Body Fat Percentage

How the U.S. Navy Body Fat Method Works

Body fat percentage is the share of your total body weight that’s made up of fat, as opposed to lean mass like muscle, bone, and water — the U.S. Navy Circumference Method estimates it from a few tape-measure readings instead of a scale, calipers, or scan. Developed for military body composition assessments, it uses only your height, neck, and waist measurements (plus a hip measurement for the female formula), and this calculator estimates your body fat percentage and which standard category it falls into.

This method is widely used precisely because it’s simple and repeatable, but it’s still an estimate based on a statistical relationship between circumferences and body fat, not a direct measurement. More precise methods (DEXA scans, hydrostatic weighing, or skinfold calipers used by a trained professional) exist, but require specialized equipment this calculator can’t replicate.

Key Factors to Consider

• Measurement technique makes a real, meaningful difference in the result. A tape measured too loosely or compressed too tightly against the skin, or measured at the wrong landmark, can shift the circumference reading enough to noticeably change the estimated body fat percentage — measure carefully, and consider having someone else take the measurement for consistency.
• This method can be less accurate for body types outside the population it was validated on. Very muscular or very lean individuals are more likely to see a meaningfully off estimate from any circumference-based method, since these formulas were derived from population averages rather than a direct measurement of fat itself.
• Tracking the trend over time is often more useful than any single reading. Since measurement technique and normal day-to-day body fluctuations both affect the result, taking measurements consistently (same time of day, same technique) and watching the trend over weeks or months gives more reliable information than treating any one measurement as precise.
• Essential body fat differs meaningfully between men and women, and it isn’t optional to reduce. A certain amount of body fat is necessary for normal hormonal and physiological function — women naturally carry more essential fat than men, which is reflected in the different category thresholds used for each.

Interpreting Your Results

The percentage this calculator returns is compared against five standard categories (American Council on Exercise) — the exact cutoffs differ between men and women, since a healthy amount of essential fat is higher for women:

A few things worth keeping in mind when reading which category you land in:

• “Essential Fat” is a floor, not a target. It’s the minimum body fat needed for normal hormone production, organ protection, and temperature regulation — sitting at or below it isn’t automatically a fitness achievement, and sustained levels this low can disrupt hormonal function.
• “Athletes” and “Fitness” describe body composition, not actual athletic performance. Landing in one of these bands from a tape measurement alone doesn’t mean you train at that level, and vice versa — plenty of genuinely fit people sit in the “Average” band.
• The category is a snapshot, not a diagnosis. It’s a useful reference point for where your estimated body fat sits relative to these commonly-cited ranges, not a medical assessment — see a healthcare provider for a clinical body composition evaluation.

Common Mistakes

• Measuring at the wrong landmark. The neck measurement goes just below the larynx (Adam’s apple), and the waist measurement goes at the navel — measuring an inch or two off from either landmark can shift the result more than most people expect, since the formula is sensitive to small circumference changes.
• Measuring right after eating, drinking, or exercising. A full stomach, water retention, or post-workout bloating can all temporarily inflate your waist measurement, pushing the estimate higher than your actual, rested body fat percentage.
• Sucking in your stomach or pulling the tape too tight. Both understate your true waist circumference, which will make this method report a lower body fat percentage than is actually accurate — measure standing relaxed, with the tape snug but not compressing the skin.
• Comparing results across different measurement methods as if they’re interchangeable. A circumference-based estimate, a body fat scale (bioelectrical impedance), and a DEXA scan can each give a meaningfully different number for the same person, since they measure body composition in fundamentally different ways — track your trend using one method consistently rather than comparing absolute numbers across methods.

The Formula

Men:

$$\text{Body Fat \%} = 86.010 \times \log_{10}\left(\vA{\text{Waist}} - \vB{\text{Neck}}\right) - 70.041 \times \log_{10}\left(\vC{\text{Height}}\right) + 36.76$$

Women:

$$\text{Body Fat \%} = 163.205 \times \log_{10}\left(\vA{\text{Waist}} + \vD{\text{Hip}} - \vB{\text{Neck}}\right) - 97.684 \times \log_{10}\left(\vC{\text{Height}}\right) - 78.387$$

All measurements use the same unit (inches, in the formula’s original published form — this calculator converts automatically regardless of which unit you enter in). $\vA{\text{Waist}}$ is measured at the navel; $\vB{\text{Neck}}$ just below the larynx; $\vD{\text{Hip}}$ at its widest point (women’s formula only).

The result is then compared against the standard body-fat-percentage categories (American Council on Exercise): Essential Fat, Athletes, Fitness, Average, and Obese — the ranges differ between men and women since essential (health-necessary) body fat levels differ by gender.

Worked Examples

Male, height 180 cm, neck 38 cm, waist 85 cm:

1. Converted to inches: height ≈ 70.9 in, neck ≈ 15.0 in, waist ≈ 33.5 in.
2. $86.010 \times \log_{10}\left(\vA{33.5} - \vB{15.0}\right) - 70.041 \times \log_{10}\left(\vC{70.9}\right) + 36.76 \approx 16.2\%$.
3. 16.2% falls in the Fitness category for men (14–17%).

Female, height 165 cm, neck 32 cm, waist 75 cm, hip 97 cm:

1. Converted to inches: height ≈ 65.0 in, neck ≈ 12.6 in, waist ≈ 29.5 in, hip ≈ 38.2 in.
2. $163.205 \times \log_{10}\left(\vA{29.5} + \vD{38.2} - \vB{12.6}\right) - 97.684 \times \log_{10}\left(\vC{65.0}\right) - 78.387 \approx 28.7\%$.
3. 28.7% falls in the Average category for women (25–31%).

Useful to Know

Measure first thing in the morning, before eating or drinking, for the most consistent readings. Your waist circumference naturally fluctuates throughout the day with food, fluid intake, and even how long you’ve been standing — measuring at roughly the same time each day (most people find right after waking, before breakfast, the easiest to repeat) removes one of the biggest sources of day-to-day variation, so a change in your tracked trend is more likely to reflect a real change in body composition rather than just when you happened to measure.