What is calculated
Estimated lean-mass change between two measurements.
GLP-1 Body Composition Change Calculator
Estimated lean-mass change
Enter your values, then calculate to see a verified result.
Reference range
How this result is calculated
A clear calculation path based on your inputs.
Inputs usedReview the information used for this result.
Full calculation and sourcesOpen Full calculation and sources to review the method, assumptions, boundaries, and evidence.
Versioned calculationFormula v1.0.0
Method
Convert both weight/body-fat pairs to lean mass and subtract.
Important boundary
Body-fat measurement error can be larger than the reported change.
Use comparable measurementsUse the labeled field and matching unit.
Calculate lean mass twiceThe calculator applies the displayed method at full precision.
Interpret measurement error firstKeep the stated boundary beside the result.
Interpretation
A negative result means the current estimate is lower.
Starting and current lean mass remain visible.
The tool cannot identify muscle function, cause, or treatment risk.
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Related calculators and resources
Method and test recordFormula v1.0.0
- Recorded scope
- health-evidence-completion
- Publisher
- CalculatorGeek
- Recorded review date
- 2026-10-05
- Next source review
- 2027-04-05
- Definition fixtures
- 1 configured scenarios
- Published examples
- 1 shown below
Recorded method
Verify both lean-mass calculations, signed change, zero-loss handling, unit parity, and non-causal wording.
These records describe the published model and reference tests. A test-case count is not a certification of every possible input or an independent specialist review. Editorial policy
Known limitations
- Body-fat methods have meaningful error.
- Lean mass is not identical to skeletal muscle.
- Hydration and glycogen affect estimates.
- Does not predict future loss or prescribe protein, exercise, dose, or medicine changes.
Method sources
- FDA Wegovy prescribing information (2026)
- FDA Zepbound prescribing information (2026)
- Kouri et al. fat-free mass index study
Latest model update
2026-10-05 - Initial evidence-limited shared-system implementation with fixtures, original support content, and contextual links.
Reference inputs and expected results
Up to 12 examples from the configured definition fixtures are shown. Expected values use the stated output units; invalid inputs are intended to be rejected.
| Case | Inputs | Expected result |
|---|---|---|
| two kilogram estimated decrease | Starting weight: 100 kg; Starting body-fat estimate: 35 percent; Current weight: 90 kg; Current body-fat estimate: 30 percent | Starting estimated lean mass: 65 kg; Current estimated lean mass: 63 kg; Estimated lean-mass change: -2 kg; Estimated lean share of weight lost: 20 %; classification: estimated-lean-mass-decrease (allowed numeric tolerance: 0.0001) |
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On this page
What the GLP-1 Body Composition Change Calculator calculates
Estimate lean-mass change between two comparable weight and body-fat measurements.
The route answers a measurement-change question and does not claim to predict GLP-1 muscle-loss risk.
Start with the Health calculators hub when a neighboring workflow better matches the question.
How to use the GLP-1 Body Composition Change Calculator
- Use the same body-composition method at both dates.
- Enter starting weight and body-fat estimate.
- Enter current weight and body-fat estimate.
- Compare the change with expected measurement error and functional measures.
Continue with the Protein Calculator. Plan protein separately from medicine decisions.
Formula, variables and calculation order
lean mass = weight x (1 - body fat / 100); change = current lean mass - starting lean mass.
Inputs are validated and normalized before the calculation. Working precision is retained until display rounding.
For a separate method, open the TDEE Calculator. Estimate whole-day energy expenditure.
Worked example and boundary check
Worked example
100 kg at 35% estimates 65 kg lean mass; 90 kg at 30% estimates 63 kg, a -2 kg change.
Boundary check
A positive scale-weight loss with no estimated lean loss produces a 0% lean share; no weight loss also reports 0%.
How to interpret the result
Use repeated standardized measurements and strength/function context before drawing conclusions.
- Body-fat methods have meaningful error.
- Lean mass is not identical to skeletal muscle.
- Hydration and glycogen affect estimates.
- Does not predict future loss or prescribe protein, exercise, dose, or medicine changes.
Compare the result with the Body Fat Calculator. Estimate body composition with a named method.
Frequently asked questions
Is lean mass the same as muscle?
No.
Does a negative value prove medication caused it?
No.
Why use the same method?
Method changes can be larger than the biological change.
Does the percentage diagnose risk?
No.
Primary sources and evidence limits
- FDA Wegovy prescribing information (2026)
- FDA Zepbound prescribing information (2026)
- Kouri et al. fat-free mass index study
Sources and version-sensitive boundaries were checked 2026-10-05. The page does not replace diagnosis, prescribing, emergency care, or individualized professional judgment.