Body Fat Calculator

A formula developed by Navy researchers (Hodgdon & Beckett, 1984) that estimates body fat percentage from a few tape-measure circumferences plus height. It needs neck and waist for men, and neck, waist, and hip for women. It correlates well with hydrostatic weighing — typically within ±3–4% body fat for adults with typical fat distribution.

Good — but it's still an estimate. The Navy method assumes you store fat in roughly average locations. Very muscular men with thick necks or very lean people with low-volume waists can get readings several percentage points off. For precise measurement, DEXA, hydrostatic weighing, or BodPod are the gold standards.

Use a flexible cloth tape, snug but not compressing the skin, and keep the tape level all the way around. Neck: just below the larynx. Waist (men): at the navel. Waist (women): at the narrowest point. Hip (women): at the widest point. Measure first thing in the morning before eating for the most consistent readings, and average two measurements.

American Council on Exercise scale — men: essential fat <6%, athletes 6–13%, fitness 14–17%, average 18–24%, above average ≥25%. Women: essential fat <14%, athletes 14–20%, fitness 21–24%, average 25–31%, above average ≥32%. Higher essential ranges in women reflect reproductive physiology. The right range for any one person depends on age, training level, and goals.

The Navy formula was fit separately for men and women. The men's equation uses just waist − neck, while the women's equation uses waist + hip − neck because women's body fat distributes differently and the hip circumference adds predictive power. Without it, the women's formula loses accuracy.

For men: 14-24% is considered healthy. Below 6% is dangerously low for sustained periods. Above 25% indicates obesity. For women: 21-31% is healthy. Below 14% is dangerously low and can cause hormonal disruption. Above 32% indicates obesity. Athletes and bodybuilders often operate below these ranges temporarily for competition.

A safe, sustainable rate is 0.5-1% body fat per month for most people. Faster loss (2-3% per month) is possible with aggressive dieting but typically causes muscle loss and rebound. Body recomposition (losing fat while gaining muscle) happens slowly — typically 2-4 months to see noticeable change. Be skeptical of programs promising rapid dramatic changes.

Hydration is the biggest factor. Water makes up about 60% of lean mass — when you're dehydrated, scales and tape-measure calculations both show inflated body fat percentages. Time of day matters too (you're typically leanest in the morning). Sodium, recent carb intake, and menstrual cycle (for women) also cause 1-3% fluctuations. Measure at the same time, same conditions weekly — don't trust daily changes.

Yes — it's called "skinny fat." Someone can have a low BMI (normal weight) but high body fat percentage if they have very little muscle mass. Common in sedentary people who never exercised but didn't overeat. Health risks are similar to obesity despite a normal-looking body weight. The fix is strength training, not more dieting.

Less accurate. The Navy method tends to overestimate body fat in muscular individuals (because muscular shoulders and necks throw off the formula) and underestimate it in skinny-fat individuals. For athletes, a DEXA scan or Bod Pod is significantly more accurate. For general fitness tracking, the Navy method works well — just don't compare your number to professional athletes measured with different methods.