A wealth calculator is a controlled thought experiment. Its job is not to tell you what will happen. Its job is to show what follows mathematically if the inputs happen.
Separate facts from assumptions
| Input type | Examples | How to treat it |
|---|---|---|
| Known today | Current balance, monthly contribution, current BTC owned | Verify against records |
| Chosen assumption | Future return, future BTC price, inflation, withdrawal rate | Run a range |
| Often omitted | Fees, taxes, pauses, withdrawals, bad timing | Add a margin of safety |
The output can be mathematically correct and still be practically misleading if the assumptions are unrealistic. The more uncertain the input, the less confidence the final number deserves.
Use a three-scenario ladder
Instead of entering one return or price target, build a downside, middle, and upside case. Label them by assumption—not by emotional words like “safe” or “guaranteed.”
Lower return, higher fees, shorter timeline, or lower asset price.
A planning case you can explain without relying on perfect conditions.
A stronger outcome that remains possible but is not required for the plan to work.
The distance between the three outputs measures sensitivity. A plan that collapses under a small input change deserves more scrutiny than a plan that remains workable across the range.
Nominal dollars are not today’s dollars
Most calculators show nominal future values. Inflation can reduce future purchasing power. One simple approximation is to use a lower “real” return that subtracts expected inflation from the nominal return, though exact real-return math is slightly more complex.
Approximate real return ≈ nominal return − inflation rate
Taxes and fees should also be considered separately. A gross portfolio value is not automatically the amount available to spend.
Watch the timeline convention
Monthly contributions may be modeled at the beginning or end of each month. Returns may compound monthly or annually. These details can create different results even when the headline inputs look the same.
The 60-second result audit
- Can you restate every input without looking?
- Which input creates the largest change?
- How much is contributed versus modeled growth?
- Are fees, inflation, taxes, and withdrawals included?
- What happens in a downside scenario?
- Does another reputable calculator produce a similar answer?
Frequently asked questions
Why do two calculators give different results?
They may use different contribution timing, compounding frequency, rounding, fees, inflation, or tax assumptions.
Should I trust a precise dollar result?
The precision comes from arithmetic, not certainty. A result of $1,023,417 can still rest on very uncertain assumptions.
How often should I rerun a scenario?
Rerun it when the balance, contribution, timeline, costs, or objective changes—and periodically to keep the model connected to reality.
Sources and further reading
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