% would mean

Holding lean mass at 64.90 kg, a 15% body fat reading corresponds to a body weight of 64.90 ÷ 0.85 = 76.35 kg — a loss of 5.65 kg, all of it fat. That is the useful way to set a weight goal: work backwards from a composition target and a lean mass you intend to keep, rather than picking a scale number and hoping the right tissue disappears.

How sensitive the estimate is

Three centimetres off the waist — 89 cm instead of 92 — drops the estimate to 18.70%, a change of over two percentage points. Three centimetres the other way gives 22.90%. That sensitivity is why the method works well for tracking change over time and poorly for comparing yourself against someone else's reading.

How It Compares With Other Methods

How the estimate moves with the waist measurement, at 82 kg.

Roughly 0.7 percentage points per centimetre of waist. That makes the method a genuinely useful progress tracker — a real change in composition shows up clearly — and it also means a sloppy measurement produces a meaningless number.

The conventional bands for men run: essential fat below 6%, athletic 6% to 14%, fitness 14% to 18%, average 18% to 25%, and above average beyond that. For women each band sits roughly eight points higher, because essential fat requirements are genuinely different rather than as a matter of convention. This example at 20.85% is in the average band.

Against other methods, the tape approach has a standard error of about three to four percentage points compared with hydrostatic weighing. DEXA is more accurate and costs money; bioelectrical impedance scales are cheap and swing several points with hydration alone. For tracking a trend at home, taking the same measurements the same way each fortnight beats all of them, because consistency matters more than absolute accuracy when what you care about is the direction.

Five Limits Worth Understanding

Five limits worth understanding before trusting a number.

It infers rather than measures. The equations assume a typical relationship between circumference and fat distribution. Someone who carries fat unusually — very little abdominally, or a great deal — will read wrong in a predictable direction.

Very lean and very heavy people read worst. Accuracy degrades at both extremes of the range, because the validation samples contained fewer people there. Athletes below about 10% often read high.

Technique drift looks like progress. Measuring slightly higher on the abdomen one week and slightly lower the next produces changes that have nothing to do with your body. Mark the spot, use the same time of day, and measure before eating rather than after.

The bands are conventions. The category names carry no clinical authority, and no single body fat percentage is correct for everyone. Health outcomes relate more closely to waist circumference itself and to fitness than to a percentage derived from it.

Scale weight and composition move differently. Someone training hard on a deficit can hold weight steady while losing fat and gaining muscle, which looks like failure on a scale and success here. That is precisely why the two are worth tracking together — the BMI calculator covers the weight-and-height side, and it cannot distinguish muscle from fat at all.

One practical protocol makes the method far more useful than it looks. Measure first thing in the morning, before eating or drinking, after using the bathroom, with the tape marked at the same spot each time — a small pen dot on the skin is not excessive. Take each circumference three times and use the middle value rather than the first, since the first is usually pulled tightest.

Then record the result once a fortnight rather than daily, and read the trend across three readings rather than the change between two. At roughly 0.7 percentage points per centimetre of waist, day-to-day noise from food, salt and posture is larger than a fortnight of real progress, and a chart of single readings will show you a story that did not happen.