Spreading $12,000 over a year feels safer. Investing it all at once won 63% of months since 1871.

Every month since 1871 is one possible day you got a lump of cash. Put it all in the S&P 500 today, or buy in twelve equal monthly steps? If the waiting cash earns nothing, the lump sum won 63 times in 100. Cash would have to pay about 6.5% a year before the drip wins half the time. Change the settings and watch it move.

Every month since 1871, one cell each

Each row is a decade, each thin column a start month. Blue: the lump sum finished ahead. Orange: the drip did. Darker means a bigger gap. Tap or drag across the grid, or use the arrow keys.

What that one start month looked like

Your money month by month, if you started in the selected month. The dashed line marks the last drip purchase, where the two are compared.

How big the gap gets

How far ahead or behind the lump sum finished, across all start months in view. The longer the drip, the further both tails reach.

How it is computed. Lump sum buys the whole amount at the start month's S&P 500 price. The drip buys 1/N of it at the start of each of N months, at that month's price; the part not yet invested earns the interest rate you set. Both are valued at the month of the last drip purchase. Prices are the index level without dividends, so absolute returns are understated for both strategies alike; there are no fees or taxes. This is a look at history, not a forecast and not advice about what to do with your money.

Sources

• Monthly S&P 500 price series 1871 to Sep 2026 (1,869 months): the open "datasets/s-and-p-500" file on GitHub (github.com/datasets/s-and-p-500). Secondary source: it republishes Robert Shiller's data (econ.yale.edu/~shiller/data.htm, monthly averages of daily closes, to Jun 2023) and extends it with FRED's S&P 500 series (fred.stlouisfed.org/series/SP500) for Jul 2023 onward.
• Because the figures are monthly averages, a single month's buy price is smoothed; real daily prices would show somewhat larger swings.