Body fat
calculator
Estimate body fat percentage from four tape measurements using the U.S. Navy method (Hodgdon-Beckett, 1984), with a BMI classification, a lean-mass split, and two things most calculators skip: how much a mismeasured tape moves the number, and how that number sets your Katch-McArdle BMR. Imperial or metric.
How the math works
The Navy method skips calipers and uses circumferences instead. It fits body fat to the log of a girth measurement and your height. For men it needs waist, neck, and height; for women it adds the hip. All measurements are in inches in the formula below (the calculator converts centimetres for you).
Women: %BF = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387
Source: Hodgdon & Beckett, Naval Health Research Center, 1984. Lean body mass is then just weight × (1 − body fat / 100), and fat mass is the rest. BMI is reported alongside as weight(kg) / height(m)² with WHO categories — a different number measuring a different thing (see below).
Worked example
A 40-year-old man, 6'0" (183 cm), 195 lb (88.5 kg), with a 36" (91.4 cm) waist and a 16" (40.6 cm) neck:
- Body fat: 86.010 × log10(36 − 16) − 70.041 × log10(72) + 36.76 = 18.6% (18.57% before rounding)
- Lean body mass: 195 × (1 − 0.1857) = 158.8 lb (72.0 kg)
- Fat mass: 195 × 0.1857 = 36.2 lb (16.4 kg)
- BMI: 88.5 / 1.829² = 26.4 — "overweight" by WHO, at 18.6% body fat
The women's equation runs the same way with a hip measurement. A 30-year-old woman, 5'5" (165 cm), 150 lb (68.0 kg), waist 30" (76.2 cm), hip 40" (101.6 cm), neck 13" (33.0 cm): 163.205 × log10(30 + 40 − 13) − 97.684 × log10(65) − 78.387 = 31.1% body fat, for a lean mass of 103.4 lb (46.9 kg).
When this calculator is wrong
Body fat is worth measuring mostly because lean mass drives your metabolic rate, and Katch-McArdle BMR uses lean mass directly. For lean or muscular people, that makes Katch-McArdle more accurate than a weight-and-height formula like Mifflin-St Jeor, which can underestimate by 5–10% when muscle mass is high. But that accuracy is only as good as the body fat number you feed it — and a tape estimate is a noisy input.
- The measurement error propagates. The Navy method carries about 3.5 percentage points of error against hydrostatic weighing, and it turns on log10(waist − neck): a 1-inch waist mistake alone moves the estimate by roughly 1.8 points. Push that through Katch-McArdle and the BMR shifts too. If your body fat came from a tape held loosely or at the wrong spot, Katch-McArdle is no more trustworthy than Mifflin.
- It was validated on a specific population. Hodgdon and Beckett fit the equations on U.S. Navy personnel. Very lean physique athletes, very heavy individuals, and people carrying fat mostly outside the waist all sit at the edges of the data, where the estimate drifts.
- BMI and body fat disagree on purpose. The worked example lands at "overweight" on BMI and a fitness-range body fat at the same time. BMI can't tell muscle from fat; a lifter routinely reads "overweight" by BMI while measuring lean by tape. Use them as two separate readings, not a cross-check.
What to do with the result
Treat the single number as approximate and the trend as the signal. Re-measure every 2–4 weeks, same tape, same spots, same time of day, and watch the direction — a body fat estimate that drops 1.5 points over a month is more informative than any one reading's decimal place.
Then use the lean-mass figure, which is the durable output. Feed it into a Katch-McArdle BMR to set calories, or anchor protein to lean mass in the macro calculator — protein per pound of lean mass is the more defensible target for very lean or very heavy people. If you need a body fat number for anything medical, get a DEXA scan; the tape is for tracking, not for diagnosis.
Common questions
- How accurate is the Navy body fat calculator?
- Its standard error against hydrostatic weighing is about 3.5 percentage points (Hodgdon & Beckett, 1984). That means a reading of 18% could reasonably be anywhere from about 15% to 21%. It's good enough to track change over time and far cheaper than a lab, but it is not a substitute for DEXA or hydrostatic weighing when you need an accurate absolute number.
- How do I measure my waist and neck correctly?
- Waist at the navel, tape level all the way round, relaxed — not sucked in. Neck just below the larynx, sloping slightly down at the front. Hip (women) at the widest point. Keep the tape snug but not compressing the skin, and use the same landmarks every time. Because a 1-inch waist error moves the result by roughly 1.8 points, consistent placement matters more than which formula you use.
- What's a healthy body fat percentage?
- Using the ACE population ranges: for men, 6–13% is the athlete band, 14–17% fitness, and 18–24% acceptable. For women it's 14–20% athlete, 21–24% fitness, and 25–31% acceptable. These are population norms, not goals — essential fat sits at 2–5% for men and 10–13% for women, and going below that carries health risk.
- Is BMI or body fat percentage better?
- They answer different questions. BMI is weight scaled for height and can't separate muscle from fat, so it flags many lifters as "overweight." Body fat percentage estimates the fat fraction directly. For anyone training, body fat and lean mass are the more useful pair; BMI is a population screening tool, not an individual body-composition measure.
- Can I use my body fat percentage to calculate calories?
- Yes — that's the main reason to measure it. Body fat gives you lean body mass, and the Katch-McArdle BMR formula (370 + 21.6 × lean mass in kg) uses lean mass rather than total weight, which makes it more accurate for lean or muscular people. Our TDEE calculator runs Katch-McArdle when you enter a body fat number.
- Why do different calculators give me different body fat numbers?
- Mostly because they use different methods. A Navy-tape estimate, a skinfold formula (Jackson-Pollock), a bioimpedance scale, and a DEXA scan all measure body fat a different way and disagree by several points. Even within one method, the tape reading you take today versus tomorrow can differ. Pick one method, keep the conditions constant, and compare against yourself over time rather than against another calculator.