Muscle Gain Potential Calculator

What does the Muscle Gain Potential Calculator calculate, and which formula does it use?

The Muscle Gain Potential Calculator estimates maximum natural lean body mass with the Casey Butt model, which predicts it from height, wrist, and ankle circumference: LBM = H^1.5 × (√wrist/22.667 + √ankle/17.0104) × (BF%/224 + 1), evaluated at 10% body fat. It reports how much of that ceiling you have already reached and estimates the years remaining on Lyle McDonald's yearly gain curve from your training year.

Last updated October 21, 2026.

How far off can this be?

The bone-structure formula this calculator uses has no published validation study. The measured ceiling it can be checked against: in 157 male athletes, the height-normalised fat-free mass index (FFMI) of the 74 who had never used anabolic steroids extended up to a well-defined limit of 25.0, and 20 pre-steroid-era Mr. America winners averaged 25.4.

For the example (a man, 178 cm), the calculator's maximum of 80.3 kg lean mass is a normalised FFMI of 25.5, above the highest value drug-free athletes reached in that study. Read it as an upper bound, not a target.

The study measured one sample of male athletes and its authors call the findings preliminary; there is no comparable ceiling for women in it.

Source: Kouri, Pope, Katz & Oliva 1995, Clin J Sport Med, “Fat-free mass index in users and nonusers of anabolic-androgenic steroids” .

How to use it

1. Enter your sex, height, wrist circumference (measured at the narrowest point just below the wrist bone), and ankle circumference (measured at the narrowest point above the ankle bone). These bone structure measurements are the foundation of the Casey Butt model, developed through analysis of top-level drug-tested bodybuilders and published in his book 'Your Muscular Potential.' The model uses wrist and ankle circumference as proxies for overall skeletal frame size because these sites have minimal soft tissue coverage and do not change with training, making them stable predictors of the skeleton's capacity to support muscle mass. A 1 cm difference in wrist circumference shifts the predicted maximum lean body mass by approximately 2-3 kg, and a similar difference in ankle circumference shifts it by 1-2 kg. Measure carefully using a flexible tape pulled snug but not compressing the tissue, and take the average of two measurements.
2. Add your current body weight and estimated body fat percentage to calculate your current lean body mass and compare it against your predicted genetic maximum. The calculator derives your current lean mass as: body weight x (1 - body fat% / 100), then compares this against the Casey Butt prediction for your frame. Separately, the Lyle McDonald model provides a complementary perspective by estimating the maximum rate of muscle gain by training year: approximately 9-11 kg in year one, 4.5-5.5 kg in year two, 2-3 kg in year three, and 1-1.5 kg in year four and beyond, assuming optimal training, nutrition, and recovery. In McDonald's model women gain at roughly half the male rate. Alan Aragon's model offers similar estimates scaled by training status: beginners can gain 1-1.5% of bodyweight per month in lean mass, intermediates 0.5-1%, and advanced trainees 0.25-0.5%.
3. Read your percentage of genetic potential reached, the calculator's main output: your current lean mass as a share of the Casey Butt model's predicted maximum for your frame. There are no validated benchmarks for where lifters sit by training age, so use it to set expectations rather than to grade yourself: the closer you are to the model's ceiling, the more slowly each further kilogram comes, and a lifter well below it after a couple of years usually has training consistency or nutrition to fix first.
4. Review the estimated timeline for remaining gains, which applies the Lyle McDonald diminishing returns model to your specific situation. Enter your years of consistent training so the curve starts where you are: with 6 kg of lean mass remaining after two years of training, the model projects about 5 more years, because each year adds less than the one before. The timeline assumes optimized conditions: protein intake at 1.6-2.2 g/kg/day (per Morton et al. 2018 meta-analysis), a caloric surplus of 200-500 calories above maintenance during hypertrophy phases, progressive overload in training with 10-20 hard sets per muscle group per week, and adequate sleep of 7-9 hours nightly. Significant deviations from any of these conditions will extend the timeline. Age is also a factor: testosterone levels decline by approximately 1% per year after age 30, and research by Bhasin et al. (2001) showed that this hormonal decline correlates with reduced hypertrophy rates, though the effect is modest until the 50s and 60s.
5. Validate your inputs and cross-reference with other body composition metrics for the most complete picture. The Casey Butt model was derived from drug-tested male bodybuilders and has less validation data for women and for individuals outside the typical bodybuilder physique archetype. The model assumes you are training primarily for hypertrophy with full-body coverage. Individuals who train predominantly for strength (powerlifters) or sport-specific performance may carry less total muscle mass but more mass in specific areas, which the model does not account for. Cross-reference your FFMI from the FFMI Calculator: your predicted maximum lean mass from the Casey Butt model should produce an FFMI in the 24-26 range for men, which aligns with the Kouri et al. (1995) natural ceiling research. If the two calculators produce wildly different predictions, re-check your body fat percentage estimate, as this is the most common source of error across all body composition tools.

Questions people usually ask

Related calculators

• Body composition Body Fat Percentage Calculator

  Estimate body fat percentage using the U.S. Navy circumference method.

• Nutrition TDEE Calculator

  Estimate your daily energy expenditure with Mifflin-St Jeor + activity factors.

• Nutrition Macro Calculator

  Convert calorie targets into protein, carbs, and fat grams for your goal.

• Planning Calorie Deficit Calculator

  Estimate required daily calorie deficit for a target timeline and bodyweight change.

Articles

• 11 min read FFMI 25 Natural Limit: The Kouri 1995 Study, Reappraised

  FFMI 25 as the natural muscular limit: what Kouri's 1995 study of 157 male athletes measured, and what later collegiate cohorts show about the upper end.

• 11 min read Aggressive Bulk vs Lean Bulk: The Math (Garthe RCT)

  Aggressive bulk vs lean bulk, run through the Garthe RCT: a 600-cal surplus added 1.7 kg lean and 1.1 kg fat in 12 weeks vs 1.2 kg lean, zero fat.

• 7 min read Muscle Gain Potential: A 178cm Narrow-Frame Male Case

  Muscle gain ceiling 80.3 kg for a 178 cm, 17.5 wrist / 22 ankle male. 16.3 kg remaining, 79.7% of potential reached, ~3 years to ceiling. Casey Butt math.

Use it from an agent or code

Any agent can run this calculator by URL: put the inputs in the query string and get JSON back, with the inputs that used defaults and a line to cite (add format=md for markdown). MCP clients reach it through https://aifithub.io/mcp.

https://aifithub.io/api/run/muscle-gain-potential-calculator?sex=male&height_cm=178&wrist_cm=17.5&ankle_cm=22&current_weight_kg=80&current_body_fat_pct=18&training_years=0

In code, import the same module; it runs where you import it, with no request or key.

import { compute } from "https://aifithub.io/engines/muscle-gain-potential-calculator.js";

Input and output contract, TypeScript types and the guide for agents.