Estimate your maximum natural measurements from your bone structure. Casey Butt model, validated on pre-steroid-era athletes.
📐 How it is calculated
🧪 Cited studies
Casey Butt (2008) — Your Muscular Potential: model of maximum lean mass and measurements based on frame size, derived from drug-free records.
Lyle McDonald — model of declining annual muscle gains (a reference in the evidence-based community).
Martin Berkhan — rule of thumb height (cm) − 100 ≈ maximum lean bodyweight (kg) for a lean man.
📋 Worked example
Worked example. A man of 180 cm with a 17.5 cm wrist and a 22.5 cm ankle has a lean mass potential, in a lean condition, of about (Casey Butt model) — i.e. a ceiling FFMI of around 25.
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How much muscle can you build naturally? This calculator relies on the Casey Butt model, derived from the analysis of elite drug-free bodybuilders and strength athletes. Its principle: your maximum muscular potential depends on your frame, measured at the wrist and ankle (the “leanest” joints, a good proxy for skeletal size), combined with your height.
From these measurements, the model estimates your maximum lean mass in a lean condition, then derives your ceiling muscle girths (arms, chest, thighs…). The anatomical logic: a larger frame supports and feeds more muscle mass than a slight frame, at equal height.
This is a ceiling estimate, not a prediction of what you will reach “easily”. Most lifters never hit this ceiling, for lack of long-term consistency — it mainly serves to set realistic goals and to put “inspiring” physiques, often enhanced, into perspective.
🎯 How to use it
Your potential is built over several years, at a declining pace. Benchmarks for net muscle gains (Lyle McDonald model, men):
Training year
Realistic muscle gain
1st year
9–11 kg
2nd year
4–5 kg
3rd year
2–3 kg
4th year and beyond
1–2 kg / year
Set a target: compare your current lean mass (via the FFMI) to your estimated ceiling to know how many kilos of margin you have left.
Prioritize consistency: as you approach the ceiling, gains slow down — adherence over 2-4 years matters more than the “perfect program”.
Controlled surplus: a 5-15% surplus is enough; beyond that, you mostly gain fat without accelerating muscle synthesis (already maxed out).
Keep role models in perspective: a physique far beyond your estimated ceiling, at a comparable frame, is probably enhanced — a useful benchmark to stay motivated without comparing yourself to the impossible.
❓ Frequently asked questions
How do I estimate my maximum muscular potential?+
The Casey Butt model uses your wrist and ankle circumferences (a reflection of your frame) and your height to predict your maximum natural lean mass, then your ceiling measurements. Other approaches exist (Berkhan: height in cm − 100 ≈ max lean bodyweight in kg; Lyle McDonald for annual gains), but Casey Butt is the most personalized because it is based on frame size.
How much muscle can you gain in the first year?+
A beginner can gain 9 to 11 kg of muscle in the first year with optimal training and nutrition. This pace then drops: 4-5 kg in the 2nd year, 2-3 kg in the 3rd, then 1-2 kg per year. Genetics, age and above all consistency determine where you fall within these ranges.
Does genetics really limit muscle gains?+
Yes: fiber distribution, insertion length, testosterone levels and androgen sensitivity set a ceiling. But it is a non-issue for 95% of lifters, who quit or lack consistency well before approaching this ceiling. The real limit is almost always adherence, not genetics.
Why measure the wrist and ankle?+
These are joints with very little muscle and fat: their circumference directly reflects the size of your skeleton. A large frame (thick wrist/ankle) can support more muscle mass than a slight frame, at equal height — hence their use in the Casey Butt model.
Can you exceed your estimated natural potential?+
Very rarely and only slightly in a natural context: the models are calibrated on elite athletes already at the top. A physique clearly beyond the ceiling, at a comparable frame, suggests pharmacological assistance. See the estimate as a realistic upper limit, with a small genetic margin.
82 kg
If he currently carries 74 kg of lean mass, he has ~8 kg of muscle left to build. At realistic rates (2-4 kg/year for an intermediate), that is a goal of 2 to 4 years of structured training. A slighter frame (15.5 cm wrist) would lower this ceiling by several kilos: hence the value of calibrating the goal to HIS frame.
⚠ Limitations
Ceiling estimate: it indicates an upper limit, not a guaranteed trajectory or a timeline.
Male-based models: calibrated mostly on men; interpret with caution for women (lower absolute potential, different dynamics).
Factors beyond the frame: muscle insertions, muscle belly length and hormones modulate the real result beyond the wrist and ankle.
Measuring: measure the wrist just above the protruding bone and the ankle at its narrowest point, without pulling tight.