Calculator · Goal Weight Calculator
What bodyweight would correspond to a selected body-fat percentage?
This calculator creates a theoretical scenario: it estimates current lean mass and calculates what total bodyweight would result if exactly that lean mass stayed unchanged at the selected target body-fat percentage.
Result
- Estimated target weight
- 78.0kg
- Change
- 7.0kg
- Current fat mass
- 18.7kg
- Lean body mass
- 66.3kg
Assumes lean mass is preserved.
to lose
Theoretical scenario assuming constant lean mass. Body-fat measurement error and real changes in fat-free mass can shift actual bodyweight substantially.
Method
How the calculation works
Current fat-free mass is estimated from bodyweight and entered body-fat percentage. That fat-free mass is then divided by the fat-free fraction of the target body-fat percentage.
The key assumption is that lean mass stays completely constant. The calculator does not model muscle gain, muscle loss, water changes or body-fat measurement error.
Very low target body-fat values trigger a conservative warning because a general tool cannot assess the individual health suitability of such targets.
Methodology: formulas, assumptions and estimate confidence →
Estimate confidence
How reliable is this estimate?
Low
The algebra is unambiguous, but the result depends heavily on an often-estimated body-fat value and the unrealistic assumption that lean mass remains completely constant.
Interpretation
How to read your result
The result is a mathematical scenario, not an ideal or medically recommended bodyweight. It only answers what weight follows from the constant-lean-mass assumption.
Because starting body fat is often itself an estimate, even a modest measurement error can meaningfully shift the calculated target. Treat the value as a rough comparison point rather than an exact destination.
Limits
Where this calculator ends
The strongest assumption is constant lean mass. During real dieting or gaining phases, water, glycogen, muscle and other fat-free tissues can change.
Body-fat methods outside imaging-based laboratory methods have method- and application-dependent error. The calculator cannot verify the quality of the body-fat value you enter.
The calculator does not define 'healthy', 'ideal' or medically safe body-fat ranges. Very low targets require individualized professional context.
Common mistakes
Common mistakes
- 01Treating the result as a medically ideal bodyweight.
- 02Treating the entered body-fat percentage as an exact measurement.
- 03Assuming lean mass is guaranteed to stay constant during weight change.
- 04Assuming very low target body-fat values are suitable as long as no warning appears.
- 05Over-interpreting small changes in the calculated target weight.
Example
A worked example
Illustrative scenario: 80 kg at an estimated 20 % body fat → 15 % target
Estimated lean mass is 64 kg. If those 64 kg stayed completely constant, 64 ÷ 0.85 ≈ 75.3 kg. That is a calculation scenario, not a prediction of a real diet trajectory.
FAQ
Frequently asked questions
- Is the result my ideal target weight?
- No. It is only the weight that follows mathematically if your entered lean mass stayed exactly constant at the target body-fat percentage.
- Does lean mass really stay constant during a diet?
- Not necessarily. Water, glycogen, muscle and other fat-free components can change. That is why the result is a scenario.
- How accurate does my body-fat estimate need to be?
- The less accurate the starting value, the less accurate the target weight. Use the same method under similar conditions where possible and interpret trends cautiously.
- Which target body-fat values are healthy?
- This calculator deliberately does not define them. Health suitability depends on the individual and context and cannot be inferred from a general calculator threshold.
- Why does the tool warn at very low targets?
- The warning limits the scope of a general tool. It does not mean a value just above the warning threshold is automatically suitable or safe.
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