How the US Navy body fat method works (simple but powerful)

The US Navy body fat method is sometimes called the “tape test.” It estimates body fat percentage from a small set of body circumferences. The logic is straightforward: when people gain fat, certain circumferences tend to grow faster than others. When people lose fat, those same areas often shrink. Instead of trying to directly measure fat with lab equipment, the Navy equations look at the relationship between your height and your circumferences and convert that relationship into an estimated percentage.

That’s why the inputs look a little unusual. For men, the key signal is the difference between waist and neck. A bigger waist relative to neck usually corresponds to a higher body fat percentage. For women, the model adds hips because fat distribution patterns differ, and the waist + hip combination captures that distribution better.

Your body weight is made up of fat mass (stored fat) and fat‑free mass (muscle, bones, organs, water, etc.). Body fat percentage is simply:

• Body Fat % = (fat mass ÷ total body mass) × 100

This calculator is an estimator. It’s most useful when you treat it as a consistent tracking tool: if your body fat % drops over months while your strength and energy remain good, you’re likely making real progress, even if the exact number is off by a couple of percentage points.

People love body composition tools because they’re immediately actionable: the result is a single number, but it suggests a story (“I’m leaner than I thought” or “okay, I’ve got a plan”). It’s also easy to share because the inputs are simple. If you’re posting fitness updates, you can screenshot your result and compare “cut” vs “bulk” phases without needing fancy devices.

The exact Navy equations (and what to watch out for)

The Navy method uses logarithms (log base 10). Don’t worry—your tape measure doesn’t need to know math. The calculator handles the log steps for you. What matters is that the inputs are valid and consistent.

These traditional Navy equations are based on inches. If you enter centimeters, we convert cm → inches internally before calculating. That means you can pick whatever unit is comfortable and still get a correct result.

• BF% = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76

• BF% = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387

• For men: waist must be larger than neck (otherwise waist − neck ≤ 0 and log10 breaks).
• For women: waist + hip must be larger than neck (also to keep the log term positive).
• Height must be a positive number.
• Try to avoid rounding too aggressively—use one decimal place if possible.

If you enter weight, we compute:

• Fat mass = weight × (BF% ÷ 100)
• Lean mass = weight − fat mass

These two values are incredibly motivating because they show progress even when scale weight is messy (water retention, glycogen, etc.). If you lose 2 lb of fat and gain 1 lb of muscle, the scale barely moves—but your composition improved.

Worked examples you can copy

Below are realistic examples to show how the inputs translate into an output. You don’t have to do the math yourself— the point is to understand what “moves the needle.” The most influential term is usually waist relative to neck (and hips for women).

Let’s say a man measures: height 70 in, neck 15.5 in, waist 34.0 in. Waist − neck = 18.5 in (positive, good). When you run the calculation, you’ll get a body fat estimate in the typical range for an active adult. If his waist drops to 32 in (with the same height/neck), the body fat % will drop noticeably because the waist − neck term shrinks.

A woman measures: height 64 in, neck 13.5 in, waist 30.0 in, hips 40.0 in. Waist + hip − neck = 56.5 in. The calculator converts this relationship into a body fat estimate. If her waist decreases by 2 inches while hips stay similar, her body fat % trend usually improves.

If you prefer centimeters, just enter the same measurements in cm. For example, height 178 cm, neck 39.4 cm, waist 86.4 cm (these correspond roughly to the male example above). Internally we convert to inches, then compute BF% using the same Navy equations. You get the same result trend without manual conversion.

Suppose your body fat is 22% and your weight is 180 lb. Your estimated fat mass is 180 × 0.22 ≈ 39.6 lb of fat. Your estimated lean mass is 180 − 39.6 ≈ 140.4 lb of lean mass. If two months later you’re still 180 lb but your estimate drops to 19%, your fat mass becomes 34.2 lb and lean mass becomes 145.8 lb—same scale weight, much better composition story.

How to interpret your result (without overthinking it)

Body fat % ranges depend on sex, age, and goals. The labels below are not medical categories—just a practical snapshot. Many athletes sit in lean ranges; many healthy people sit in typical ranges. The most useful thing is your trend line over time.

• Very lean: under ~10%
• Lean/athletic: ~10–17%
• Typical: ~18–24%
• Higher: ~25%+

• Very lean: under ~18%
• Lean/athletic: ~18–24%
• Typical: ~25–31%
• Higher: ~32%+

• Use it as a progress metric, not a self-worth metric.
• Pair it with strength, energy, sleep, and how you feel day-to-day.
• If you’re dieting hard and your performance crashes, “lower %” might not be better right now.
• If your goal is health, your habits matter more than a single estimate.

Frequently Asked Questions

• Is the Navy method accurate?

  It’s an estimate. In real life, accuracy depends heavily on consistent measuring technique and your body shape. The Navy method tends to be “good enough” for trends (getting leaner or gaining fat) and is widely used because it’s simple and repeatable at home.

• Where exactly do I measure my waist, neck, and hips?

  For waist, many people use the level of the navel. For neck, measure just below the larynx. For women’s hips, measure around the widest part of the buttocks. The most important rule is consistency: use the same landmarks every time you measure.

• Why does the formula use log10?

  The logarithm helps the equation fit real-world body shapes better across different sizes. It essentially “compresses” very large circumference values so the relationship stays realistic rather than linear.

• My result changed a lot from last week. Did I gain fat?

  Not necessarily. Small tape differences, hydration, and posture can change measurements. That’s why a weekly average (or measuring at the same time each week) is better than reacting to a single reading.

• Should I use this or BMI?

  They answer different questions. BMI is a height-to-weight screening metric. Body fat % is closer to composition, because it estimates how much of your body is fat vs lean mass. If you lift weights, body fat % is often more informative.

• Can I use centimeters?

  Yes. Select “cm” and enter your values. We convert to inches internally because the traditional Navy formulas use inches.

• Why do women need a hip measurement?

  Fat distribution patterns differ on average. The women’s equation includes hips to capture that distribution more accurately than waist and neck alone.

• What if waist is smaller than neck?

  That’s rare and usually indicates a measurement issue (or you entered the wrong unit). The equation requires a positive value inside the log term. Re-measure and double-check units.