Key Takeaways
- Casey Butt's model predicts your drug-free muscular ceiling from skeletal frame size — height, wrist, and ankle circumference.[1]
- Most genetically typical trainees can realistically reach about 95% of the predicted maximum with years of consistent training.[1]
- The model complements FFMI by predicting where you could end up, while FFMI measures where you are now.[2]
What This Calculator Estimates
This calculator implements Casey Butt's nonlinear frame-based model from his research on natural bodybuilding champions. It estimates three things:
- Maximum lean body mass — the most muscle you could carry at a given body fat percentage
- Maximum bodyweight — total weight at that body fat level, including the fat mass
- Maximum muscular measurements — predicted girths for chest, biceps, forearms, neck, thighs, and calves at lean condition
The inputs are purely structural: your height and two joint circumferences that serve as proxies for bone frame size. Wrist and ankle measurements are used because they carry minimal muscle and subcutaneous fat, making them reliable indicators of skeletal dimensions.[1]
How to Measure
Accurate input produces accurate predictions. Follow these measurement procedures from the original Casey Butt methodology.[1]
| Body Part | Procedure |
|---|---|
| Height | Stand barefoot against a wall. Look straight ahead. Mark the top of your head and measure down to the floor. |
| Wrist | Wrap tape around your wrist on the hand side of the styloid process (the bony bump on the outside of your wrist). Keep tape snug but not compressing skin. |
| Ankle | Wrap tape around the narrowest point of your lower leg, just above the ankle bone. Stand with weight evenly distributed. |
| Chest | Arms relaxed at sides. Tape under armpits, across the widest part of the chest. Breathe normally; measure at end of a normal exhale. |
| Biceps | Flex your arm fully. Measure around the largest point of the contracted biceps. |
| Forearms | Clench your fist with arm straight. Measure around the largest point of the forearm. |
| Neck | Measure just below the Adam’s apple at the narrowest point. Keep tape level and snug. |
| Thighs | Stand relaxed. Measure midway between the hip crease and the top of the kneecap. |
| Calves | Stand relaxed with weight on both feet. Measure around the largest point of the calf. |
How the Casey Butt Formula Works
Maximum Lean Body Mass
Maximum Bodyweight
Maximum Muscular Girths (inches)
Variable Glossary
| H | Height in inches |
| W | Wrist circumference in inches (hand side of styloid process) |
| A | Ankle circumference in inches (smallest point) |
| %BF | Target body fat percentage (numeric, e.g. 10) |
Worked Example
Height power
69^1.5 = 573.15
573.15
Frame factor
√7.0/22.667 + √8.7/17.01
0.2902
BF adjustment
10/224 + 1
1.0446
Max LBM
573.15 × 0.2902 × 1.0446
173.7 lb
Max Bodyweight
173.7 / (100 − 10) × 100
193.0 lb
The 95% realistic tier for this example would be 165.0 lb lean mass and 183.4 lb bodyweight.
Interpreting Your Results
100% Maximum = Genetic Ceiling
This is the empirical upper bound from elite drug-free bodybuilders with comparable frame sizes. Very few trainees will reach it fully.
95% = Realistic Lifetime Goal
Most genetically typical, dedicated natural trainees can approach this level over many years of consistent training and nutrition.
Body Fat Changes Only Total Weight
The lean body mass prediction stays the same regardless of body fat input. Adjusting body fat only changes the total bodyweight prediction.
Not a Promise
Individual variation in muscle belly length, fiber type distribution, training history, and health conditions means actual results will differ.
Methodology & Limitations
Casey Butt's model is an empirical regression derived from approximately 300 male drug-free bodybuilding champions combined with anthropometric reference data. It represents an upper-bound prediction, not a guaranteed outcome.[1]
Ectomorphic traineesmay plateau near 94–95% of the prediction, while some mesomorphic outliers may exceed it by roughly 3–5%. The model does not account for muscle belly length, fiber type composition, training history, or health conditions.
This model is distinct from FFMI research. FFMI measures current muscularity relative to height based on population studies, while Butt's model predicts theoretical maximums from frame dimensions. The two approaches are complementary — use our FFMI Calculator to track current status and this tool to estimate your ceiling.[2]
The female extrapolationapplies post-hoc scaling factors (LBM ×0.68, girths ×0.82) derived from Janssen et al. (2000) skeletal muscle sex differences.[3] These are notfrom Casey Butt's original regression sample and should be treated as rough estimates.
Casey Butt's original methodology does not include a waist formula. We do not invent unsupported measurements.
Frequently Asked Questions
Sources & References
- Your Maximum Muscular Bodyweight and Measurements. Casey Butt, Ph.D. (2010)
- Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Kouri EM, Pope HG Jr, Hudson JI, et al.. Clinical Journal of Sport Medicine (1995)
- Skeletal muscle mass and distribution in 468 men and women aged 18–88 yr. Janssen I, Heymsfield SB, Wang ZM, Ross R.. Journal of Applied Physiology (2000)
- The Keys to the Inner Universe (anthropometric bodybuilding standards). McCallum JC, Willoughby DP. (1970)
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Disclaimer: This tool is for informational and personal journaling purposes only. Results are estimates based on published formulas and may vary based on individual factors such as genetics, training history, and measurement technique.
This tool is for informational purposes only.
It provides estimates based on published research, not guarantees of individual outcomes. I am NOT a doctor.