How to calculate body fat percentage
You do not need a lab. A tape measure and the US Navy formula give a solid estimate in two minutes. Here is the method, a worked example, and what the number actually means.
Estimate your body fat percentage using the US Navy tape method and a few measurements.
Body fat percentage tells you something the bathroom scale never can: how much of your weight is actually fat, rather than muscle, bone and water. Two people can weigh the same and look completely different, and this number is why. The good news is you do not need an expensive scan to get a usable estimate. A cloth tape measure and the US Navy formula will get you within a few percent, and a free body fat calculator does the maths for you. Here is how it works. This is general information, not medical advice.
In short: the most practical at-home method is the US Navy circumference method. You measure your height, neck and waist (plus hips for women), put them into a formula, and out comes an estimate of your body fat percentage. In the original military validation it typically landed within about 3 to 4 percentage points of underwater weighing. Individual results can be further off, so treat it as a trend tool, not an exact reading.
What body fat percentage actually means
Your body fat percentage is simply the share of your total body weight that is fat. If you weigh 80 kg and have 16 kg of fat, that is 20 percent body fat. The rest, your "lean mass", is muscle, organs, bone and water. This is why it beats weight alone: lose 2 kg of fat and gain 2 kg of muscle and the scale does not move, but your body fat percentage drops and you look leaner. It is also why it is more useful than BMI for fit people, since BMI cannot tell muscle from fat.
The US Navy method, step by step
This method estimates body fat from body measurements, no special equipment beyond a tape measure. Take each measurement snugly but without squashing the skin, and measure three times and average for accuracy:
- Height, standing straight.
- Neck, just below the larynx (Adam's apple), with the tape sloping slightly down at the front.
- Waist. For men, around the navel. For women, at the narrowest point.
- Hips (women only), at the widest point.
Then the formulas, using inches, are:
- Men: body fat percent = 86.010 x log10(waist - neck) - 70.041 x log10(height) + 36.76
- Women: body fat percent = 163.205 x log10(waist + hip - neck) - 97.684 x log10(height) - 78.387
Nobody expects you to work logarithms by hand, which is exactly what the calculator is for (it also accepts centimetres and converts for you). But it helps to see one worked through.
A worked example
Take a man who is 70 inches tall (5 feet 10), with a 36 inch waist and a 16 inch neck. Waist minus neck is 20 inches. Putting those into the men's formula:
86.010 x log10(20) - 70.041 x log10(70) + 36.76 = 86.010 x 1.301 - 70.041 x 1.845 + 36.76, which works out to about 19.4 percent. Looking at the chart below, that falls in the "average" band for men. If he trimmed his waist to 33 inches, the same formula gives about 13.4 percent, the top of the athlete band. Three inches off the waist moved the estimate by roughly six points, which shows how much the waist measurement drives the result.
And a worked example for women
Take a woman who is 65 inches tall (5 feet 5), with a 30 inch waist, 40 inch hips and a 13.5 inch neck. Waist plus hips minus neck is 56.5 inches. The women's formula, 163.205 x log10(56.5) - 97.684 x log10(65) - 78.387, gives about 30.5 percent, which sits in the "average" band for women. Notice the women's formula adds the hip measurement, which is why the two formulas look different.
Body fat percentage chart: what the categories mean
Body fat norms differ for men and women, because women naturally carry more essential fat. A widely used set of categories from the American Council on Exercise (these are fitness labels, not clinical diagnoses):
| Category | Men | Women |
|---|---|---|
| Essential fat | 2 to 5 percent | 10 to 13 percent |
| Athletes | 6 to 13 percent | 14 to 20 percent |
| Fitness | 14 to 17 percent | 21 to 24 percent |
| Average | 18 to 24 percent | 25 to 31 percent |
| Obese | 25 percent and over | 32 percent and over |
A few honest caveats. These bands are general guides, not diagnoses, and healthy ranges drift a little higher with age. "Essential fat" is the minimum your body needs to function; going below it is not a goal, it is a health risk. And the single number hides where the fat sits, which matters, fat around the organs is riskier than fat on the hips. Treat the category as a rough label, not a verdict.
A health note. These numbers are general information, not medical advice or a diagnosis, and the ACE labels are fitness categories rather than health thresholds. Most adults do not need to reach the athlete range, and staying very lean for a long time can harm health; women in particular who drop much below the athlete range can lose their periods and bone density. This method is for adults. If you are pregnant, under 18, living with an eating disorder, or managing a medical condition, talk to a doctor before using any body fat number to set goals.
Which measuring method should you trust?
The tape-measure method is the most practical, but it is not the only one, and they trade convenience against accuracy:
| Method | How it works | Accuracy |
|---|---|---|
| Tape measure (US Navy) | Circumference formula | Typically within 3 to 4 points in validation studies; can be off more for very lean, very muscular or higher-fat bodies. Free and repeatable |
| Skinfold calipers | Pinch fat at several sites | Good with a trained tester; operator-dependent |
| Bioelectrical impedance (smart scales) | Tiny current through the body | Convenient but swings with hydration and time of day |
| DEXA scan | Low-dose X-ray | Widely used as the practical reference, with repeat-scan precision around 1 to 2 points; still varies by machine and hydration. Costs money |
For everyday use, the most important thing is not which method is most accurate in absolute terms, but that you use the same method under the same conditions each time. A tape measure taken every few weeks, first thing in the morning, tracks your real trend far better than hopping between a smart scale, a caliper and a one-off DEXA.
Working in centimetres? The calculator accepts either unit, but if you want to use the formula by hand in centimetres it needs different constants, so the simplest route is to convert your measurements to inches first (1 inch = 2.54 cm) and use the formulas above.
When the tape method is less reliable
The US Navy method is quick and free, but it is an estimate built on averages, so it is least accurate at the extremes:
- Very muscular builds. A thick, muscular neck can skew the maths and overstate fat, because the formula reads neck size as a proxy that does not hold for everyone.
- Higher body-fat ranges, where the results tend to compress and under-read.
- A result below the essential-fat range (under about 2 percent for men or 10 percent for women) almost always means a mis-measurement, not a real value, so re-measure.
In those cases a DEXA scan or skinfold test with a trained tester gives a truer figure. For most people in the middle of the range, the tape method tracks change just fine.
Common mistakes to avoid
| Mistake | Do instead |
|---|---|
| Pulling the tape tight | Snug against the skin, not compressing it |
| Measuring at different times of day | Pick one time, usually morning, and stick to it |
| Chasing the leanest range | Most adults do not need the athlete range; staying very low for long can harm health, and women who go much below it may lose their periods |
| Trusting a single reading | Measure three times, average, and watch the trend |
| Treating the estimate as exact | It is an estimate; the change over time is what counts |
If you are using this to guide weight loss or training, pair it with how many calories you need: our guides on calories to lose weight and calculating your macros turn a target body fat into a daily plan.
Sources: the circumference formula comes from the US Navy body composition research (Hodgdon and Beckett); the category bands are the American Council on Exercise body fat norms. Worked examples checked with a calculator.