Body Composition
As of 2026-04-24
How Muscle Gain Potential Calculator works
Methodology for the Muscle Gain Potential Calculator: formulas, coefficients, data sources, assumptions, and known limitations.
Scope
Estimates natural muscular potential at low body fat using the Casey Butt model, mapping wrist and ankle sizes to limit body circumferences.
Formula
LBM_max = height^1.5 * (sqrt(wrist) / 22.667 + sqrt(ankle) / 17.010) * (1 + bodyfat_target/224) — abbreviated Casey Butt equation.
Coefficients
| Parameter | Value | Note |
|---|---|---|
| Wrist normalizer | 22.667 | |
| Ankle normalizer | 17.010 | |
| Bodyfat term | 1 + bf/224 |
Data sources
- Butt C. Your Muscular Potential. weightrainer.net, 2006 (aggregated from historical natural-bodybuilder measurement data). — Original aggregate-regression publication that fit the formula on natural-bodybuilder competition data.
- Kouri EM, Pope HG Jr, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med. 1995;5(4):223-228. — PMID 8580417. Provides the natural-lifter FFMI ceiling (~25.0 adjusted) cross-checked against Casey Butt's predictions.
- Helms ER, Aragon AA, Fitschen PJ. Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. J Int Soc Sports Nutr. 2014;11:20. — PMID 24864135. Modern review of natural-physique outcomes that contextualizes the Casey Butt limit.
Assumptions
- Wrist and ankle circumferences are accurate proxies for frame size.
Approximation range
Casey Butt's model estimates the natural ceiling within ~5% for lifters with typical frame geometry.
Limitations
- Not a prescription — it is an upper-bound estimate, not a forecast for a given training block.
- Fit on historical competition data; modern athletes may exceed the natural ceiling with better nutrition and training.
Reproducibility
Height 180 cm, wrist 17 cm, ankle 22 cm, target 8% bf: formula returns LBM ~84 kg → total BW ~91 kg at 8%.
Change log
- 2026-04-24: methodology page first published.
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