Same starting point, three paths
Use $500,000 over ten years. The figures below come from the same Money Stack engine used by the calculator.
| Scenario | Rate | Monthly contribution | Monthly cost | Ending capital |
|---|---|---|---|---|
| Base | 8% | $15,000 | $0 | $3,872,305 |
| Upside | 10% | $20,000 | $0 | $5,484,561 |
| Stress | 4% | $5,000 | $3,000 | $1,042,370 |
The large currency amounts are deliberately illustrative. The comparison is about sensitivity, not a market prediction.
Choose assumptions you can defend
A base rate should be explainable, not desired. In the stress case, lower the rate and contribution while adding a cost or missed-contribution period. An upside case should not turn a risky historical average into a smooth guaranteed return.
What makes a plan resilient
A resilient plan does not sacrifice essential goals in the stress case, and the recurring contribution fits actual free cash flow. Check your liquid reserve in Cash Years before assigning the same cash to a long horizon.
Scenario-analysis mistakes
- changing only the rate while every other assumption remains perfect;
- counting contributions as return;
- ignoring liquidity and possible capital loss;
- adjusting inputs repeatedly until the target appears;
- failing to record why an assumption changed.
Cross-check constant-growth arithmetic with the Investor.gov compound interest calculator. Product selection and risk assessment remain outside this model.