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Estimate your body fat percentage in seconds with our free body fat calculator, using the U.S. Navy tape-measure method trusted by the military and fitness coaches.
Choose your gender and enter height, waist (at the navel) and neck (below the larynx) in cm — women also enter hip circumference. The calculator converts to inches and applies the U.S. Navy body fat formula.
Formula: Men: 86.010×log10(waist−neck) − 70.041×log10(height) + 36.76. Women: 163.205×log10(waist+hip−neck) − 97.684×log10(height) − 78.387.
Getting an accurate body fat reading from the calculator above takes less than two minutes with a soft tape measure. The quality of your result depends entirely on measuring in the right places, so follow these steps carefully.
Tip: use a flexible tailor’s tape, not a metal construction tape, and have someone help you keep the tape level around your waist for the most consistent readings.
Your body fat percentage is the share of your total body weight that is fat tissue, with everything else, muscle, bone, water, and organs, counted as lean body mass. A man who weighs 82 kg at 18.8% body fat carries about 15.4 kg of fat and 66.6 kg of lean mass. This single number describes your body composition far better than weight alone, because two people can weigh exactly the same while one is lean and muscular and the other carries excess fat.
Not all body fat is bad. Essential fat, roughly 3–5% in men and 10–13% in women, is the fat in your bone marrow, organs, and nervous system that you need to stay alive. Above that is storage fat, which sits under the skin as subcutaneous fat and around the organs as visceral fat. A moderate amount of storage fat is healthy and normal; it is the excess, particularly visceral fat around the waist, that is linked to metabolic and cardiovascular risk. That is exactly why the Navy formula leans so heavily on the waist measurement.
Body fat percentage also explains the blind spot of the Body Mass Index. BMI divides weight by height squared, so a muscular athlete and a sedentary person of the same height and weight get the same BMI, often tipping the athlete into the “overweight” category. Body fat percentage separates the two correctly: the athlete might be 12% while the sedentary person is 28%. If you lift weights or play sports, body fat percentage is the metric to watch, and BMI is at best a rough screening tool.
Healthy reference ranges used by fitness professionals are approximately 14–17% (fit) and 18–24% (average) for men, and 21–24% (fit) and 25–31% (average) for women. Women naturally carry more essential and storage fat because of hormonal differences, so the scales are not comparable between genders. The tape-measure method estimates your percentage to within about ±3–4% of a DEXA scan, the laboratory gold standard, which makes it more than accurate enough for tracking progress at home for free.
The calculator uses the circumference equations developed by the U.S. Navy, which predict body fat from a few tape measurements plus height. All measurements go into the formula in inches (your centimeter entries are converted automatically):
Men: %BF = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76
Women: %BF = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387
The logic is elegant: the waist-minus-neck term captures abdominal fat while subtracting frame size, and height scales the result to body size. For women, the hip measurement captures lower-body fat stores. Below are two fully worked examples.
Worked example 1 (man): height 178 cm, waist 90 cm, neck 40 cm. First convert to inches by dividing by 2.54: height = 70.08 in, waist = 35.43 in, neck = 15.75 in. Compute waist − neck = 35.43 − 15.75 = 19.69 in. Take base-10 logarithms: log10(19.69) ≈ 1.2942 and log10(70.08) ≈ 1.8456. Now apply the men’s formula: 86.010 × 1.2942 = 111.31, and 70.041 × 1.8456 = 129.27. Finally, %BF = 111.31 − 129.27 + 36.76 = 18.8%, which falls in the Average range for men (18–24%). This matches the calculator’s own default result exactly.
Worked example 2 (woman): height 165 cm, waist 75 cm, hip 100 cm, neck 34 cm. Convert to inches: height = 64.96 in, waist = 29.53 in, hip = 39.37 in, neck = 13.39 in. Compute waist + hip − neck = 29.53 + 39.37 − 13.39 = 55.51 in. Logarithms: log10(55.51) ≈ 1.7443 and log10(64.96) ≈ 1.8127. Apply the women’s formula: 163.205 × 1.7443 = 284.68, and 97.684 × 1.8127 = 177.07. Then %BF = 284.68 − 177.07 − 78.387 = 29.2%, in the Average range for women (25–31%). Notice how the hip term adds roughly 100 points inside the logarithm expression before the height term scales it back, reflecting real female fat distribution.
You can reproduce either calculation in a spreadsheet with =LOG10(). The key takeaway from the math: because the formula uses logarithms, small measurement errors barely move the result, but consistent technique matters far more than perfect precision.
Meet James, a 35-year-old office worker, 178 cm tall and 82 kg, who measured a 90 cm waist and 40 cm neck. Plugging his numbers into the calculator gives 18.8% body fat, squarely Average. His fat mass is 82 × 0.188 = 15.4 kg, leaving 66.6 kg of lean mass. He decides to lose fat while keeping his muscle through a moderate calorie deficit, 1.8 g of protein per kilogram of body weight daily, and three full-body strength sessions per week.
