RetireFire Research
Why FIRE calculators disagree
Seven assumptions that make FIRE calculators produce different answers, plus a checklist for comparing tools without mistaking precision for truth.
Two FIRE calculators can receive the same balance and spending but return different dates. That does not automatically mean one is broken. They may be answering different questions with different timing, inflation, contribution, and withdrawal conventions.
A useful calculator publishes those conventions. A useful comparison changes one assumption at a time.
Seven assumptions to compare
| Assumption | Version A | Version B | Why it matters |
|---|---|---|---|
| Returns | 7% nominal | 5% real | Inflation may be counted differently |
| Contributions | Beginning of year | End of year | Earlier deposits compound longer |
| Target | 25× spending | 33.3× spending | 4% versus 3% planning rate |
| Income | Ignores benefits | Includes later income | Changes the portfolio-funded gap |
| Path | Constant return | Monte Carlo / history | Sequence risk changes ranges |
| Taxes and fees | Excluded | Estimated | Gross withdrawals can exceed spending |
| Stopping rule | Fractional year | Whole year | Displayed date may differ |
A fair comparison protocol
- Use the same current portfolio, contribution, spending, and target.
- Convert nominal and inflation assumptions into one real return.
- Match contribution timing and withdrawal timing.
- Disable Social Security, pensions, taxes, and fees unless both tools model them.
- Compare deterministic results first, then compare stress-test methodology separately.
Precision is not accuracy
A result of 12.43 years can still rely on uncertain returns and spending. More decimal places do not make the future more knowable. Use outputs as sensitivity maps and decision triggers.
Frequently asked questions
- Which FIRE calculator is correct?
- A calculator is correct relative to its formula and inputs. The more important question is whether its assumptions match the decision you are making.
- Should I use real or nominal returns?
- Either can work if inflation is handled consistently. RetireFire uses real returns by default so targets stay in today’s dollars.
- Why does contribution timing matter?
- Money contributed earlier receives more compounding. End-of-year contributions are a conservative simplification for many planning models.
Keep planning
Educational illustration only — not financial, investment, tax, legal, medical, or insurance advice. Calculations use simplified assumptions and do not predict future returns or benefits.