Can agents run this deterministically?

Yes. Supply weight_kg, height_cm, and body_fat_pct as JSON and receive ffmi, adjusted_ffmi, fat_free_mass_kg, and interpretation deterministically.

Body Composition

FFMI Calculator

Calculate your Fat-Free Mass Index to gauge muscularity relative to height and compare against natural training benchmarks.

Results

Fat-Free Mass: 68 kg
FFMI: 21.5
Adjusted FFMI: 21.6

Adjusted FFMI Position

Adjusted FFMI is shown against broad training and muscularity reference bands.

Adjusted FFMI

Above average — consistent training

21.6

1430

Adjusted FFMI normalised to 1.80 m reference height. FFMI ≥ 25 is widely cited as the approximate natural limit for men. Women's benchmarks are approximately 3–5 points lower. This is a screening estimate, not a diagnosis.

How to use it

Enter height, weight, and body fat percentage. FFMI normalizes muscle mass for height — removing body size from the comparison the way BMI removes it for total weight.
FFMI score interpretation: below 18 = below average muscle development, 18–20 = average, 20–22 = above average, 22–25 = athletic to advanced, above 25 = exceptional and rare in drug-free athletes.
FFMI above 25 has been proposed in research (Kouri et al., 1995) as a threshold above which drug-free status becomes statistically improbable. The vast majority of advanced natural lifters peak between 22–24.
To raise FFMI you must gain lean mass. Moving from FFMI 22 to 25 at fixed height typically requires 15–25 lbs of additional muscle — representing 5–8 years of serious dedicated natural training.
Accurate body fat input is critical: a 5-percentage-point error in body fat changes FFMI by 1.5–2 points. Use the most accurate measurement method available. DEXA > skinfold calipers > Navy formula.

AI Integrations

Contract, discovery endpoints, and developer notes for agent use.

Always available for agents

Tool contract JSON

https://aifithub.io/contracts/ffmi-calculator.json

Stable input and output contract for this exact tool.

Human review

People can use the browser page to sense-check outputs and charts, but agents should still execute against the contract and discovery endpoints.

/agent-tools.json - machine-readable tool index.
/llms.txt - human-readable model discovery guide.
/.well-known/webmcp.json - WebMCP capabilities manifest.
/.well-known/ai-plugin.json - plugin-style discovery manifest.

{
  "tool": "ffmi",
  "weight_kg": 80,
  "height_cm": 178,
  "body_fat_pct": 15
}

Open tool contract JSON Open AI integrations guide

Expand developer notes

Agent playbook

Resolve FFMI Calculator from /agent-tools.json and open its contract before execution.
Validate inputs against the contract schema instead of scraping labels from the page UI.
Open the browser page only when a person wants to review charts, assumptions, or related tools.

Agent FAQ

Should ChatGPT, Claude, or another agent click through the UI?

No. Start with /agent-tools.json, then follow the tool's contract URL. The page UI is for human review, not parameter discovery.

When do tools show Quick and Advanced?

Every tool opens in Quick Start first. Advanced Controls keeps the same scenario, reveals more assumptions or diagnostics, and every tool keeps AI integrations inline below the instructions.

When should an agent still open the browser page?

Open it when a human wants to sense-check the output, review the chart, or keep exploring related tools after the calculation finishes.

Questions people usually ask

What is FFMI and how is it different from BMI?

FFMI (Fat-Free Mass Index) measures lean mass relative to height, while BMI uses total weight. FFMI is more useful for muscular individuals because BMI penalises muscle the same as fat.

What does 'adjusted FFMI' mean?

Adjusted FFMI normalises your score to a reference height of 1.80 m, allowing fair comparison between people of different heights.

What is the natural FFMI limit?

Research (Kouri et al., 1995) suggests an adjusted FFMI of ~25 as the approximate ceiling for natural male trainees. Women's benchmarks are roughly 3–5 points lower.

Is this tool free and private to use?

Yes. AI Fit Hub tools are free, no-signup browser tools. Inputs stay in your browser unless you choose to share a URL.

Related Resources

Learn the decision before you act

Every link here is tied directly to FFMI Calculator. Use the explanation, formula, examples, and benchmarks to pressure-test the calculator output from first principles.

Browse all 18 resources

Guides · 1 Learn · 1 Mistakes · 1 Formulas · 7 Examples · 6

How To Use

5 STEPS

How to Use FFMI Calculator

reveal your muscle-building potential with the FFMI Calculator. Learn how to accurately input your data, understand your muscularity, and benchmark your progress against natural limits.

Read

Guide

6 MIN READ

How to Measure Body Fat at Home: 4 Methods Compared

Compare the Navy tape method, smart scales, calipers, and visual estimation for home body fat measurement. Learn which is most accurate and how to do each correctly.

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Formula

3 VARIABLES

BMI Formula

Calculate your Body Mass Index (BMI) with this clear guide. Learn the formula, measure weight and height correctly, follow a detailed example, and explore essential variations.

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Formula

2 VARIABLES

Body Fat Formula

reveal your body fat percentage with the Siri equation! Learn to calculate it using body density, understand variables, and explore common measurement variations for accurate fitness tracking.

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Formula

4 VARIABLES

Body Recomposition Formula

Master the Body Recomposition Formula to simultaneously build muscle and lose fat. Learn to calculate your ideal calorie target by adjusting TDEE for optimal results.

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Formula

3 VARIABLES

FFMI Formula

Calculate your Fat-Free Mass Index (FFMI) to assess lean body mass relative to height. Discover the formula, variables, and a practical example from AI Fit Hub.

Read