Methodology
How the calculator turns assumptions into a retirement estimate.
The WhenCanIRetire.net calculator is an educational planning model. It is meant to make the moving parts of retirement easier to compare, not to predict a single guaranteed date. Every result depends on the numbers you enter and on simplified assumptions about investment returns, inflation, spending, taxes, Social Security, and market volatility.
Return to the calculator to test these assumptions against your own scenario.
FIRE number and safe withdrawal rate
The FIRE number is the portfolio target implied by your expected retirement spending and withdrawal rate. A simple version divides annual spending by the safe withdrawal rate. For example, a 4% withdrawal rate points to a target of about 25 times annual spending.
The safe withdrawal rate is not a promise. It is a planning assumption about how much of a portfolio might be withdrawn in the first year of retirement, then adjusted over time. Lower rates usually require a larger portfolio but leave more room for uncertainty. For more background, see the safe withdrawal rate guide.
Contributions and projected portfolio
Before retirement, the calculator projects your portfolio by combining your current balance, ongoing contributions, and assumed investment growth. Contributions are treated as regular additions to the portfolio, so higher saving rates can shorten the time needed to reach the target.
The projection is deliberately simple. It does not know your exact account mix, payroll timing, employer match rules, tax treatment, future salary changes, or whether contributions will pause. The result is best read as a directional estimate that becomes more useful when you compare several scenarios side by side.
Inflation and spending power
Inflation matters because a retirement dollar in the future may buy less than a dollar today. The calculator uses inflation assumptions to translate long-term spending and portfolio needs into a more realistic planning frame.
Small changes in inflation can have large effects over decades. If your estimate feels sensitive to this input, that is useful information: it means your plan may need a larger margin of safety, lower fixed expenses, or periodic reviews. The inflation and retirement guide explains this risk in more detail.
Social Security estimate handling
When Social Security is included, the calculator treats it as a future income source that can reduce the amount your portfolio must cover after benefits begin. That can lower the required portfolio draw in later retirement years, but it does not remove the need to fund the years before benefits start.
Social Security estimates are inherently uncertain. Actual benefits depend on your earnings record, claiming age, future law, cost-of-living adjustments, taxes, and household circumstances. Use the calculator's estimate as a placeholder, then compare it with your official statement and the tradeoffs described in the Social Security timing guide.
Monte Carlo simulation and volatility
A straight-line projection assumes the same return pattern every year. Real portfolios do not behave that neatly, so the calculator also uses Monte Carlo simulation to show a range of possible outcomes. Instead of one smooth path, the model tests many randomized paths using return and volatility assumptions.
Volatility affects the order and size of returns. Two portfolios can earn similar average returns and still produce different retirement outcomes if losses arrive early or withdrawals happen during market declines. This is closely related to sequence risk, which is covered in the sequence of returns risk guide.
Scenario comparison
The calculator is most useful when you compare choices rather than stare at one output. Changing retirement spending, contribution amounts, withdrawal rates, expected returns, inflation, or Social Security timing can reveal which assumptions have the biggest effect on your date and probability range.
Scenario comparison can also separate decisions you control from conditions you can only plan around. Savings rate, spending level, and retirement age are usually more controllable than market returns or inflation. The retirement planning basics guide is a good starting point for choosing practical inputs.
Limitations and no financial advice
This calculator does not model every real-world detail. It may omit or simplify taxes, investment fees, account withdrawal order, required minimum distributions, pensions, healthcare costs, insurance, home equity, debt, estate planning, market regime changes, and changes in personal circumstances.
Nothing on WhenCanIRetire.net is financial, investment, tax, legal, or retirement advice. The site provides educational estimates only. Before making major financial decisions, review your numbers carefully and consider speaking with a qualified professional who can evaluate your full situation.