Body Fat Calculator
Estimate your body fat percentage with nothing but a tape measure. This calculator uses the U.S. Navy circumference method and shows healthy ranges for men and women.
Built and reviewed by Jesse · Last reviewed
Enter your measurements to estimate your body fat percentage.
- Measure with a soft tape. Neck and waist for everyone; women also measure the hips. Measure relaxed, at the end of a normal breath.
- Enter your height and measurements. In centimeters or inches — keep units consistent.
- Read your body-fat estimate. The result is a percentage with a category label for your sex.
This calculator uses the U.S. Navy circumference method, developed so body composition could be checked with nothing more than a tape measure. It combines your height with the circumference of your neck and waist (and hips for women) to estimate the percentage of your body weight that is fat.
Because it relies on where you carry size rather than weight alone, it captures something BMI cannot: two people at the same weight and height can have very different body fat. It won't match a lab scan exactly, but it's consistent and free.
The Navy equation does its work in two steps, and knowing which step is which explains most of its behaviour. First it combines your circumferences into a single number — waist minus neck for men, waist plus hips minus neck for women — and uses it to predict your body density. Then it converts that density into a fat percentage using Siri's equation, 495 / density − 450, which is where the two odd-looking constants in the formula come from.
| Step | Man, 178 cm | Woman, 165 cm |
|---|---|---|
| Circumference value | 90 − 38 = 52 cm | 75 + 98 − 32 = 141 cm |
| Body density the equation predicts | 1.053 g/cm³ | 1.034 g/cm³ |
| Fat percentage after Siri's conversion | 20.1% | 28.9% |
| Category this calculator shows | Average | Average |
Computed with the same function this calculator uses. The density row is Siri's conversion run backwards from the percentage.
Two consequences follow. The first is that the circumference value carries almost all the information. When circumference equations were tested against DEXA scans in 496 young male soldiers, the relationship to measured body fat improved steadily from BMI, to waist alone, to waist minus neck — the neck term earns its place by standing in for frame size and lean tissue, so a broad-shouldered man is not scored as fat simply for being large. The flip side is that the neck is a lever on your result: for the man above, adding 3 cm of neck and changing nothing else drops the estimate by 2.1 points.
The second is that Siri's conversion assumes fixed densities for fat and for everything else — about 0.90 and 1.10 g/cm³. Bone density and body water content vary between people, so that assumption adds its own error on top of the tape work, and it is part of why a tape estimate and a scan rarely land on the same number.
The formula is fixed; the only thing you control is the quality of the three or four numbers you feed it. Use a soft, non-stretching tape on bare skin, keep it level all the way round, and pull it firm against the skin without compressing it.
| Site | Where the tape goes | What usually goes wrong |
|---|---|---|
| Neck | Just below the larynx, with the tape sloping slightly downward at the front. Shoulders relaxed, not shrugged. | Measuring the widest part of the neck instead of the narrowest. A larger neck lowers your estimate. |
| Waist (men) | Horizontally at the level of the navel, at the end of a normal exhale. | Pulling the stomach in, or looking down while reading the tape, which tilts it out of level. |
| Waist (women) | At the narrowest part of the torso, usually a little above the navel. | Using the navel instead of the narrowest point. That is a different site, and it gives a different answer. |
| Hips (women) | Around the widest part of the buttocks, feet together. | Measuring at the bony shelf of the hips, which sits higher and is narrower than the widest point. |
Two habits matter more than anything else in that table:
- Take three readings at each site and use the middle one. If your highest and lowest differ by more than about a centimetre, your tape placement is drifting — reset and measure again.
- Standardise the conditions. Morning, before eating or drinking, after using the bathroom, same tape every time. Gut contents alone can move a waist measurement, and the waist is the measurement the formula leans on hardest.
- Write the raw centimetres down, not just the percentage. If you later change how you measure, the raw numbers let you see it; the percentage on its own hides it.
