How Maximum Muscular Potential Is Estimated

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How much muscle can you build naturally? This calculator combines four independent models to estimate your maximum muscular potential — the genetic ceiling for lean mass in drug-free lifters. The models use different inputs (skeletal frame, height, body composition, training experience) to triangulate a consensus range.

No single model is definitive. By combining frame-based, height-based, FFMI-based, and rate-of-gain approaches, the calculator provides a range that accounts for each model’s strengths and limitations.

Cum funcționează

The calculator runs four models simultaneously:

1. Casey Butt — uses your skeletal frame (wrist and ankle circumference) plus height to predict maximum lean body mass. The most personalised model.
2. Martin Berkhan — a simple height-based formula for contest weight at 5% body fat, validated against drug-tested competitors.
3. FFMI-based — uses the Kouri et al. (1995) finding that a Fat-Free Mass Index above 25 is extremely rare in natural male athletes.
4. Lyle McDonald — predicts how much muscle you can gain per year of proper training, with diminishing returns each year.

The consensus range is the spread across models 1–3 (which all estimate maximum lean mass). Model 4 (McDonald) tells you how quickly you can get there.

The formulas

Casey Butt (frame-based)

This is the simplified linear formula from the earlier edition of Casey Butt’s “Your Muscular Potential”. The 4th edition uses a nonlinear variant (H^1.5, sqrt wrist/ankle, BF/224) that produces results within 1–2% for average male frames. Both versions are based on measurements of ~300 drug-free bodybuilding champions (1947–2010).

Body part circumferences at genetic potential are derived from wrist and ankle size:

Female adjustment: Casey Butt’s research studied males only. For females, lean mass is multiplied by 0.68 and circumferences by 0.82, based on Janssen et al. (2000) sex-linked skeletal muscle mass differences.

Martin Berkhan (height-based)

Predicts maximum weight at contest condition (5–6% body fat). Validated against drug-tested natural competitors. Most accurate for men 170–190 cm. Berkhan notes the formula is not perfectly linear — shorter athletes tend to slightly exceed it, taller athletes fall slightly below.

Lean mass at contest = contest weight × 0.95 (since contest BF ≈ 5%).

Female adjustment: contest weight × 0.68 (Janssen et al., 2000).

FFMI-based (Kouri et al., 1995)

Based on a study of 157 male athletes (83 steroid users, 74 non-users). Non-users’ FFMI topped out at 25.0. Pre-steroid era Mr. America winners (1939–1959) had a mean FFMI of 25.4, providing independent validation.

The female limit of 21 is estimated from limited literature and is less well-established.

Lyle McDonald (rate-of-gain)

Total lifetime potential: ~19 kg (male), ~10 kg (female). These are mid-range values; McDonald’s original ranges are 18–23 kg for males.

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Assumptions and limitations

• Casey Butt’s formula was developed from male drug-free bodybuilding champions — people who trained optimally for years. Your personal ceiling may be slightly lower if you don’t train at that level, or slightly higher if you’re a genetic outlier.
• The FFMI limit of 25 is not a hard ceiling. Kouri et al. described it as “extremely rare” in their sample, not impossible. Some researchers (Nuckols, RP Strength) argue the true natural limit is closer to 26–28 for elite genetics.
• Female estimates use sex-adjustment multipliers (0.68 for lean mass, 0.5 for McDonald rates) derived from population studies, not bodybuilding-specific data. These are reasonable approximations with wider uncertainty than male estimates.
• Wrist and ankle are proxies for skeletal frame size. They are not perfect predictors — muscle insertion points, limb proportions, and hormonal profiles also matter.
• McDonald’s rate-of-gain assumes “proper training” — structured progressive overload, adequate protein (1.6–2.2 g/kg), and sufficient recovery. Sub-optimal training will reduce actual gains below these projections.