Can agents run this deterministically?

Yes. Supply sex, weight_kg, and height_cm as JSON to receive lbm_kg, body_fat_pct, and per-formula breakdowns deterministically.

Body Composition

Lean Body Mass Calculator

Estimate lean body mass (fat-free mass) using Boer, James, Hume, and Peters formulas from height and weight.

Lean Body Mass Estimates

Average Lean Body Mass

55 kg

Est. body fat: 31.2%

Formula Comparison

Boer: 60.9 kg(23.9% fat)
James: 62.1 kg(22.3% fat)
Hume: 57.1 kg(28.6% fat)
Peters: 39.9 kg(50.1% fat)

Estimated Body Composition

Average lean body mass and estimated fat mass together explain total bodyweight.

Current bodyweight

80 kg

Lean mass

55 kg

Fat mass

25 kg

Lean body mass (LBM) is your total weight minus fat mass — including muscle, bone, organs, and water. These formulas estimate LBM from height and weight only. For precision, use body composition testing (DEXA, hydrostatic).

How to use it

Enter total body weight and body fat percentage. If you do not know your body fat %, run the Body Fat Percentage Calculator first.
LBM = total weight minus fat mass. A 185 lb person at 20% body fat has 148 lbs of lean mass. This is your metabolically active tissue — it drives BMR, protein needs, and strength potential.
Use LBM × 1.0–1.2 for daily protein target during a fat loss phase. This is more accurate than using total body weight because it accounts for the actual muscle mass at risk of catabolism.
During a successful cut, LBM should decrease by no more than 20–25% of total weight lost. Losing 10 lbs while maintaining 7.5+ lbs of LBM indicates effective fat-focused cutting. Larger LBM losses signal too steep a deficit or insufficient protein.
Re-run monthly and compare LBM across months: rising LBM while weight holds or drops = muscle gain or successful recomposition. Stable LBM while body fat drops = textbook fat loss.

AI Integrations

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

Always available for agents

Tool contract JSON

https://aifithub.io/contracts/lean-body-mass-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": "lean_body_mass",
  "sex": "male",
  "weight_kg": 80,
  "height_cm": 178
}

Open tool contract JSON Open AI integrations guide

Expand developer notes

Agent playbook

Resolve Lean Body Mass 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 lean body mass?

Lean body mass (LBM) is your total body weight minus fat mass. It includes muscle, bone, organs, water, and connective tissue — everything except stored body fat.

Which formula should I trust most?

All four formulas (Boer, James, Hume, Peters) are validated estimates. The average provides a reasonable middle ground. For precision, use DEXA or hydrostatic weighing.

Why do results differ between formulas?

Each formula was derived from different study populations and uses different regression coefficients. Variation of 1-3 kg between formulas is normal.

How is this different from body fat percentage calculators?

Body fat calculators typically require circumference measurements (Navy method). LBM calculators estimate fat-free mass from height and weight only — simpler inputs but less precise.

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 Lean Body Mass Calculator. Use the explanation, formula, examples, and benchmarks to pressure-test the calculator output from first principles.

Browse all 14 resources

Guides · 1 Checklists · 1 Learn · 1 Tips · 1 Mistakes · 2

How To Use

5 STEPS

How to Use Lean Body Mass Calculator

reveal your fitness potential by calculating your lean body mass. This guide shows you how to use the LBM calculator to track progress, optimize training, and set realistic body composition goals for a healthier you.

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.

Read

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.

Read

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

Formula

3 VARIABLES

Lean Body Mass Formula

Calculate your Lean Body Mass (LBM) using a simple formula incorporating your weight and height. Understand your body composition for better fitness and health insights with AI Fit Hub.

Read

Examples

4 EXAMPLES

Body Fat Examples

Explore diverse real-world body fat examples, from sedentary professionals to athletes and new mothers, revealing unique insights into health, performance, and body composition.

Read