What is calculated
Current fat mass, current lean mass, and the target weight implied by two explicit composition assumptions.
Body Recomposition Calculator
Scenario target weight from fat and lean mass assumptions
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 sourcesReview the equation, normalized inputs, assumptions, and version.
Versioned calculationFormula v1.0.0
Method
Add the selected lean-mass change, then divide target lean mass by one minus target body-fat fraction.
Important boundary
Body-fat measurements contain error, and simultaneous fat loss and lean gain cannot be guaranteed.
Split current massCalculate estimated fat mass and lean mass.
80 kg at 25% gives 20 kg fat and 60 kg lean.Set the hypothetical endpointAdd selected lean mass and choose a lower body-fat percentage.
62 kg lean at 20%.Solve target weightDivide target lean mass by the target lean fraction.
62 / 0.80 = 77.5 kg.Interpretation
kg is the weight implied by the entered endpoint assumptions.
Fat-mass and scale changes are shown separately.
No rate, calorie plan, or muscle-gain promise is produced.
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Related calculators and resources
Method and test recordFormula v1.0.0
- Recorded scope
- health-nutrition-goals
- Publisher
- CalculatorGeek
- Recorded review date
- 2026-10-04
- Next source review
- 2027-10-04
- Definition fixtures
- 3 configured scenarios
- Published examples
- 3 shown below
Recorded method
Verify fat/lean split, target-weight algebra, implied changes, unit parity, and target-direction validation.
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 can disagree materially.
- Lean mass includes water and other fat-free tissues, not only muscle.
- The model supplies no calorie target, training program, or timeline.
- A scenario can be mathematically consistent without being realistic or appropriate.
Method sources
- Vargas-Molina et al.: nutrition protocols and body recomposition
- Morton et al.: protein supplementation and resistance-training meta-analysis
- NIDDK: Dynamic body-weight model materials
Latest model update
2026-10-04 - Initial shared-system implementation with distinct workflow, evidence boundaries, fixtures, and authored support content.
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 |
|---|---|---|
| reference recomposition | Current body weight: 80 kg; Current estimated body fat: 25 percent; Scenario target body fat: 20 percent; Scenario lean-mass change: 2 kg | Estimated current fat mass: 20 kg; Estimated current lean mass: 60 kg; Scenario target weight: 77.5 kg; Scenario target fat mass: 15.5 kg; Implied fat-mass change: -4.5 kg; Implied scale-weight change: -2.5 kg (allowed numeric tolerance: 0.0001) |
| zero lean change boundary | Current body weight: 70 kg; Current estimated body fat: 20 percent; Scenario target body fat: 15 percent; Scenario lean-mass change: 0 kg | Estimated current lean mass: 56 kg; Scenario target weight: 65.8823529 kg (allowed numeric tolerance: 0.0001) |
| invalid target direction | Current body weight: 80 kg; Current estimated body fat: 20 percent; Scenario target body fat: 25 percent; Scenario lean-mass change: 2 kg | Reject invalid input: current_body_fat_percent |
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On this page
What the Body Recomposition Calculator answers
Translate current body weight and estimated body fat into a hypothetical endpoint using an explicit lean-mass change and target body-fat percentage.
The calculator separates fat mass, lean mass, and scale weight so users can see why recomposition does not necessarily mean a large scale change.
Use the Health calculators hub to move between nutrition and body-composition workflows.
How to use the Body Recomposition Calculator
- Enter current weight and an estimated current body-fat percentage.
- Choose a lower scenario body-fat percentage.
- Enter a hypothetical lean-mass change.
- Read the endpoint as composition arithmetic, not a forecast.
For the closest upstream calculation, open the Body Fat Calculator. Estimate body-fat percentage with a separate method.
Formula, variables and calculation order
current fat mass = weight x body-fat%; current lean mass = weight - fat mass.
target weight = (current lean mass + selected lean change) / (1 - target body-fat%).
To compare the method with a neighboring workflow, use the Lean Body Mass Calculator. Compare formula-based lean-mass estimates.
Worked example and boundary check
80 kg, 25% to 20%, plus 2 kg lean mass
Current fat mass is 20 kg and lean mass is 60 kg. The scenario uses 62 kg lean mass; at 20% body fat, target weight is 77.5 kg and target fat mass is 15.5 kg.
Boundary check
A target body-fat percentage at or above the current estimate is rejected because it does not match this loss-plus-lean-gain workflow.
How to interpret the result
The primary output is the scale weight implied by the selected composition endpoint. It does not predict the program or time needed to reach it.
- Body-fat methods can disagree materially.
- Lean mass includes water and other fat-free tissues, not only muscle.
- The model supplies no calorie target, training program, or timeline.
- A scenario can be mathematically consistent without being realistic or appropriate.
For another useful cross-check, continue with the Weight Loss Timeline Calculator. Inspect separate energy-gap timeline arithmetic.
Inputs, outputs and result meaning
Separates fat, lean, and scale mass; treats lean gain as an input; and excludes automatic calorie cycling or tissue forecasts.
Inputs
- Current body weight
- Use a current measured body weight.
- Current estimated body fat
- Use one consistent measurement method and treat the result as an estimate.
- Scenario target body fat
- This is a scenario assumption, not a recommended target.
- Scenario lean-mass change
- Enter a hypothetical lean-mass change; the calculator does not predict it.
Outputs
- Scenario target weight
- Primary result in kg.
- Estimated current fat mass
- Supporting result in kg.
- Estimated current lean mass
- Supporting result in kg.
- Scenario target fat mass
- Supporting result in kg.
- Implied fat-mass change
- Supporting result in kg.
- Implied scale-weight change
- Supporting result in kg.
Frequently asked questions
Is lean mass the same as muscle?
No. Lean mass includes water, organs, bone, and other fat-free tissue.
Does the calculator predict muscle gain?
No. The lean-mass change is an input, not a forecast.
Why might target weight be lower despite added lean mass?
The assumed fat-mass decrease can be larger than the assumed lean-mass increase.
Which body-fat method should I use?
Use one consistent method and focus on trends; different methods can produce different estimates.
Does this give calories?
No. It intentionally separates endpoint composition arithmetic from energy planning.
Primary sources and evidence limits
- Vargas-Molina et al.: nutrition protocols and body recomposition
- Morton et al.: protein supplementation and resistance-training meta-analysis
- NIDDK: Dynamic body-weight model materials
Sources were checked 2026-10-04. They support the calculation method or its evidence boundary; they do not turn a planning scenario into medical nutrition advice.