Small tape errors are not harmless here. The body fat percentage chart sets out how far a centimetre in the wrong place moves the result, and how that compares with the error in the method itself.
A percentage on its own is hard to act on. Multiply it by your body weight and you get fat mass in kilograms; the remainder is lean mass — muscle, bone, organs, and water. Do that at two points in time and the scale stops being ambiguous. Here is the same man twelve weeks apart, down 10 kg:
| Measurement | Start | Week 12 | Change |
|---|---|---|---|
| Body weight | 95.0 kg | 85.0 kg | −10.0 kg |
| Waist | 102 cm | 92 cm | −10 cm |
| Neck | 39 cm | 38 cm | −1 cm |
| Estimated body fat | 27.4% | 21.5% | −5.9 points |
| Fat mass | 26.0 kg | 18.3 kg | −7.7 kg |
| Lean mass | 69.0 kg | 66.7 kg | −2.3 kg |
Fat mass is body weight times the estimated percentage; lean mass is the remainder.
The scale says 10 kg. The two measurements together say 7.7 kg of that was fat and 2.3 kg was lean mass — a 77% fat share, which is a good outcome for a deficit of that size. The waist falling by 10 cm is what makes it a good outcome, and it is worth seeing the alternative: had he lost the same 10 kg with his waist down only 5 cm, the split would read 4.8 kg of fat against 5.2 kg of lean mass — barely half the loss coming from fat.
Treat the split as a direction, not a measurement: with several points of error in each estimate, a change of 5.9 points over three months is real, but the exact kilograms are soft. To keep the ratio on the right side, pair a moderate calorie deficit with enough protein and resistance training.
The calculator labels your percentage with one of five categories — essential fat, athlete, fitness, average, or above average — using the American Council on Exercise ranges for your sex. Those are descriptive population bands, not diagnoses, and the boundaries are round numbers rather than thresholds where something changes. Since the method carries several points of error, landing a point either side of a boundary tells you very little. The body fat percentage chart lists all the bands for both sexes and what each range looks like in practice.
Use the category as context, not a verdict. A "fitness" or "average" reading is perfectly healthy for most people, and chasing very low body fat can harm hormones, energy, and mood. If your goal is to lower body fat, combine a modest calorie deficit with enough protein — see the protein calculator — and resistance training to keep the weight you lose coming from fat, not muscle.
The equation was fitted on adult service members, and it only knows the three or four numbers you gave it. It will still return an answer in each of these cases, but the answer means less:
- Anyone under 18. Body fat in children and adolescents is assessed against age- and sex-specific growth references, which this method has no way to apply.
- Pregnancy, or the months after it. The waist measurement is not reporting abdominal fat, so the estimate is not interpretable.
- Very muscular or very lean bodies. There is no term for muscularity: a well-developed abdominal wall widens the waist and is read as fat, and neck training moves the answer without any change in body composition.
- After a large weight loss. Loose skin sits inside the tape, so the waist reading — and therefore the estimate — stays higher than the fat that remains.
- Alongside a different method. Impedance scales, skinfold calipers, and DEXA each answer the question differently. Comparing your tape estimate with a scan is not a check on your progress; comparing your tape estimate with your own tape estimate is.
None of this is a reason to skip the measurement. It is a reason to read it as one line of evidence next to your weight trend, your waist in centimetres, and how your training is going — and to leave medical interpretation to a clinician.
U.S. Navy body-fat method (metric)
Men: %BF = 495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450
Women: %BF = 495 / (1.29579 - 0.35004 x log10(waist + hip - neck) + 0.22100 x log10(height)) - 450- waist
- = waist circumference in centimeters
- neck
- = neck circumference in centimeters
- hip
- = hip circumference in centimeters (women)
- height
- = height in centimeters
The circumference method estimates body fat within roughly 3-4 percentage points for most people; it is a screening estimate, not a DEXA-grade measurement.
