Hey all, after about an hours sleep yesterday after arriving back from Darwin, I just didn't have the energy to train, so decided to do some research instead. I've always wondered what my genetric max would be and if there was research around it.
I ended up finding the Casey Butt model. It estimates the maximum amount of muscle a natural lifter can carry. It’s based on a few hundred pre-steroid era bodybuilders and uses height, wrist size, and ankle size to predict how much lean mass you can realistically hold at ~8–10% body fat.
If you want to check your own numbers, here’s a calculator I found:
Casey Butt Calculator
The formula (simplified) is:
View attachment 5447
where H = height in cm, A = ankle circumference in cm, W = wrist circumference in cm.
My stats
- Age: 45
- Height: 174 cm
- Wrist: 19 cm
- Ankle: 23 cm
- Current weight: 85 kg
- Body fat: ~13%
That puts my lean body mass at around 74 kg, with an FFMI of ~24.4.
FFMI = Fat-Free Mass Index. It’s basically like BMI but only counts lean mass (muscle, bones, organs), ignoring fat.
Research on elite natural lifters shows FFMI rarely exceeds 25. Essentially, 25 is like a “genetic ceiling” for natural muscle mass.
What the model says for me
- Max lean bodyweight @ 8–10% BF: 78–81 kg
- Arms: ~41–42 cm
- Thighs: ~61–62 cm
- Chest: ~108–110 cm
- Calves: ~39–40 cm
If I cut down to 10% right now, I’d probably sit around 78–80 kg. So the prediction lines up really well with where I actually am.
My goal
- 90 kg @ 10% BF
- That means 81 kg lean mass → about +7 kg lean tissue above where I am now.
- That would give me an FFMI of ~26.7, which is well past the typical natural ceiling of ~25.
The reality check
The Casey Butt model basically confirms what I already suspected: I’m pretty much at my natural limit. To push beyond it and actually hit 90 @ 10%, I’d need to run steroids and running a TRT dose of test would likely only allow a slight increase over my genetic max. It actually makes me feel better to have chose to to use gear, because my goal is very likely impossible for my genetics otherwise.
Looking forward to getting over these niggling injuries and getting my next order in from
@ozsteroids
If anyone’s used the calculator or done a similar comparison, I’d love to hear your numbers and how far off the model you think you are.