Maximum muscular potential calculator
Wrist and ankle circumference are proxies for skeletal frame size, and frame size predicts how much muscle you can ultimately carry. This is a lifetime ceiling, not a target for this year.
Maximum Muscular Potential Calculator
The Casey Butt formula predicts maximum drug-free lean body mass from height, wrist circumference and ankle circumference, because wrist and ankle are stable proxies for skeletal frame size that training does not change. The result is a lifetime ceiling reached only by well-trained natural competitors, not a target for a single year of training.
Narrowest point, just below the wrist bone.
Narrowest point above the ankle bone.
Defaults to 8% for men, 18% for women.
80.2 kg of lean mass
Predicted maximum girths (approximate)
Method notes (5)
- Casey Butt 2008 formula — fit on natural-champion bodybuilders. Predicts the upper bound of drug-free lean mass; most lifters land below it.
- Wrist + ankle are stable bone-frame proxies that don't change with training; the result is a lifetime ceiling, not a near-term target.
- Body-fat term: LBM falls slightly at stage-lean BF% vs off-season. The reference_band shows max weight at typical conditions.
- Predicted_max_circumferences_cm use Park/Casey-Butt rule-of-thumb ratios (forearm 1.16×wrist, bicep 1.96×wrist, neck 2.5×wrist, chest 6.5×wrist, calf 1.92×ankle, thigh 4.65×ankle). Rougher than the LBM number — flag as approximate.
- Measurement matters: wrist at the narrowest point distal to the styloid; ankle at the narrowest point above the lateral malleolus. A 0.5 cm tape error shifts the result a couple of kg.
Metric or imperial · runs the same code as the app · nothing is sent anywhere
How it works
Casey Butt's 2008 formula, fitted to a population of natural champion bodybuilders: LBM(lb) = height(in)^1.5 × (√wrist/22.667 + √ankle/17.0104) × (BF%/224 + 0.984). Wrist and ankle stand in for skeletal frame size, which training does not change, so the output is a lifetime ceiling.
The body-fat term exists because lean mass peaks slightly off-season and dips at stage condition, which is why the reference band shows a different maximum weight at each condition. For women a 0.85 scalar is applied to the male-derived fit, a heuristic from the natural bodybuilding literature where population data is thinner.
The girth predictions use the Park and Casey Butt rule-of-thumb ratios against wrist and ankle. They are rougher than the lean-mass figure, so treat them as indicative.
Common questions
Is the Casey Butt formula realistic?
It predicts the upper bound reached by drug-free competitors with excellent training and nutrition over many years. Most lifters land well below it, so read it as a ceiling rather than a forecast.
Why wrist and ankle?
They are almost pure bone with very little soft tissue over them, so they are stable proxies for frame size and do not change as you gain or lose weight.
How precisely do I need to measure?
Half a centimetre of error shifts the result by a couple of kilos. Measure at the narrowest point, with the tape snug but not compressing.
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