Body Fat Percentage Calculator

What Is Body Fat Percentage?

Body fat percentage is the proportion of your total body mass that's made up of fat tissue. A 180lb person at 15% body fat carries 27lbs of fat and 153lbs of lean mass (muscle, bone, water, organs).

Unlike BMI, body fat percentage directly measures composition. Two people can weigh the same but look completely different — the one with lower body fat will appear leaner and more muscular because they carry more lean mass relative to fat.

How Is It Calculated?

This calculator uses two validated methods:

• US Navy Method (Hodgdon & Beckett, 1984): Uses circumference measurements (neck, waist, and hip for women) along with height. Validated against hydrostatic weighing with an accuracy of ±3-4%. This is the primary method the US military uses for fitness assessments.
• BMI Method (Deurenberg et al., 1991): Estimates body fat from BMI, age, and gender using the formula 1.20 × BMI + 0.23 × age - 10.8 × gender - 5.4. Less accurate for muscular individuals but requires no measurements beyond height and weight.

The Navy method is generally more accurate for lifters because it accounts for body shape, not just weight. If you carry significant muscle mass, the BMI method will overestimate your body fat.

ACE Body Fat Categories

The American Council on Exercise (ACE) classifies body fat into five categories based on gender:

• Essential Fat: 2-6% men, 10-14% women. The minimum fat needed for basic physiological function. Competitive bodybuilders reach this range briefly for stage. Not sustainable long-term.
• Athletic: 6-14% men, 14-21% women. Lean, visible muscle definition. Common in competitive athletes and dedicated lifters.
• Fitness: 14-18% men, 21-25% women. Healthy, active appearance. Most people training consistently land here.
• Average: 18-25% men, 25-32% women. The general population average. No visible abs, but not overweight by health metrics.
• Obese: 25%+ men, 32%+ women. Associated with increased health risks including cardiovascular disease and type 2 diabetes.

Measurement Tips for Accuracy

• Neck: Measure just below the larynx (Adam's apple). Keep the tape level and snug.
• Waist: Measure at navel level. Keep the tape parallel to the floor. Don't suck in.
• Hips (women): Measure at the widest point of the buttocks. Keep the tape level.
• Take each measurement 2-3 times and use the average.
• Measure at the same time of day for consistent tracking (morning, before eating).
• Use a flexible tape measure, not a metal one.

Navy Method vs. Other Methods

There are several ways to measure body fat, each with different accuracy and practicality trade-offs:

• DEXA scan: Gold standard, ±1-2% accuracy. Costs $50-150 per scan. Shows regional fat distribution.
• Hydrostatic weighing: ±2% accuracy. Requires being submerged in water. Available at some universities and sports clinics.
• Navy method: ±3-4% accuracy. Free, requires only a tape measure. Best balance of accuracy and accessibility for regular tracking.
• Bioelectrical impedance (BIA): ±4-8% accuracy. Common in smart scales. Results vary significantly with hydration status.
• Skinfold calipers: ±3-5% accuracy. Requires trained administrator. Multiple protocols exist (Jackson-Pollock 3-site, 7-site).

For regular tracking, the Navy method hits the sweet spot: accurate enough to detect real changes, free, and reproducible at home. Track your measurements every 2-4 weeks to see trends.

Why Body Fat Matters for Training

Your body fat percentage affects training decisions. During a cut (caloric deficit), higher body fat means you can lose fat faster without losing muscle — Helms et al. (2014) recommend a 0.5-1% bodyweight loss per week for lean athletes. During a bulk, starting leaner (under 15% for men) means better nutrient partitioning — more calories go to muscle, fewer to fat.

RepStack uses your body composition data alongside your training progress to make smarter recommendations about when to push harder and when to pull back.