Body fat calculator — US Navy method and BMI estimate
Estimate body fat percentage from tape measurements with the US Navy formula, cross-check it against the BMI method, and see fat and lean mass.
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These equations were built from male and female reference groups — pick the one closest to your physiology.
5 ft 10.1 in
176.4 lb · 12 st 8.4 lb
Used by the BMI-based method only.
Just below the larynx, tape sloping slightly downward at the front.
At the navel, relaxed, at the end of a normal breath out.
Circumference methods estimate body fat, they do not measure it. Compared with a DEXA scan they are typically within 3 to 4 percentage points. Their real value is tracking change over weeks when you measure the same way each time.
Where you sit
The formula, with your numbers
- US Navy
- 495 ÷ (1.0324 − 0.19077·log₁₀(waist − neck) + 0.15456·log₁₀(height)) − 450
- waist − neck = 88 − 38 = 50 cm, height = 178 cm = 18.7%
- BMI method
- (1.20 × BMI) + (0.23 × age) − (10.8 × sex) − 5.4
- (1.20 × 25.2) + (0.23 × 30) − (10.8 × 1) − 5.4 = 21%
- Fat mass
- 80 kg × 18.7% = 15 kg
- Lean mass
- 80 kg − 15 kg = 65 kg
The Navy equations are defined in centimetres; imperial measurements are converted before the logarithms are taken, so both unit systems give the same answer.
What this means
About 18.7 percent of your body weight is fat — roughly 15 kg of fat and 65 kg of lean mass. That puts you in the “Average” band for men.
The two methods differ by 2.3 percentage points. That gap is informative: the BMI method only sees height, weight, age and sex, so it reads muscle as fat. If the Navy figure is the lower one, you carry more muscle than average for your weight.
Your waist-to-height ratio is 0.49 — that is 88 cm of waist against 178 cm of height. Keeping it under 0.5 is one of the simplest and best-supported markers of metabolic health, and it works across heights and builds where BMI does not.
Knowing your lean mass also unlocks the Katch-McArdle equation: it puts your basal metabolic rate at about 1,775 kcal a day.
| Category | Body fat |
|---|---|
| Essential fatBelow this is not survivable long term | under 6 % |
| AthletesVisible abdominal definition | 6 – 14 % |
| FitnessLean and healthy | 14 – 18 % |
| AverageYouTypical adult male range | 18 – 25 % |
| Obese | 25 % and above |
How to measure yourself properly
The US Navy method is only as good as the tape work. Five minutes of care here is worth more than any refinement to the equation.
- Use a flexible, non-stretch tape. A cloth tailor's tape is ideal. A metal builder's tape will not follow the body's curves.
- Measure first thing in the morning, after the bathroom, before eating or drinking. Food and fluid change waist circumference by a centimetre or more.
- Bare skin, tape snug but not compressing. If it leaves a mark, it is too tight. If it sags, it is too loose.
- Take each measurement three times and use the average. Re-seat the tape between attempts rather than reading it twice.
- Stand relaxed, arms at your sides, breathing normally. Do not suck in.
Where each measurement goes
| Site | Placement |
|---|---|
| Neck | Just below the larynx, tape sloping slightly downward toward the front. Do not include the shoulders. |
| Waist (men) | Horizontally at the navel, at the end of a normal breath out. |
| Waist (women) | At the narrowest point of the torso, usually a little above the navel. |
| Hips (women) | At the widest point of the buttocks, feet together, tape level all the way round. |
The two methods on this page
US Navy circumference method
Developed by the Naval Health Research Center in 1984 as a practical field standard. It uses the logarithm of a circumference difference, which captures how fat distributes around the trunk:
Men: %BF = 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450
Women: %BF = 495 ÷ (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450
All measurements are in centimetres. Imperial entries are converted before the logarithms are taken, so both unit systems give identical results.
BMI method (Deurenberg equation)
%BF = (1.20 × BMI) + (0.23 × age) − (10.8 × sex) − 5.4, where sex is 1 for men and 0 for women. Published by Deurenberg and colleagues in 1991, it needs no tape at all — but it only knows height, weight, age and sex, so it cannot distinguish muscle from fat. It is included here as a cross-check, not as the primary figure.
When the two disagree, the gap is informative. If the Navy figure is lower, you carry more muscle than average for your weight. If it is higher, you carry more fat than your BMI suggests — the "normal weight obesity" pattern.
How the methods compare on accuracy
| Method | Typical error vs DEXA | Cost and practicality |
|---|---|---|
| DEXA scan | Reference standard (±1–2%) | Clinic visit, £50–150 per scan |
| Hydrostatic weighing | ±2% | Specialist lab, uncomfortable |
| Bod Pod (air displacement) | ±2–3% | Specialist lab |
| Skinfold callipers (7-site) | ±3–4% with a skilled tester | Cheap, but technique-dependent |
| US Navy tape method | ±3–4% | Free, two minutes, repeatable at home |
| Bioimpedance scales | ±4–8%, very sensitive to hydration | Cheap and instant, but drifts with fluid status |
| BMI-based estimate | ±5–8%, worse for athletes | Needs no equipment at all |
Notice that the tape method is competitive with equipment costing hundreds of times more. Its real strength is repeatability: measured the same way each fortnight, it tracks change accurately even if the absolute number is a couple of points off.
Body fat categories
| Category | Men | Women |
|---|---|---|
| Essential fat | 2 – 5% | 10 – 13% |
| Athletes | 6 – 13% | 14 – 20% |
| Fitness | 14 – 17% | 21 – 24% |
| Average | 18 – 24% | 25 – 31% |
| Obese | 25% and above | 32% and above |
These are the American Council on Exercise bands. Women carry more essential fat than men for reproductive and hormonal function, which is why every threshold is roughly 8 to 10 points higher. Sustained very low body fat — under about 5 percent in men or 12 percent in women — disrupts hormones, bone density and immune function, and is not a target worth chasing.
Waist-to-height ratio: the two-second version
If you only ever measure one thing, measure your waist and divide it by your height. Keep it under 0.5. That single ratio predicts cardiometabolic risk at least as well as BMI, works across heights and populations, and needs no equations at all. It is shown alongside your body fat results above.
Tracking change over time
Body fat moves slowly. A realistic rate of loss is 0.5 to 1 percentage point a month while training and eating in a modest deficit. Measure every two to four weeks, always under the same conditions, and compare the trend rather than individual readings. A tape measure combined with progress photos and a lifting log will tell you more about body composition than any single scan.
Questions
How accurate is the US Navy body fat formula?
Against a DEXA scan it is typically within about 3 to 4 percentage points for people of average build, and less accurate at the extremes. Its real strength is consistency: measured the same way each time, it tracks change reliably.
Where exactly do I measure?
Neck: just below the larynx, tape sloping slightly down at the front. Waist: at the navel for men, at the narrowest point for women. Hips: at the widest point of the buttocks. Keep the tape snug but not compressing, and measure first thing in the morning.
What is a healthy body fat percentage?
The American Council on Exercise puts the fitness range at 14 to 17 percent for men and 21 to 24 percent for women, with essential fat at 2 to 5 and 10 to 13 percent. Under about 5 percent for men and 12 for women is not sustainable.
Why do the two methods disagree?
The BMI-based Deurenberg estimate only knows height, weight, age and sex, so it reads muscle as fat. The Navy method uses circumferences and reflects where the mass actually sits. If you are muscular, trust the Navy number more.