Muscular Potential Calculator (Casey Butt formula)
Use height, wrist, ankle, weight and a body-fat estimate to compare your current lean mass with the Casey Butt frame model — the regression built on drug-free champion bodybuilders. You get the lean-mass ceiling your skeleton supports, the bodyweight it maps to at 8-15% body fat, and the girths that come with it. Free, no signup.
The formula
All inputs in inches (the calculator converts for you):
LBM = H^1.5 × ( √W / 22.6670 + √A / 17.0104 ) × ( %bf / 224 + 1 )
Bodyweight = LBM / (100 − %bf) × 100
Maximum girths at 8-10% body fat, where H = height, W = wrist girth, A = ankle girth:
- Chest = 1.6817W + 1.3759A + 0.3314H
- Biceps = 1.2033W + 0.1236H
- Forearm = 0.9626W + 0.0989H
- Neck = 1.1424W + 0.1236H
- Thigh = 1.3868A + 0.1805H
- Calf = 0.9298A + 0.1210H
Source: Casey Butt, PhD — “Your Maximum Muscular Bodyweight and Measurements”.
Read it honestly
The model describes a balanced elite natural physique on an average skeleton, not a hard biological law. It is male-derived, assumes accurate body-fat and bone measurements, and allows about 3% for small joints or long muscle bellies. Garbage body-fat estimate in, garbage ceiling out.
How to measure
- Wrist. Just below the styloid process — the bony lump on the outside of the wrist — on the hand side. Snug tape, arm relaxed.
- Ankle. The smallest circumference of the lower leg, just above the ankle bones. Stand relaxed, weight even on both feet.
- Body fat. Use a caliper, a DEXA scan or our free AI Fat% scan. This single number moves your result the most, so do not guess low.
Questions people actually ask
What is the Casey Butt muscular potential formula?
Casey Butt, PhD, regressed the measurements of drug-free champion bodybuilders (largely pre-steroid era and drug-tested naturals) against their skeletal frame size. The result predicts the maximum lean body mass a natural lifter with average proportions can carry: LBM (lb) = H^1.5 × (√wrist / 22.6670 + √ankle / 17.0104) × (body-fat% / 224 + 1), with every measurement in inches.
Why wrist and ankle instead of bone scans?
Wrist and ankle girths are almost pure bone and connective tissue — they barely change with training or body fat, which makes them the best cheap proxies for skeletal frame size. A bigger frame supports more muscle mass, which is exactly what the regression captures.
How do I measure my wrist and ankle correctly?
Wrist: wrap the tape immediately below the styloid process (the bony lump on the outside of your wrist), on the hand side of it. Ankle: find the smallest circumference of the lower leg, just above the ankle bones. Snug tape, no compression, and measure both sides — use the average if they differ.
Is the calculator accurate for women?
No — the underlying dataset is male champions, so we keep this calculator male-only rather than invent a scaling factor. Women can still use the general principle (frame size caps muscle mass), but the absolute numbers would be misleading.
Can I beat my predicted maximum?
Butt himself notes that people with uncharacteristically small joints for their frame, or unusually long muscle bellies, can exceed the prediction by roughly 3% — which is why we show a ±3% band. Anabolic drug use, of course, moves the ceiling far beyond what this natural model describes.
What if the calculator says I am already at 100%?
First re-check your body-fat estimate: underestimating body fat inflates your lean mass and can push you artificially past the ceiling. Then re-measure your wrist and ankle. If the numbers hold, you have built an elite natural physique — the remaining work is refinement, not mass.
How much muscle can I actually gain per year?
Roughly 8-12 kg of lean mass in the first year of serious training, 4-5 kg in the second, 2-3 kg in the third, and under 1-2 kg per year after that. The closer you are to your predicted ceiling, the slower and more precise the process becomes.
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