Natural Muscular Potential Calculator

How it works

Casey Butt: max lean mass (lb) = H^1.5 × (√W ÷ 22.667 + √A ÷ 17.0104) × (BF% ÷ 224 + 1), with H, W, A = height, wrist, ankle in inches · max weight = lean mass ÷ (1 − BF%) · Berkhan: stage weight (kg) ≈ height (cm) − 100 · FFMI 25: lean mass = 25 × height (m)²

Casey Butt fitted his formula to drug-free bodybuilding champions from before the steroid era: taller people and those with thicker wrists and ankles (a bigger frame) can carry more muscle. It predicts the ceiling after many years of optimal training, not a short-term goal. Martin Berkhan's rule of thumb and the fat-free mass index limit of about 25 (Kouri et al., 1995) are shown alongside as cross-checks; most lifters reach 90–95% of these figures with a decade of consistent training.

Worked example

A lifter 178 cm tall with a 17.5 cm wrist and 22.5 cm ankle, at 10% body fat: H = 70.08 in, W = 6.89 in, A = 8.86 in → lean mass = 586.7 × (0.1158 + 0.1750) × 1.0446 = 178.2 lb = 80.8 kg, so the maximum is 89.8 kg at 10% body fat. Berkhan's rule gives 78.0 kg at about 5% body fat and FFMI 25 gives 79.2 kg of lean mass.

Sources: Kouri EM et al. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med 1995

Frequently asked questions