Tools.
The math the books teach — run it yourself.
Every book on this site teaches a concept. The calculators below let you run that concept against your own numbers — interactive companions to the ideas the authors are arguing for.
Pick a calculator.
Compound Interest Calculator
Plug in your starting amount, monthly contribution, expected return, and time horizon — see what your portfolio compounds to over the years that matter.
Dollar-Cost Averaging Simulator
Model the real-world impact of investing a fixed amount on a regular schedule — see how consistent contributions smooth out market volatility over time.
FIRE Number Calculator
Multiply your annual expenses by 25 to find the portfolio size that funds retirement indefinitely — the central equation behind the Financial Independence, Retire Early movement.
Good Debt Calculator
Run the 4-number ROI test against a specific loan, line, or advance — all-in cost of capital, expected cash-flow lift, and whether a downside scenario still survives.
Lump Sum vs. DCA Calculator
Should you invest a windfall all at once or spread it out over time? Compare expected final values, historical win rates, and drawdown exposure for both strategies.
Retire-at-X Calculator
Forecast your retirement balance at any target age and see whether it covers your annual expenses at a safe withdrawal rate — the Trinity Study 4% rule made concrete.
Rule of 72 Calculator
Enter an expected return and see how many years it takes to double your money — the famous Rule of 72 shortcut alongside the mathematically exact answer.
Savings Rate Calculator
Calculate what percentage of your income you're saving — the single metric that predicts how many years until financial independence faster than any other.
Withdrawal Rate Calculator
Model how long your portfolio lasts at different withdrawal rates — stress-test the 4% rule against your actual spending, timeline, and allocation.