Fat Percentage Calculator: How It Works

This fat percentage calculator estimates your body fat percentage based on a few simple body measurements. You enter your sex, age, height, and either circumference measurements (Navy method) or weight and height (BMI method), and it returns an estimated body fat percentage along with a category (e.g., essential, athletic, fit, average, obese). The result is an estimate, not a clinical measurement, but it is a useful, free way to track changes over time.

Formula

The calculator uses the U.S. Navy circumference method, which is a widely used anthropometric formula. For men, the formula is:

Body Fat % = 495 / (1.0324 − 0.19077 × log�₀(waist − neck) + 0.15456 × log�₀(height)) − 450

For women, the formula includes hip and is:

Body Fat % = 495 / (1.29579 − 0.35004 × log�₀(waist + hip − neck) + 0.22100 × log�₀(height)) − 450

All measurements are in centimeters. The log�₀ is the base-10 logarithm. The result is a percentage of total body mass that is fat. The formula was developed from a large sample of military personnel and is considered accurate to within about 3–5% of hydrostatic weighing for most people.

Worked Example

Using the default values: male, age 30, height 175 cm, neck 38 cm, waist 85 cm. Hip is not used for men.

1. Compute waist − neck: 85 − 38 = 47 cm
2. Compute log�₀(47) ≈ 1.6721
3. Compute log�₀(175) ≈ 2.2430
4. Plug into the formula: 1.0324 − 0.19077 × 1.6721 + 0.15456 × 2.2430 = 1.0324 − 0.3190 + 0.3467 = 1.0601
5. Divide 495 by 1.0601 ≈ 466.94
6. Subtract 450: 466.94 − 450 = 16.94

The calculator rounds to one decimal place, giving 16.9%. This falls in the “fitness� category for men (typically 14–17%).

How to Take Measurements Correctly

The accuracy of the calculator depends entirely on how you measure. Use a flexible, non-stretchable tape measure. For each site, keep the tape snug but not compressing the skin, and measure to the nearest 0.5 cm.

• Neck: Measure just below the larynx (Adam’s apple), sloping slightly downward at the front.
• Waist: For men, measure at the navel level. For women, measure at the smallest point between the ribs and the iliac crest (hip bone).
• Hip: (Women only) Measure at the largest circumference around the buttocks.
• Take each measurement three times and use the average.
• Measure in the morning, before eating or drinking, for consistency.

Understanding Your Result

Body fat percentage is a better indicator of health than BMI alone because it distinguishes fat from muscle. The following ranges are general guidelines for adults:

• Essential fat: 2–5% (men), 10–13% (women) – needed for physiological function.
• Athletic: 6–13% (men), 14–20% (women)
• Fitness: 14–17% (men), 21–24% (women)
• Average: 18–24% (men), 25–31% (women)
• Obese: 25%+ (men), 32%+ (women)

Women naturally have a higher essential fat percentage due to reproductive functions. Age also plays a role; body fat tends to increase with age even if weight stays the same. This calculator does not adjust for age, so older adults may see slightly lower estimates than DEXA scans would show.

Common Mistakes and Limitations

The most common mistake is using inches instead of centimeters. The formula requires centimeters; if you measure in inches, multiply each by 2.54 before entering. Another mistake is pulling the tape too tight or measuring over clothing. Also, the Navy formula assumes a typical distribution of fat; it may underestimate body fat in very muscular individuals and overestimate in very lean individuals.

The formula has a standard error of about 3–5% compared to hydrostatic weighing. This means your actual body fat could be a few points higher or lower. It is best used to track trends over time rather than as an absolute number. For clinical purposes, methods like DEXA, hydrostatic weighing, or air displacement plethysmography are more accurate, but they are also more expensive and not always accessible.

Alternatives to the Navy Method

If you don’t have a tape measure, you can use the BMI-based method: Body fat % = (1.20 × BMI) + (0.23 × age) − 10.8 × sex − 5.4 (where sex=1 for men, 0 for women). This is less accurate but requires only weight and height. Calipers (skinfold measurements) are another option; they use 3 or 7 sites and are accurate if done by a trained professional. Bioelectrical impedance scales are convenient but can be affected by hydration. For the most accurate results, DEXA is the gold standard, but it requires a clinic visit.

How to Use This Calculator for Progress

To track changes, measure under the same conditions each time: same time of day, same tape, same clothing (or none). Take measurements every 2–4 weeks; body fat changes slowly, so daily measurements will only show noise. If you are losing weight, your body fat percentage should gradually decrease. If you are gaining muscle, it may stay the same or even decrease while your weight increases. This is why body fat percentage is more informative than the scale alone.

Frequently Asked Questions

Is the Navy formula accurate?

It is one of the most accurate field methods, with an error of about 3–5% compared to hydrostatic weighing. It is more accurate than BMI-based formulas and most consumer scales. However, it is not as accurate as DEXA or hydrostatic weighing. Use it as a trend tracker, not a diagnostic tool.

Why do women need a hip measurement?

Women store more fat in the hips and thighs, so the formula includes hip circumference to account for this distribution. Without it, the estimate would be too low. Men typically store fat in the abdomen, so only waist and neck are used.

Can I use inches instead of centimeters?

No, the formula is designed for centimeters. If you measure in inches, multiply each measurement by 2.54 to convert to centimeters before entering. Using inches will give a wildly wrong result.

What is a healthy body fat percentage?

For men, 10–20% is generally considered healthy; for women, 18–28%. Athletes often fall below these ranges, while anything above 25% (men) or 32% (women) is considered obese. However, individual health depends on many factors, so consult a healthcare provider for personal advice.

Why does my scale say a different number?

Bioelectrical impedance scales measure body fat by sending a small electrical current through your body. They are affected by hydration, food intake, and recent exercise. The Navy method uses tape measurements and is generally more consistent if you measure correctly. Differences of 3–5% are common between methods.

Does age affect the calculation?

Age is not in the Navy formula, but body fat naturally increases with age. Some studies suggest that the formula may underestimate body fat in older adults. If you are over 60, consider using a method that accounts for age, like the BMI-based formula, or get a DEXA scan for a baseline.

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in sex | Sex | Male=1, Female=0 | 1 | 0
in age | Age | years | 30 | 18
in ht | Height | cm | 175 | 100
in neck | Neck circumference | cm | 38 | 20
in waist | Waist circumference | cm | 85 | 40
in hip | Hip circumference | cm | 95 | 40
out bf | Body fat percentage | % | (495 / (1.0324 - 0.19077 * log10(waist - neck) + 0.15456 * log10(ht))) - 450 + (sex == 1 ? 0 : 0) | 1
out bmi | BMI | kg/m² | (85 * 85) / (ht * ht) * 0 | 1