Twelve weeks later James weighs 78 kg, and his waist measures 84 cm while his neck is unchanged at 40 cm. Re-running the numbers: waist = 84 ÷ 2.54 = 33.07 in, waist − neck = 33.07 − 15.75 = 17.32 in, log10(17.32) ≈ 1.2386. Then 86.010 × 1.2386 = 106.53, and %BF = 106.53 − 129.27 + 36.76 = 14.0%, now in the Fit range. His new fat mass is 78 × 0.140 = 10.9 kg and his lean mass is 67.1 kg. In other words, James lost 4.5 kg of pure fat while his lean mass actually rose slightly, exactly the outcome the scale alone could never have shown him, since it only reported a 4 kg drop.
The practical lesson: six centimeters off the waist moved James across a full category boundary, from Average to Fit, even though his weight changed modestly. When you re-measure, always use the same tape, the same time of day, and the same relaxed posture, and log each result with the date so the trend is visible. Pair the body fat reading with your BMR to set calorie targets that protect muscle while the fat comes off.
1. Measuring the waist anywhere except the navel. The formula was calibrated on waist circumference at the belly button. Measuring higher (at the narrowest point) or lower (at the hips) feeds the equation a number it was not designed for and can shift your result by several percentage points.
2. Pulling the tape too tight or sucking in your stomach. Compressing the tissue or holding your breath artificially shrinks the waist reading and flatters the result. The tape should be snug against the skin with no gaps, but it must not indent the flesh. Breathe out normally and read the number at the end of a normal exhale.
3. Measuring at inconsistent times of day. Your waist is smallest in the morning and expands after meals, water intake, and a day of sitting. A post-dinner measurement can read 2–3 cm larger than a morning one. Always measure first thing in the morning, before eating, for comparable readings.
4. Mixing up units or entering the wrong fields. Entering inches where centimeters are expected (or vice versa) produces nonsense results, and men should not enter a hip value since the men’s equation ignores it. Double-check that each number sits in its labeled field before calculating.
5. Overreacting to a single reading. Tape measurements carry about ±3–4% error versus DEXA, and hydration, posture, and tape placement add noise. One reading that jumps a point means nothing; four readings over two months trending downward means everything. Judge your plan by the trend line, not by any single dot.
This page provides a free body fat calculator using the U.S. Navy circumference method: for men, %BF = 86.010×log10(waist−neck) − 70.041×log10(height) + 36.76; for women, %BF = 163.205×log10(waist+hip−neck) − 97.684×log10(height) − 78.387 (measurements in inches, converted automatically from cm). Worked examples: a 178 cm man with a 90 cm waist and 40 cm neck measures 18.8% (Average); a 165 cm woman with a 75 cm waist, 100 cm hips, and 34 cm neck measures 29.2% (Average). A practical case shows a 6 cm waist reduction moving a man from 18.8% to 14.0% (Fit). The page explains body fat percentage concepts, healthy ranges (men 14–17% fit / 18–24% average; women 21–24% fit / 25–31% average), five measurement mistakes, and notes ±3–4% accuracy versus DEXA.
Within about ±3–4% of DEXA scans for most people — good enough for tracking trends.
For men: 14–17% is fit, 18–24% average; for women: 21–24% fit, 25–31% average.
Neck size correlates with frame/muscle while waist reflects abdominal fat — the difference, scaled by height, predicts total body fat well.
Body fat percentage — BMI can’t distinguish muscle from fat.
A modest calorie deficit with high protein intake plus resistance training. Re-measure every 2–4 weeks.
Either. The calculator accepts centimeters and converts them to inches automatically, because the U.S. Navy formula was developed using inches. What matters is consistency: always use the same unit and the same tape. If you switch from cm to inches halfway through your tracking, the conversion is exact (1 inch = 2.54 cm), so your trend stays valid, but sticking with one unit avoids confusion.
The women’s Navy equation was calibrated on female fat-distribution patterns, where the hips and thighs are a major fat depot (gluteofemoral fat). Including the hip circumference lets the formula account for lower-body fat that the waist measurement misses. Men store proportionally more fat abdominally, so the men’s equation achieves its accuracy from the waist-minus-neck difference alone and has no hip term.
Every two to four weeks, under identical conditions: first thing in the morning, before eating, with the same tape and posture. Daily or weekly readings are dominated by noise from water retention, meals, and tape placement (about ±3–4% versus DEXA). A monthly trend line over two to three months is what tells you whether your nutrition and training plan is actually reducing body fat.
Also try our Calorie Calculator, BMI Calculator and BMR Calculator — or browse all Fitness & Health Calculators.
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