Body Fat Calculator
Estimate your body fat percentage with the U.S. Navy tape-measure method. Enter height, neck and waist (plus hips for women) to see your result and category.
📏 Body Fat Calculator
Results
💡 How the Body Fat Calculator Works
This calculator estimates the percentage of your body weight that is fat, using only a tape measure. It applies the U.S. Navy circumference method, developed by Hodgdon and Beckett at the Naval Health Research Center in 1984: men enter height, neck and waist; women also enter hips. With the default inputs (man, 170 cm, waist 85 cm, neck 38 cm) the estimate is 17.8%, in the Fit range.
How to measure yourself accurately
The result depends almost entirely on your measurements, so a few millimetres matter. Use a flexible, non-stretch tape, stand relaxed, and keep the tape level and snug without compressing the skin.
- Height: without shoes, standing straight against a wall.
- Neck: just below the larynx (Adam's apple), with the tape sloping slightly downward toward the front. Don't flare your neck outward.
- Waist (men): horizontally at the level of the navel, at the end of a normal breath out. Don't pull your stomach in.
- Waist (women): at the narrowest point of the torso, roughly midway between the bottom of the ribs and the navel.
- Hips (women): around the widest part of the buttocks, with feet together.
Measure each site two or three times and use the average. For tracking change, measure at the same time of day, ideally in the morning before eating.
The Navy body fat formula
The equations predict body density from your measurements, then convert density to body fat with the Siri equation. In centimetres:
Women: Body fat % = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
A version of the formula for inches is also in common use. It gives results within a few tenths of a percentage point of the centimetre version.
Worked examples
- Man, 170 cm, waist 85 cm, neck 38 cm: waist − neck = 47 cm, giving 17.8% body fat (Fit on the chart below). If he weighs 70 kg, that is about 12.5 kg of fat mass and 57.5 kg of lean mass.
- Woman, 165 cm, waist 75 cm, hips 100 cm, neck 33 cm: waist + hip − neck = 142 cm, giving 29.4% body fat (Average).
Body fat percentage chart
The American Council on Exercise (ACE) publishes these general ranges, and the calculator's categories follow them. They are broad guides for adults, not medical targets.
| 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 |
Essential fat is the minimum the body needs for normal function, including hormone production and protecting organs. Women carry more essential fat than men for reproductive reasons, which is why every range is higher for women. ACE notes that levels below about 6% for men or 14% for women may be dangerously low.
How waist size changes the estimate
Waist is the measurement that moves the result most. These tables hold the other inputs constant to show the effect.
Men (height 178 cm, neck 38 cm):
| Waist | Body fat | Category |
|---|---|---|
| 76 cm (29.9 in) | 8.8% | Athletic |
| 80 cm (31.5 in) | 12.4% | Athletic |
| 84 cm (33.1 in) | 15.7% | Fit |
| 88 cm (34.6 in) | 18.7% | Average |
| 92 cm (36.2 in) | 21.5% | Average |
| 96 cm (37.8 in) | 24.2% | Average |
| 100 cm (39.4 in) | 26.7% | Obese |
| 104 cm (40.9 in) | 29.1% | Obese |
| 110 cm (43.3 in) | 32.5% | Obese |
Women (height 165 cm, neck 33 cm, hips 100 cm):
| Waist | Body fat | Category |
|---|---|---|
| 64 cm (25.2 in) | 23.8% | Fit |
| 68 cm (26.8 in) | 25.9% | Average |
| 72 cm (28.3 in) | 27.9% | Average |
| 76 cm (29.9 in) | 29.9% | Average |
| 80 cm (31.5 in) | 31.9% | Average |
| 84 cm (33.1 in) | 33.8% | Obese |
| 88 cm (34.6 in) | 35.7% | Obese |
| 92 cm (36.2 in) | 37.6% | Obese |
| 98 cm (38.6 in) | 40.3% | Obese |
How accurate is the Navy method?
In the original Navy studies, the equations were checked against underwater (hydrostatic) weighing, then a laboratory reference method. The standard error was about 3.5 percentage points for men (602 men) and 3.7 percentage points for women (214 women). In practice, for an estimate of 20%, the true value is within about 3.5 points (roughly 16.5–23.5%) about two times in three.
The method tends to be least accurate for people whose shape differs from the original study groups, such as very muscular people with thick necks, people with very low or very high body fat, and older adults. In 2023 the U.S. Army replaced its multi-site tape test with a single abdominal measurement, after its own research found the older equations varied in accuracy with body type.
| Method | What it involves | Practical notes |
|---|---|---|
| DEXA (DXA) scan | Low-dose X-ray of the whole body | Clinical reference method; also shows where fat is stored |
| Underwater weighing | Weighing while fully submerged | Classic laboratory reference; needs specialist equipment |
| Air displacement (Bod Pod) | Sitting in a sealed chamber | Laboratory accuracy without submersion |
| Bioelectrical impedance (smart scales) | Weak current through the body | Convenient, but readings shift with hydration and recent meals |
| Skinfold calipers | Pinching fat at several sites | Accuracy depends heavily on the tester's skill |
| Navy tape method (this calculator) | Tape measurements and height | Free and repeatable; best for tracking change over time |
Body fat percentage vs BMI
BMI uses only height and weight, so it can't tell muscle from fat: a muscular athlete may be classed as overweight, while someone with little muscle may have a normal BMI but a high body fat percentage. Body fat estimates and waist measurements add information that BMI misses.
Waist size on its own is also a health signal. The U.S. National Heart, Lung, and Blood Institute notes that a waist of more than 35 inches (about 89 cm) for women or 40 inches (about 102 cm) for men raises the risk of conditions such as type 2 diabetes and heart disease. The waist-to-height ratio calculator is another quick check.
Using your result
- Track the trend, not one reading. Because each estimate can be off by a few points, the change over several weeks is more meaningful than a single number.
- Pair it with other measures. Your lean body mass and FFMI show muscle; the TDEE calculator estimates daily energy needs.
- Talk to a professional before making big changes to diet or training, especially if your result is very low or very high, or if you have a health condition.
This calculator is for adults and is not a medical diagnosis. It isn't designed for children, pregnant women or people with conditions that affect body shape or fluid balance.
Sources
- Hodgdon & Beckett (1984), Prediction of percent body fat for U.S. Navy men from body circumferences and height, NHRC Report 84-11
- Hodgdon & Beckett (1984), Prediction of percent body fat for U.S. Navy women from body circumferences and height, NHRC Report 84-29
- American Council on Exercise (ACE), body fat percentage ranges
- National Heart, Lung, and Blood Institute (NIH), waist circumference and health risk
- U.S. Army (2023), Evolving body composition standards