How the FI/RE calculator works
Checked Oct 24, 2026
Your FI/RE number is the savings that would pay your retirement spending, plus tax on it, at your withdrawal rate. The calculator grows your savings year by year at your returns and contributions and reports the age you reach that number. Everything is in today’s dollars.
Step by step
1. Your number
Retirement spending is grossed up for the tax rate you expect in retirement, then divided by the withdrawal rate. At 4%, that is 25 times the yearly amount.
2. Each year
Savings grow by the return on your mix of stocks, bonds and cash, plus income less spending, plus or minus any extra income or expense streams for that age. Income grows by your raise rate; spending stays flat in today’s dollars.
3. Fixed returns
Uses the stock and bond returns you enter, after inflation. Cash earns nothing after inflation.
4. Historical returns
Replays your plan starting in every year from 1871, using real (after-inflation) S&P 500 total returns and 10-year Treasury returns from Robert Shiller’s data, and reports the range of ages across all those starts.
5. Monte Carlo
Draws each year’s returns at random from a normal distribution with your average return and volatility, runs the plan many times, and reports the median age and the spread.
Figures used
- Historical returns
- 1871 to 2024
- Projection runs to age
- 100
Assumed
- All amounts are in today’s dollars, so returns are after inflation.
- Contributions and returns happen once a year.
- One portfolio holds everything; account types and their tax rules are not separated.
Not modeled
- Social Security and pensions, unless you add them as income streams.
- Taxes while working, and the order you would draw from taxable, tax-deferred and Roth accounts.
- Whether the money lasts after you reach the number; the calculator stops at the age you reach it.
- Investment fees, unless you lower your returns to include them.
Sources
These are estimates for education, not tax or investment advice. If a number looks wrong, report a problem with this calculation and say what you entered.