How the numbers are computed.

The plan being replayed

A fixed dollar amount is invested on the first trading day of every month into the S&P 500 (S&P Composite) index, with dividends reinvested and nothing sold, from the chosen start month until Sep 2023. Fractional shares are assumed. No taxes, fund fees, commissions or cash drag are included — real results would be lower.

Data

Robert J. Shiller, Yale University — Irrational Exuberance online data (S&P Composite monthly average price, dividends, CPI). Source →

• Series: S&P 500 (S&P Composite) total return, dividends reinvested, nominal.
• Convention: Monthly average of daily closes (Shiller). Contributions modeled on the first trading day of each month at that month's value.
• Coverage: Jan 1926 through Sep 2023 (1173 months). Figures update when the series is refreshed; the “data through” date is shown beside every result.
• “Today's dollars” uses the CPI series in the same dataset; contributions are held constant in real terms and all values are stated in Sep 2023 dollars.

What is shown

• You contributed — the sum of monthly amounts.
• Historical value — shares accumulated × index value at the end month.
• Historical return — value ÷ contributed − 1; and an approximate money-weighted annual rate (internal rate of return of the monthly contributions against the final value).
• The low — the month in which value fell furthest below the amount contributed, and how many months the plan was worth less than the money put in.
• Worst 10-year window — the 120-month window, out of every one since 1926, that ended with the lowest value relative to contributions. Across all windows the outcome ranged from −29% to +224%, median +76%; about 2% of windows ended below contributions.

What it is not

• It is not a prediction, a forecast, or a projection. The future can differ from every period shown.
• An index cannot be bought directly. Index funds track it with fees and small differences; results in an account would differ.
• It shows one index. Other markets, single companies and other periods behaved differently — including much worse.
• It does not account for when you might need the money, taxes, or your ability to keep contributing through a decline.