What is Body Fat Percentage?
Body fat percentage is the proportion of your total weight that is fat tissue. The remaining percentage is lean body mass — everything that is not fat: muscles, bones, organs, blood, and water. Unlike BMI, which only considers height and weight, body fat percentage gives you direct information about your body composition.
Two people can weigh exactly the same and have the same BMI while having very different body compositions. A person with 15% body fat and one with 30% body fat at the same weight represent completely different health and fitness profiles.
The US Navy Circumference Method
This calculator uses the US Navy body fat formula, one of the most widely validated field-measurement methods. It requires only a measuring tape — no calipers, no scale, no water tank.
For Men:
BF% = 495 / (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450
For Women:
BF% = 495 / (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450
All measurements in inches; height in inches.
Why does the women's formula include hip measurement? Women naturally accumulate more fat in the hip and thigh region. Including the hip circumference in the formula provides a more accurate estimate of total body fat for women, where hip fat is a significant component of the fat distribution pattern.
Body Fat Categories
The American Council on Exercise (ACE) defines the following body fat categories:
| Category | Men | Women | Notes |
|---|---|---|---|
| Essential Fat | 2–5% | 10–13% | Minimum for organ protection; below this is dangerous |
| Athletic | 6–13% | 14–20% | Competitive athletes, high physical demands |
| Fitness | 14–17% | 21–24% | Active lifestyle, good health markers |
| Acceptable | 18–24% | 25–31% | Generally healthy; some health risk factors may appear |
| Obese | 25%+ | 32%+ | Elevated risk for metabolic disease, cardiovascular issues |
How to Take Accurate Measurements
The accuracy of the US Navy method depends entirely on consistent, correct measurements. Follow these guidelines:
Neck: Measure just below the larynx (the Adam's apple), with the tape perpendicular to the long axis of the neck. Do not flex the neck muscles. The tape should be snug but not compressing skin.
Waist (men): Measure at the navel (belly button level). Stand relaxed, exhale normally, and measure without sucking in or tensing the abdomen. This measurement represents where men typically carry central fat.
Waist (women): Measure at the narrowest point of the abdomen, which is typically 1–2 inches above the navel. Exhale normally before measuring.
Hip (women only): Measure at the widest point of the hips and buttocks. Keep the tape level and parallel to the floor. Take the measurement with the feet together.
For best results, take three measurements and use the average. Measure at the same time of day, as body measurements fluctuate with hydration, digestion, and swelling.
Lean Body Mass and Why It Matters
Lean body mass (LBM) = Total body weight × (1 − Body fat % / 100)
LBM is important for several reasons. In medicine, drug dosages (antibiotics, anesthetics) are often calculated based on lean body mass rather than total weight to avoid overdosing obese patients. In nutrition, protein requirements scale with lean body mass — 1.6–2.2 g of protein per kg of LBM is the research-supported range for muscle protein synthesis. In fitness, tracking LBM over time reveals whether you are gaining muscle, losing fat, or both.
Frequently Asked Questions
Which body fat measurement method is most accurate?
From most to least accurate: (1) Hydrostatic weighing — gold standard, measures water displacement, error ±1.5-2%. (2) DEXA scan — X-ray based, also shows bone density, error ±1-2%. (3) Air displacement plethysmography (Bod Pod) — measures air displacement, error ±2-3%. (4) Skinfold calipers — trained technique required, error ±3-4%. (5) Bioelectrical impedance (body fat scales) — highly variable, error ±3-8% depending on hydration. (6) US Navy circumference method — tape measure only, error ±3-4%. The Navy method compares well with calipers and is far easier to do at home consistently.
Can I reduce body fat in specific areas (spot reduction)?
No. Spot reduction is a fitness myth. Exercise burns fat systemically — your body chooses where to mobilize fat based on genetics and hormones, not which muscles you exercise. High reps of crunches will strengthen your abdominal muscles but will not preferentially burn belly fat. Overall fat loss through calorie deficit combined with resistance training reduces fat across the body, with the pattern varying per individual.
How often should I measure body fat?
Body fat percentage changes slowly. Measuring more than once every 2–4 weeks creates noise that obscures the signal. Meaningful changes require weeks to months. Use the same measurement method each time, under the same conditions (time of day, hydration state), to ensure valid comparisons. Don't chase daily fluctuations — they are mostly water.
Why do body fat scales give different results each time?
Bioelectrical impedance scales measure electrical resistance through your body. Hydration affects electrical conductivity dramatically — being dehydrated, or having drunk a large amount of water, can shift the reading by 3–5%. Time of day also matters: first thing in the morning (fasted, after using the bathroom) gives the most consistent results. That's why circumference methods are often more practically consistent for home tracking.