Body Fat Percentage Calculator (US Navy Method)

What body fat percentage measures

Body fat percentage is the share of your total body mass made up of fat tissue (the rest is “lean body mass” — muscle, bone, water, organs). Two people who weigh the same can have wildly different body-fat percentages, which is why pure body-weight goals are misleading. Body fat percentage is a much better composition signal than weight or BMI alone.

Reference ranges (American Council on Exercise, 2009):

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%+	32%+

These bands are descriptive, not prescriptive. Long-term health depends much more on metabolic markers, fitness, and behavior than on hitting a specific category.

The US Navy circumference method

In 1984, Hodgdon and Beckett at the Naval Health Research Center published a circumference-based body-fat estimation method validated against hydrostatic weighing for active-duty service members. It has remained the de facto field method for decades because it requires only a tape measure and produces respectable accuracy.

For men

Body fat % = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76

All measurements in inches. The formula uses common (base-10) logarithms.

For women

Body fat % = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387

Again, inches and base-10 logs. The hip measurement improves accuracy for the female fat-distribution pattern.

In metric units, convert cm to inches (1 in = 2.54 cm) before plugging in. This calculator handles the conversion automatically.

Where and how to measure

The single biggest source of error is sloppy measurement. Use a flexible (cloth or fiberglass) tape, not a rigid one.

• Neck: Wrap the tape just below your larynx (Adam’s apple), perpendicular to the long axis of the neck. Keep your head straight and your gaze level. Don’t flex the neck or shrug.
• Waist: At the level of your navel, tape horizontal, after a normal exhale. Don’t suck in. Don’t push out. Stand naturally.
• Hips (women): At the widest part of your buttocks, feet together, tape horizontal.

Take each measurement three times in a row, dropping the tape between each, and average. If readings differ by more than 0.5 cm, try again with more attention to tape angle and tension.

A worked example

A 30-year-old man, 178 cm tall, with a 90-cm waist and 38-cm neck:

Convert to inches: 178 cm = 70.08 in, 90 cm = 35.43 in, 38 cm = 14.96 in.

Body fat % = 86.010 × log10(35.43 − 14.96) − 70.041 × log10(70.08) + 36.76
           = 86.010 × log10(20.47) − 70.041 × log10(70.08) + 36.76
           = 86.010 × 1.3113 − 70.041 × 1.8456 + 36.76
           = 112.76 − 129.27 + 36.76
           = 20.25%

This man would be in the “Average” category — a healthy starting point, with body recomposition (build muscle, lose fat) likely taking him to “Fitness” or “Athletes” with consistent training.

How to use this calculator

1. Toggle to metric or imperial.
2. Enter your height, neck, and waist measurements (and hip if female).
3. The result appears immediately, with your category band.
4. Track the same three measurements weekly to watch the trend over time.

Tracking changes over time

A single body-fat percentage estimate is noisy. The change over time is what matters. A practical workflow:

1. Measure first thing in the morning, fasted, after using the bathroom.
2. Measure on the same day of the week (Monday morning is a common choice).
3. Log all three (or four) circumferences, not just the body-fat percentage. Circumference is the underlying signal; the percentage is a derived number.
4. Compare 4-week rolling averages, not week-to-week. Day-to-day fluctuations of 1–2 cm in waist circumference are normal.

If your waist drops by 1 cm over four weeks while your weight stays flat, you’re recompositioning — gaining muscle while losing fat. That’s exactly what most lifters want to see.

When the Navy method is least accurate

• Body fat below 8% (men) or 14% (women). The formula systematically overestimates. Use a 7-site skinfold or DXA.
• Body fat above 35% (men) or 42% (women). The formula was not validated here.
• Very tall (> 200 cm) or very short (< 150 cm) adults. The height term breaks down outside its sample.
• Pregnant women. Don’t use it.
• Bodybuilders peaking for competition. Get a DXA scan or use professional skinfold calipers.

How the Navy method compares to other methods

Method	Accuracy (typical)	Cost	Speed	Notes
DXA scan	±1–2%	$50–150 per scan	10 minutes	Reference standard; regional info
Hydrostatic weighing	±2%	$50–100	30 minutes	Reference grade; uncomfortable
Bod Pod (air displacement)	±2%	$50–100	10 minutes	Comparable to hydrostatic
7-site skinfolds (Jackson-Pollock)	±3%	Caliper one-time cost	10 minutes	Tester skill matters a lot
US Navy circumference	±3%	Free (tape only)	2 minutes	No expert required
Bioelectrical impedance (BIA)	±4–5%	$30–800 device	30 seconds	Hydration sensitive
Visual estimation	±5–10%	Free	Instant	Noisy; use for plausibility check

For most lifters and recreational athletes, the US Navy method hits the sweet spot of accuracy, accessibility, and repeatability.