Muscle Gain Calculator - Free Genetic Potential
Calculate your maximum natural muscle building potential using the Casey Butt formula. Know your genetic limits based on frame size, or track your actual progress with AI photo analysis.
The Problem with Genetic Potential Formulas
The Casey Butt formula is a self-published rule of thumb based on measurements of champion natural bodybuilders, not a peer-reviewed prediction model. It can't account for individual differences like muscle insertions, fiber types, or hormones, so treat the result as a rough ceiling, not a target.
Casey Butt Formula
Rule of thumb- Accounts for frame size
- Easy to calculate
- Doesn't measure actual muscle
- Large individual variation
- Based on elite bodybuilders
McDonald Model
Estimates only- Simple year-by-year projections
- Accounts for training experience
- Averages only
- Doesn't account for genetics
- No personalization
FFMI Estimates
Depends on BF% input- Normalized for height
- Good for comparisons
- Requires body fat data
- Natural limits debated
- Population bias
Track Actual Progress
Your real data- Measures YOUR body
- Tracks real changes
- Objective progress
- Requires consistent tracking
- Takes time to see trends
Casey Butt Calculator
Your Measurements
Measure below the wrist bone
At narrowest point above ankle bone
Track Real Muscle Gains
Track your progress over time
The Casey Butt formula estimates your potential—but tracking your actual body composition shows whether your training is working. PhysiqueAI estimates lean mass from photos so you can track the trend.
- Track lean mass gains month over month
- See body fat % changes with each scan
- Know if your bulk/cut is on track
- No tape measure or gym scale needed
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What the Research Says
Casey Butt's Analysis
Casey Butt analyzed measurements from champion natural bodybuilders dating back to the pre-steroid era. His self-published article "Your Maximum Muscular Bodyweight and Measurements" (archived) (not peer-reviewed) relates bone structure to maximum muscular development:
- Wrist and ankle circumference predict frame size
- Height determines total muscle mass potential
- Derived from top natural competitors, so it describes an upper limit
McDonald/Aragon Muscle Gain Model
Lyle McDonald and Alan Aragon each published coaching rules of thumb for expected muscle gain. These aren't peer-reviewed; McDonald's version, from "What's My Genetic Muscular Potential?", is:
- Year 1: 20-25 lbs (9-11 kg) potential muscle
- Year 2: 10-12 lbs (4.5-5.5 kg) potential
- Diminishing returns after year 3-4
Natural Muscle Gain Reference
Yearly Muscle Gain Potential (Men)
Beginner gains - fastest progress
Intermediate - still significant
Advanced - slower but steady
Experienced - diminishing returns
Total natural muscle potential
Natural FFMI Ranges (Men)
Untrained or underweight
Typical non-lifter physique
Regular lifter, 1-2 years
Dedicated training, 3-5 years
Near genetic limit
How to Measure
Wrist Circumference
Measure around your wrist just below the bone (where you'd wear a watch). Keep the tape snug but not tight. Wrist size doesn't change with training—it's a reliable indicator of your bone structure.
Ankle Circumference
Measure around the narrowest point of your ankle, just above the ankle bone. If you skip this measurement, the calculator estimates it as wrist × 1.25, which is accurate for most people.
Track your actual muscle gains
Knowing your genetic potential is step one. Tracking your real progress with body composition scans shows whether your training and nutrition are working.
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