Body Fat Calculator

A body fat calculator estimates the percentage of your total weight that is fat (rather than muscle, bone, organs, or water). It's a more meaningful health and fitness number than BMI because two people with identical BMIs can have radically different body compositions — one lean and muscular, the other carrying excess fat.

This tool uses the US Navy circumference method, originally developed by Hodgdon and Beckett at the Naval Health Research Center. It needs only a tape measure and produces results within a few percentage points of DEXA — close enough to track progress through a cut or bulk without spending $100+ on a scan.

A 75 kg man, 178 cm tall, with a 85 cm waist and 38 cm neck:

• log₁₀(85 − 38) = log₁₀(47) = 1.6721
• log₁₀(178) = 2.2504
• Denominator: 1.0324 − 0.19077 × 1.6721 + 0.15456 × 2.2504 = 1.0610
• %BF = 495 / 1.0610 − 450 = 16.5%
• Fat mass = 75 × 0.165 = 12.4 kg
• Lean mass = 75 − 12.4 = 62.6 kg

That puts him in the "fitness" category. If after a 12-week cut his weight drops to 70 kg with the same waist of 81 cm, his body fat would fall to roughly 13% — a clear, measurable improvement that pure scale weight wouldn't show.

• Tracking a cut or bulk — re-measure every 2–3 weeks at the same time of day to see real composition changes.
• Setting a fitness goal — a target like "14% body fat" is more meaningful than a target weight.
• Distinguishing muscle gain from fat gain — the scale going up while waist stays flat means you're gaining lean mass.
• Health screening — body fat above 25% (men) or 32% (women) is associated with elevated cardiovascular and metabolic risk.
• Athletic performance — endurance athletes typically perform best between 8–15% (men) or 14–20% (women).

• Measuring inconsistently. Measuring waist after a meal or hip over jeans introduces 2–4 cm of error, which can swing %BF by several points.
• Trusting a single reading. Take the same measurements weekly and trend the average — daily fluctuations are noise.
• Comparing methods. A DEXA, BIA scale, and Navy method will give three different numbers for the same person. Pick one method and stick with it.
• Targeting essential fat. Going below 5% (men) or 13% (women) suppresses hormones, immunity, and recovery. Bodybuilders briefly hit these levels for competitions, not as a lifestyle.
• Ignoring waist circumference itself. Even apart from %BF, waist over 102 cm (men) or 88 cm (women) is independently linked to metabolic disease.