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Maximum Muscular Potential Calculator (Casey Butt)

Estimate your drug-free genetic ceiling — max lean body mass, bodyweight and measurements — from height, wrist and ankle using Casey Butt's formula.

A drug-free genetic-ceiling estimate. General fitness information, not medical advice.

About this tool

The Maximum Muscular Potential Calculator estimates the muscular ceiling a lean, drug-free lifter could realistically reach, based on Casey Butt's research on natural bodybuilding champions. It uses your frame — height, wrist and ankle circumference — plus a target body-fat level. All maths runs locally in your browser and nothing is uploaded.

Maximum lean body mass in pounds is LBM = height^1.5 × (√wrist ÷ 22.6667 + √ankle ÷ 17.0104) × (body-fat% ÷ 224 + 1), with height, wrist and ankle in inches (centimetres are converted first). Your maximum muscular bodyweight at the chosen body-fat is then LBM ÷ (1 − body-fat% ÷ 100). The predicted 'max' measurements are scaled from wrist circumference using Casey Butt's published multipliers — chest 6.5×, thigh 3.445×, neck 2.405×, biceps 2.34×, calf 2.21×, forearm 1.885× and waist 4.55× your wrist.

This is a genetic-ceiling estimate for natural (drug-free) lifters, not a promise or a target date — bone structure, training history and genetics all vary, and almost nobody reaches the exact number. It is general fitness information, not medical advice.

Frequently asked questions

Why use wrist and ankle size?
Wrist and ankle circumference are good proxies for bone structure, which strongly limits how much muscle a frame can hold. Bigger joints support more lean mass, so Casey Butt built them into the formula.
Is this an achievable goal or a hard limit?
It is an estimated ceiling for a lean, drug-free lifter after years of training — a realistic upper bound, not a target you should expect to hit. Most people top out somewhat below it.
What target body-fat should I enter?
The formula was calibrated around contest-lean conditions, so 8–10% is typical for men. A lower body-fat gives a leaner predicted bodyweight, while lean body mass itself changes only slightly.
Does this apply to enhanced lifters?
No. The model is built from natural bodybuilding data, so it estimates a drug-free ceiling only. It is general fitness information, not medical advice, and individual results vary widely.

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