How does this body fat calculator work?
It uses the U.S. Navy circumference method, which estimates body fat percentage from your height and tape measurements: neck and waist for men, plus hips for women. No calipers, scales, or scans required — just a soft tape measure.
How accurate is the Navy body fat method?
The Navy method is accurate to roughly ±3–4% for most people, which is good for an at-home tape measurement. It's less precise than a DEXA scan or hydrostatic weighing, but far more accessible. Its real strength is consistency: measured the same way each time, it tracks changes reliably.
How do I measure correctly?
Use a flexible tape and measure on bare skin. Neck: just below the larynx, tape sloping slightly downward at the front. Waist: at the navel for men, at the narrowest point for women. Hips (women): at the widest point of the buttocks. Keep the tape snug but not compressing the skin, and don't hold your breath or suck in.
What is a healthy body fat percentage?
For men, roughly 14–24% is typical (6–13% is athletic). For women, roughly 21–31% is typical (14–20% is athletic). Women naturally carry more essential fat than men. Very low body fat isn't necessarily healthier — some fat is essential for hormones and organ function.
Why is my body fat different from my bathroom scale reading?
Smart scales use bioelectrical impedance, which is sensitive to hydration, meals, and skin temperature and can swing several percent between readings. The Navy tape method and impedance scales use completely different approaches, so they often disagree. Pick one method and track the trend rather than comparing across methods.
Should men and women use different ranges?
Yes. Women require more essential fat for reproductive and hormonal health, so healthy ranges sit several points higher than for men. This calculator applies sex-specific categories automatically and asks women for a hip measurement, which the formula requires.
How often should I remeasure?
Every 2–4 weeks is plenty. Body fat changes slowly, and measuring more often mostly captures noise. Measure under the same conditions each time — same time of day, same tape, ideally before eating — so the comparison is meaningful.
Is body fat percentage better than BMI?
For assessing body composition, yes — body fat percentage distinguishes fat from muscle, which BMI cannot. BMI is a quick screening tool based only on height and weight. If you're muscular, body fat percentage will give a far more accurate picture of your health and fitness.
Why does the formula subtract my neck measurement?
The neck stands in for frame size and lean tissue, so the equation can read the waist as a signal of abdominal fat rather than of overall size. It has a real effect on the answer: for a 178 cm man with a 90 cm waist, adding 3 cm of neck with no change at the waist lowers the estimate by 2.1 points. That is a quirk of the method, not fat loss.
How do I turn a body fat percentage into kilograms of fat?
Multiply your body weight by the percentage. At 85 kg and 21.5% body fat, fat mass is 85 × 0.215 = 18.3 kg, and the remaining 66.7 kg is lean mass — muscle, bone, organs, and water. Tracking those two numbers separately tells you far more than the scale alone.
My weight has not changed but my body fat estimate dropped. What happened?
Most likely you lost fat and gained a similar amount of lean mass, which is common in the first months of resistance training. The scale cannot see that trade; a waist that shrinks at a steady body weight is the signature of it. Confirm it over two or three measurements before trusting it, since the method carries several points of error.
Does the Navy method work for very muscular or very lean people?
Less well. The equation has no term for muscularity, so a well-developed abdominal wall widens the waist and reads as fat, while someone lean who carries what fat they have on the legs rather than the abdomen reads lower than they are. If you are at either extreme, use the estimate to track your own changes rather than to compare yourself with anyone else.
References
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men and women from body circumferences and height. Naval Health Research Center, 1984.
- Siri WE. Body composition from fluid spaces and density: analysis of methods. 1961. Reprinted in Nutrition. 1993;9(5):480-491.
- Friedl KE, Vogel JA. Validity of percent body fat predicted from circumferences: classification of men for weight control regulations. Mil Med. 1997;162(3):194-200.
FitCalcs calculators provide general estimates for healthy adults and are not medical advice.
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