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Body Fat Percentage Calculator

Percent body fat equals total weight minus lean mass divided by total weight times 100

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How It Works

Body fat percentage from lean body mass and total weight uses the simple identity: BF% = (W − LBM) / W × 100, where W is your total body weight and LBM is your lean body mass (everything that isn't fat — muscle, bone, water, organs). This page does NOT derive lean mass from skinfold calipers or DEXA scans; you must provide it. If you have a DEXA, BodPod, or hydrostatic measurement of lean mass, this calculator converts the relationship into a percentage with the work shown.

Example Problem

An athlete weighs 70 kg total. A DEXA scan reports 55 kg of lean body mass. What is the body fat percentage?

  1. Use the formula: BF% = (W − LBM) / W × 100.
  2. Substitute: BF% = (70 − 55) / 70 × 100.
  3. Subtract: 70 − 55 = 15.
  4. Divide: 15 / 70 ≈ 0.2143.
  5. Multiply by 100: 0.2143 × 100 ≈ 21.4%.

Key Concepts

Body fat percentage is a better health indicator than BMI for athletes and people with high muscle mass, but it requires an actual measurement of lean body mass — not an estimate. Common measurement methods (most → least accurate): hydrostatic weighing, DEXA, BodPod (air displacement), bioelectrical impedance, skinfold calipers (3- or 7-site Jackson-Pollock equations). Each has a margin of error: DEXA ±1-2%, skinfolds ±3-5% in trained hands, consumer scales using bioimpedance ±4-8%. American Council on Exercise (ACE) classifications: essential fat 10-13% (women) / 2-5% (men), athletes 14-20% / 6-13%, fitness 21-24% / 14-17%, average 25-31% / 18-24%, obese 32%+ / 25%+.

Applications

  • Tracking body composition changes during a cut or recomposition phase.
  • Calibrating sports performance training to lean mass rather than total weight.
  • Clinical screening for sarcopenic obesity (low lean mass + high body fat).
  • Setting realistic physique goals (a body fat target is more meaningful than a scale weight).
  • Cross-checking BMI in muscular populations where BMI overestimates body-fat risk.

Common Mistakes

  • Treating a consumer bathroom scale's body-fat reading as ground truth. Bioimpedance is heavily affected by hydration; ±5% variability across a day is common.
  • Using skinfold-based body fat without proper training in caliper technique. Even small site-location errors compound across the 3-site or 7-site equations.
  • Confusing body fat percentage with BMI. BMI is a ratio of mass to height; body fat is a ratio of fat mass to total mass. They correlate but measure different things.
  • Aiming for 'essential fat' levels (10-13% women, 2-5% men) for general health. These are short-duration competition values; sustaining them is unhealthy.
  • Mismatching units — entering lean mass in lb and total weight in kg. Set both selectors to the same unit (the calculator uses one mass unit for both).

Frequently Asked Questions

How do you calculate body fat percentage?

If you know your total weight (W) and lean body mass (LBM, the non-fat tissue): BF% = (W − LBM) / W × 100. Example: 70 kg total, 55 kg lean → BF% = (70 − 55) / 70 × 100 ≈ 21.4%.

What is the formula for body fat percentage?

BF% = (Total Weight − Lean Body Mass) ÷ Total Weight × 100. The formula assumes you know your lean body mass from a separate measurement (DEXA, BodPod, hydrostatic, or skinfold).

What's a healthy body fat percentage?

American Council on Exercise classifications for adults: Athletes 14-20% (women) / 6-13% (men); Fitness 21-24% / 14-17%; Average 25-31% / 18-24%; Obese 32%+ / 25%+. Below 'essential fat' (10-13% women / 2-5% men) is unhealthy long-term.

How accurate is bioelectrical impedance body fat?

Consumer bathroom scales using bioimpedance have ±4-8% error and are sensitive to hydration, food intake, and time of day. They're useful for tracking trends if you measure under consistent conditions, but a single reading shouldn't be treated as ground truth.

Is body fat percentage better than BMI?

For athletes, muscular individuals, and anyone where BMI gives a misleading classification, yes. Body fat percentage measures what BMI is trying to proxy. But BMI is cheaper and faster — no measurement needed — so it remains the standard population-level screen.

Where do I get my lean body mass measurement?

DEXA scans (most accurate, ~$50-150 at sports medicine clinics), BodPod air displacement plethysmography, hydrostatic underwater weighing, professional skinfold caliper measurements by a trained technician (Jackson-Pollock 3-site or 7-site protocol), or consumer-grade bioimpedance scales (least accurate but cheapest).

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