Number Body Fat Calculator
BMI cannot tell muscle from fat; a tape measure does better. The US Navy method estimates body fat from circumferences — neck and waist (plus hips for women) against height — and this calculator adds your category, fat mass and lean mass.
Measure with a soft tape, snug but not compressing, standing relaxed. Men leave the hip field as-is — the male formula does not use it.
How it works
The Navy formulas use logarithms of circumferences: for men, body fat % = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76; for women, 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387 (all inches). Fat mass is weight × body fat %, and lean mass is everything else — the figure that matters if you are training, because it tells you whether the scale is moving the right tissue.
Formula & worked example
Formula: US Navy method, all measurements in inches. Men: body fat % = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76. Women: 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387. Fat mass = weight × body fat %; lean mass = weight − fat mass.
Worked example: A man 70 in tall with a 34 in waist and 15 in neck measures 17.51% body fat; at 180 lb that is 31.52 lb of fat mass and 148.48 lb of lean mass. A woman 65 in tall with a 30 in waist, 38 in hips and 13 in neck measures 28.56%.
When to use this
- The scale has stalled but your waistband has not, and you want to know whether fat is actually leaving.
- You are training and need to separate fat loss from lean-mass loss instead of chasing weight alone.
- A fitness standard, sport or service uses body-fat bands and you want a tape-measure estimate before any formal test.
- Your BMI reads high because you carry muscle, and you want a shape-based check that can tell the difference.
How to use it, step by step
- Select your sex first — the female formula adds hip circumference, the male formula ignores it.
- Measure neck and waist (and hips for women) with a soft tape, snug but not compressing the skin.
- Enter height in inches and your current weight in pounds.
- Read the percentage and its category, then the fat-mass and lean-mass split of your weight.
- Retake the measurements at the same time of day every few weeks and compare the trend, not single readings.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Sex | Selects which Navy formula runs and whether hip circumference is used | Male or female |
| Height | Standing height in inches, used in both formulas | Commonly 60 to 76 inches |
| Weight | Current body weight; it splits the percentage into fat mass and lean mass but does not change the percentage itself | Morning weight is the most consistent |
| Neck | Circumference just below the Adam's apple | Often 12 to 17 inches |
| Waist | Circumference horizontal at the navel after a normal exhale | Measured, never guessed from trouser size |
| Hip | Widest point of the buttocks; women only — the male formula does not use it | Often 34 to 44 inches |
Another worked example
A woman 65 in tall with a 29 in waist, 39 in hips and a 12.5 in neck measures 29.2% body fat, inside the Average band for women. At 140 lb that works out to 40.9 lb of fat mass and 99.1 lb of lean mass — so if her weight stayed at 140 lb while the waist shrank, the lean-mass line is what would tell her the change was real progress and not just water.
Frequently asked questions
What is my body fat at a 34-inch waist and 15-inch neck?
For a man 5′10″ (70 in) tall: 17.5% — just over the top of the “Fitness” band, the first step into “Average”. At 180 lb that is 31.5 lb of fat mass and 148.5 lb of lean mass. Grow the neck or shrink the waist and the number moves fast, which is exactly what the formula is sensitive to.
What are healthy body fat ranges?
Commonly used bands for men: 2–5% essential, 6–13% athletes, 14–17% fitness, 18–24% average, 25%+ obese. For women: 10–13% essential, 14–20% athletes, 21–24% fitness, 25–31% average, 32%+ obese. Women carry more essential fat by physiology — the bands are not a double standard.
How accurate is the Navy method?
Against lab methods it is usually within roughly 3–4 percentage points when measurements are taken carefully — good enough to track change, not precise enough to argue over a single point. It beats BMI for anyone with real muscle, because it measures shape, not just weight.
Where exactly do I measure?
Neck: just below the Adam’s apple, tape angled slightly down. Waist: horizontal at the navel, after a normal exhale — not sucked in, not pushed out. Hips (women): the widest point of the buttocks. Same spots, same time of day, every time.
🥗 Feeding the lean mass?
The Macro Calculator turns a calorie target into daily protein, carb and fat grams — the numbers that decide what the lean mass does next.
Why does changing my weight not change the percentage?
The percentage comes only from circumferences and height — weight never enters the Navy formulas. Weight is used afterwards to turn that percentage into pounds: fat mass is weight multiplied by the percentage, and lean mass is the rest. Weighing more with identical measurements raises both pound figures but leaves the percentage untouched.
Why does the calculator ask me to check the measurements?
The formulas take logarithms of waist minus neck for men, or waist plus hip minus neck for women. If that difference is zero or negative the logarithm does not exist, so the tool stops instead of inventing a number. In practice it almost always means the waist and neck tapes were swapped, or the waist was measured somewhere other than the navel.