Calculators Fit
● Strength & training

Your max muscle potential, naturally.

Estimate the maximum muscle mass and measurements you can reach naturally with the Casey Butt model — based on your height and skeletal frame size (wrist and ankle).

💪
Genetic Potential — Maximum Natural Muscle Mass
Maximum natural mass and measurements (Casey Butt)
✏️

Your data

in
in
in
🏆

Your potential

— kg (max natural weight)
Casey Butt model
predicted maximum measurements, in cm
⚠️ The Casey Butt formula was developed from male bodybuilder data. For women, it overstates potential and does not apply. We use the natural FFMI ceiling (~19–20), a statistical limit on drug-free lean mass — not a bone-structure formula. Wrist and ankle are not used, and no maximum bone-based measurements are estimated.
Maximum lean body mass fat-free—
Max bodyweight at condition—
Arm (flexed)—
Chest—
Thigh—

How it's calculated

Your skeleton sets the ceiling for muscle

The Genetic Potential calculator estimates your maximum drug-free lean body mass and the maximum measurements you can reach naturally using the Casey Butt formula, which reads your skeletal frame from height, wrist and ankle circumference. For a 5'10" (178 cm) man with a 6.7" (17 cm) wrist and 8.7" (22 cm) ankle at 10% body fat, the model predicts a maximum lean mass of about 80 kg (176 lb). For women, the male Butt formula does not apply, so we use the natural FFMI ceiling (~19–20) instead.

Your skeletal frame — indicated by wrist and ankle circumference — limits how much muscle tissue your body can structurally support. Larger joints can anchor more muscle. The Casey Butt formula was validated against elite natural (drug-free) male bodybuilders competing at their peak condition. For women, that formula overstates potential, so we fall back to the natural fat-free mass index ceiling.

Max lean mass (lb) = H1.5 × (√W/22.667 + √A/17.011) × (%BF/224 + 1)
H, W and A in inches · women: max lean mass (kg) = 19.5 × height(m)²
  1. 1
    Convert height, wrist and ankle to inches (or read the female height)
    —
  2. 2
    Maximum lean body mass (lb) via the Casey Butt formula (women: FFMI ceiling)
    —
  3. 3
    Convert to kg and derive bodyweight at condition: LBM ÷ (1 − %BF)
    —
  4. 4
    Maximum measurements (cm) from wrist, ankle and height
    —

Understand the terms

Lean Body Mass (LBM)
Total body weight minus fat mass. Includes muscle, bone, organs, connective tissue and water. The key metric used by all three genetic potential models.
Natural potential
The estimated upper limit of muscle mass achievable without the use of anabolic-androgenic steroids or other performance-enhancing drugs. Differs per individual based on genetics, frame size and training consistency.
Frame size
An indicator of skeletal structure, approximated by wrist and ankle circumferences. Larger-framed individuals can typically support more muscle mass.
Fat-Free Mass Index (FFMI)
LBM (kg) divided by height² (m²). A normalised measure of muscularity. Values above 25 are rarely seen in drug-free athletes; 22–24 is typical for advanced natural lifters.
See the full glossary →

🔢 Worked example

At 1.80 m with an 18 cm wrist, 23 cm ankle and 10% body fat, the model estimates a lean-mass ceiling of around 80 kg — a statistical reference, not a hard limit.

Disclaimer: estimation tool for informational and planning purposes. The Butt, Berkhan and Aragon models were developed from analysis of elite male drug-free bodybuilders. Individual results may differ based on genetics, training quality, nutrition and measurement accuracy. Results for women may overstate potential. Does not replace assessment by a qualified coach, sports scientist or physician.

