About

Genetics dictate the ceiling; hard work determines how close you get to it. This calculator uses Dr. Casey Butt's famous formula to estimate your Maximum Muscular Potential based on skeletal structure. Research indicates that the circumference of the wrist and ankle correlates strongly with the potential thickness of the muscle belly that the bone can support.

This tool is essential for natural lifters to set realistic expectations. It prevents the frustration of chasing impossible dimensions without pharmaceutical enhancement. By inputting your bone measurements, you will receive a projection of your maximum lean body mass and ideal measurements at various body fat percentages.

Formulas

We apply the Casey Butt Formula for lean body mass potential:

H^1.5 × (√W22.66 + √A17.01) × %bf + 224224

Where H is height (inches), W is wrist size, and A is ankle size.

Reference Data

Frame Category	Wrist Circ. (cm)	Physical Characteristic
Small Frame	< 16.5	Ectomorphic. Difficult to gain mass. Pop "aesthetic" look.
Medium Frame	16.5 - 19.0	Mesomorphic. Good balance of size and definition.
Large Frame	> 19.0	Endomorphic. Strongman build. High potential for mass.

Frequently Asked Questions

No. The Casey Butt formula is derived strictly from data on elite drug-free bodybuilders (pre-steroid era champions). Enhanced athletes can surpass these limits by altering protein synthesis rates beyond natural physiology.

Not necessarily. While your absolute maximum weight might be lower than someone with huge wrists, smaller joints create a visual illusion. A 16-inch arm looks significantly larger on a 6.5-inch wrist than on an 8-inch wrist due to the "muscle pop" effect.

Ankle circumference is a strong predictor of lower body mass potential. If you have small ankles, your calves may always struggle to grow, but your thighs can still achieve significant size relative to your frame.

Body Part	Predicted Max (in)
Chest	-
Biceps	-
Forearms	-
Neck	-
Thighs	-
Calves	-