Body fat percentage is the share of your total weight that is fat. You can measure it with a DEXA scan, hydrostatic weighing or air displacement for the highest accuracy, or estimate it at home with skinfold calipers, a smart scale or a tape measure. The most practical home method is the US Navy formula, which uses height and neck, waist and (for women) hip circumferences. A man who is 5′10″ with a 15.5-inch neck and 34-inch waist comes out at about 16.5% body fat. Unlike BMI, body fat percentage separates fat from muscle, which makes it more informative for athletes and for tracking changes in body composition.
Measurement methods compared
| Method | How it works | Typical accuracy* | Cost and access |
|---|---|---|---|
| DEXA scan | Low-dose X-ray separates fat, lean tissue and bone | About ±1–2 points | $50–$150; clinics and some gyms |
| Hydrostatic (underwater) weighing | Body density from weight in and out of water | About ±2 points | University labs, some clinics |
| Air displacement (Bod Pod) | Body volume from air displaced in a sealed chamber | About ±2–3 points | Labs, sports centers |
| Skinfold calipers | Pinch thickness at 3–7 sites, converted with an equation | About ±3–4 points with a skilled tester | Cheap; skill matters |
| US Navy tape method | Circumferences and height in a formula | About ±3–4 points | Free; just a tape measure |
| Bioelectrical impedance (scales, handhelds) | Resistance to a small electrical current | ±4 points or more; varies with hydration | $30–$150 at home |
| BMI-based equations | Estimate from BMI, age and sex | Large errors for individuals | Free |
*Approximate standard errors against laboratory reference methods; individual results can be off by more.
The best method for you is often the one you can repeat consistently. A tape measure used the same way every two weeks can track change better than an occasional scan from different machines.
The US Navy formula
The Navy method, developed by the Naval Health Research Center and used by the US military, estimates body fat from circumferences measured in inches.
The body fat calculator applies these formulas in US or metric units.
How to measure
- Height: without shoes, standing tall.
- Neck: just below the larynx (Adam’s apple), tape sloping slightly down toward the front.
- Waist (men): horizontally at the level of the navel.
- Waist (women): at the narrowest point, usually above the navel.
- Hip (women): at the widest part of the buttocks.
Measure on bare skin, snug but not compressing, after a normal exhale. Take each measurement two or three times and average them.
Worked example: man
Height 70 in, neck 15.5 in, waist 34 in.
- Waist − neck = 34 − 15.5 = 18.5; log10(18.5) = 1.26717
- log10(70) = 1.84510
- %BF = 86.010 × 1.26717 − 70.041 × 1.84510 + 36.76
- = 108.99 − 129.23 + 36.76 = 16.5%
At 180 lb, that is about 29.7 lb of fat and 150.3 lb of lean mass.
Worked example: woman
Height 65 in, neck 13 in, waist 30 in, hip 40 in.
- Waist + hip − neck = 30 + 40 − 13 = 57; log10(57) = 1.75587
- log10(65) = 1.81291
- %BF = 163.205 × 1.75587 − 97.684 × 1.81291 − 78.387
- = 286.57 − 177.09 − 78.39 = 31.1%
At 150 lb, that is about 46.6 lb of fat and 103.4 lb of lean mass. Her BMI is 25.0, right at the overweight line, while her body fat sits at the upper end of the average range. The two measures answer different questions; see how BMI is calculated.
Fat mass and lean mass
Lean mass includes muscle, bone, organs and water. It drives your resting energy use, which is why the Katch–McArdle BMR equation uses it; see how to calculate BMR. The lean body mass calculator offers several formulas that estimate lean mass without a body fat measurement.
Interpreting your result
There is no single official US standard for body fat categories. These widely used fitness categories from the American Council on Exercise are a common reference:
| Category | Women | Men |
|---|---|---|
| Essential fat | 10%–13% | 2%–5% |
| Athletes | 14%–20% | 6%–13% |
| Fitness | 21%–24% | 14%–17% |
| Average | 25%–31% | 18%–24% |
| Obesity | 32% and higher | 25% and higher |
Women need more essential fat for reproductive and hormonal health. Body fat also tends to rise with age even at stable weight, as muscle declines. Very low body fat carries its own risks, including hormonal disruption and, in women, loss of menstrual periods.
BMI-based estimates and why they disagree
Some calculators estimate body fat from BMI using equations such as the Deurenberg formula: %BF ≈ 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4 (sex = 1 for men, 0 for women). For the man above (BMI 25.8, age 35), it predicts about 22.8%, six points higher than the Navy result, because it cannot see that his weight includes more muscle. Use BMI-based estimates only when no measurements are available.
Tips for tracking change
- Same conditions: measure at the same time of day, ideally in the morning before eating.
- Same method: do not compare a smart scale reading with a DEXA scan.
- Track trends: compare averages over two to four weeks, not single readings.
- Pair with simple measures: waist circumference alone is a useful health marker; NIH guidance flags more than 40 inches for men and 35 inches for women. The waist-to-hip ratio calculator adds another view of fat distribution.
This guide is for general education and is not medical advice. Body fat estimates have meaningful error, and healthy ranges vary by age, sex and individual factors; discuss your results with a healthcare professional.
Frequently asked questions
What is the most accurate way to measure body fat?
Among widely available methods, a DEXA (dual-energy X-ray absorptiometry) scan is generally considered the most accurate and also shows where fat is located. Hydrostatic weighing and air displacement (Bod Pod) are also research-grade. Multi-compartment models used in research labs are the reference standard.
How accurate is the US Navy tape measure method?
For most people it lands within roughly 3 to 4 percentage points of lab methods, similar to a skinfold test by an experienced tester. Careful, consistent measurement matters more than anything else.
What is a healthy body fat percentage?
Commonly used fitness categories put average body fat at about 25% to 31% for women and 18% to 24% for men, with 32% or more for women and 25% or more for men classed as obesity. Women naturally carry more essential fat than men.
Are smart scales accurate for body fat?
Bioelectrical impedance scales estimate body fat from how a small current passes through the body. Readings swing with hydration, meals and exercise, and individual errors can be large. They are most useful for tracking trends under the same conditions each time.
How do I calculate fat mass and lean mass from body fat percentage?
Fat mass = weight × body fat percentage ÷ 100; lean mass = weight − fat mass. A 180-pound person at 16.5% body fat carries about 29.7 pounds of fat and 150.3 pounds of lean mass.