Frequently asked questions — Genetic Potential

Which model does this max muscle potential calculator use?
The number on screen is computed with the Casey Butt formula, derived from measurements of elite drug-free bodybuilders: it predicts maximum lean body mass from height, wrist and ankle circumference. For women, the male-derived Butt formula overstates potential, so the calculator uses the natural FFMI ceiling (~19–20) instead. The Berkhan (height-based) and Aragon (rate-of-gain) references are explained on this page as cross-checks, not computed separately.
How much muscle can I gain naturally?
Natural muscle gain varies widely by genetics, frame size, age and training history. Most men can add 20–40 lb of lean mass over a lifetime of training. Year 1 typically yields 10–25 lb, year 2 around 5–10 lb, and gains slow considerably after year 4. The Casey Butt, Berkhan and Aragon models give you a research-grounded upper bound based on your frame.
How accurate are genetic potential models?
These models were derived from measurements of elite natural bodybuilders competing at their peak. They represent what is achievable with optimal training, nutrition and genetics — not an average expectation. Most natural lifters reach 80–90% of the theoretical ceiling. Expect a margin of ±5–10% around model outputs.
What is the Casey Butt formula?
Casey Butt analysed the measurements of hundreds of drug-free bodybuilders to derive a formula that predicts maximum lean body mass from height, wrist circumference and ankle circumference. These skeletal measurements correlate with muscle insertion points and the total cross-sectional area muscles can grow to. The formula: LBM = H^1.5 × (√W/22.667 + √A/17.011) × (%BF/224 + 1) where H = height in inches, W = wrist in inches, A = ankle in inches, BF = body fat % at peak.
How long does it take to reach genetic potential?
Most research and coaches estimate 8–12 years of consistent, progressive resistance training with adequate protein and sleep. The trajectory is front-loaded: the first 1–2 years bring the largest gains ("newbie gains"), while the last 10–20% of potential may take years to approach. Most lifters never fully reach their theoretical ceiling.
Does height affect muscle potential?
Yes. Taller individuals generally have longer muscle bellies with greater potential cross-sectional area, giving them a higher absolute LBM ceiling. However, shorter individuals often appear more muscular at the same LBM because mass is distributed over a shorter frame. All three models (Butt, Berkhan, Aragon) incorporate height, with Berkhan's formula being entirely height-dependent.
Are my results saved? Do I need an account?
No account or sign-up needed. Every calculation is saved automatically in your own browser, and you can also pin a result with the save button. Nothing is sent to any server: the data stays on your device and can be erased at any time with the clear button.
How do I track my progress over time?
From the fourth entry onward a “View comparison chart” button appears, opening a line chart with every entry in time order plus an indicator panel: total change, average per entry, lowest and highest value and the period covered. You can also export the history as CSV.

📚 Learn more — official sources

🔬 Study Kouri et al. (1995) — FFMI in natural vs. steroid-using bodybuilders Classic study showing that natural bodybuilders rarely exceed an FFMI of 25, providing an independent upper bound consistent with the Butt and Berkhan models. See on PubMed → 🔬 Study Haun et al. (2019) — Skeletal muscle hypertrophy rates in resistance training Review of hypertrophy literature quantifying typical muscle growth rates over the first years of training, supporting the diminishing-returns trajectory of natural potential models. See on PubMed → 🏛️ Organization NSCA — Training volume and hypertrophy National Strength and Conditioning Association overview of the physiological mechanisms driving muscle hypertrophy and how training variables affect natural gains. See official source → 🔬 Study Morton et al. (2018) — A systematic review of protein supplementation for muscle gains Meta-analysis on how dietary protein affects lean mass gains in resistance-trained individuals, relevant to approaching genetic potential ceilings. See on PubMed →

🔗 Related calculators — Strength & training

🏆 1RM 📋 Load table 📊 Training volume 🏛️ Ideal proportions 🥇 Wilks score

About the Genetic Potential Calculator

The Max Muscle Potential Calculator estimates the maximum lean body mass a natural (drug-free) athlete can achieve. The number you see is computed with the Casey Butt formula — the most anatomically grounded of the published models, because it reads wrist and ankle circumferences as proxies for bone and tendon size, which limit how much contractile tissue a skeleton can support. For women the male-derived Butt formula does not apply, so the calculator uses the natural FFMI ceiling (~19–20) instead. Two other well-known references are useful as cross-checks: the Berkhan (Leangains) guideline, a simple height-based estimate of stage-lean competition weight, and Alan Aragon's rate-of-gain model, which frames potential as diminishing monthly gains (roughly 1–1.5% of body weight per month for beginners, 0.5–1% for intermediates, 0.25–0.5% for advanced lifters).

Treat the output as a range, not a verdict. These models were fit to elite drug-free bodybuilders, and individual genetics (muscle-fiber distribution, limb lengths, tendon insertions) can move the real ceiling in either direction — which is exactly why the natural-bodybuilding community still debates them. The independent evidence line is the FFMI: Kouri and colleagues found that natural competitors rarely exceed an FFMI of about 25, a bound consistent with what the Butt formula predicts for typical frames. If your current lean body mass is well below the estimate, you have meaningful growth ahead; if you are approaching it, you are likely an advanced natural lifter. Use this tool alongside the FFMI calculator, the body fat (Navy) calculator, the lean mass calculator and the 1RM calculator to track progress relative to your individual ceiling.

By Vinicius Fonseca · Formulas checked against scientific sources · Updated Aug 2026 · How we calculate →