Natural Muscular Potential Calculator (Casey Butt & Berkhan)

Understanding the natural muscular potential calculator — concept and principle

This calculator estimates the upper limit of muscle a drug-free lifter can usually build after many years of consistent training. It uses two popular rules from the fitness world: Casey Butt's equation, based on height and the size of the wrist and ankle, and Martin Berkhan's simpler "height minus 100" rule.

Bigger wrists and ankles mean a higher ceiling: here 77.81 kg lean, 86.46 kg at 10% body fat.

How it is calculated

Butt's equation works in inches and pounds. With height H, wrist W and ankle A converted from cm to inches (÷ 2.54) and target body fat %BF:

Max lean body mass (lb) = H^1.5 × (√W ÷ 22.667 + √A ÷ 17.0104) × (%BF ÷ 224 + 1)

The result is multiplied by 0.4536 to give kilograms.
Total body weight at that body fat = lean mass ÷ (1 − %BF ÷ 100).
Berkhan's rule: very lean stage weight (kg) ≈ height (cm) − 100.

The principle

Butt fitted his equation to the measurements of drug-free bodybuilding champions from before the era of widespread steroid use. Wrist and ankle size stand for bone structure: thicker joints tend to go with a larger frame that can carry more muscle. The height term grows faster than linearly because a taller frame has longer muscles. The small body-fat factor reflects that people carrying a little more fat also tend to carry slightly more lean tissue. Berkhan's rule comes from coaching experience and describes a very lean, contest-ready look.

Worked example

Inputs: height 175 cm, wrist 17 cm, ankle 22 cm, target body fat 10%.
Convert: H = 68.90 in, W = 6.69 in, A = 8.66 in; H^1.5 = 571.88.
Bracket: √6.69 ÷ 22.667 + √8.66 ÷ 17.0104 = 0.1141 + 0.1730 = 0.2871.
Body-fat factor: 10 ÷ 224 + 1 = 1.0446. Lean mass = 571.88 × 0.2871 × 1.0446 = 171.5 lb.
In kg: 171.5 × 0.4536 = 77.81 kg lean mass.
At 10% fat: 77.81 ÷ 0.90 = 86.46 kg. Berkhan: 175 − 100 = 75 kg.

Each point is the calculator re-run with only “Height (cm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Height (cm)	Max lean weight (Butt)
43.8	9.73
87.5	27.51
131	50.54
175	77.81
219	108.75
262	142.95
350	220.09

How to read the result

Treat both figures as a ceiling that most people approach but rarely reach, typically only after five to ten years of serious training. In this example the lean mass corresponds to a fat-free mass index of about 25.4 (77.81 ÷ 1.75²), close to the value of about 25 that Kouri and colleagues observed as an upper range in drug-free athletes.

Common mistakes

Measuring the wrist or ankle loosely or at a thick point, which inflates the result.
Choosing a very high body-fat target to "unlock" more muscle; the factor is small.

Keep in mind: These are rough statistical rules from small, selected samples. Genetics vary widely, and the numbers say nothing about health. This is not medical or training advice.

Frequently asked questions

How long does it take to reach my natural limit?

Gains slow sharply after the first few years; approaching the predicted ceiling usually takes many years.

Why do Butt and Berkhan give different numbers?

Berkhan's figure is a stage-lean total weight, while Butt's lean mass excludes fat entirely and depends on your joints.

Is the calculator valid for women?

Butt's equation was built on male data, so treat the result for women as unreliable.

Sources: Butt C. Your Muscular Potential: How to Predict Your Maximum Muscular Bodyweight and Measurements. Weightrainer.net (e-book) · Kouri EM, Pope HG Jr, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med 1995;5:223–228

Checked 8 Nov 2026. Published formulas cited on the page. For information only — not medical advice.

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