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Lump Sum vs. Dollar-Cost Averaging: What 24,747 Starting Days Say

It's one of the most common investing questions: if you have a sum of money to invest, is it better to put it all in at once, or spread it out gradually over a few months or years? We tested it against nearly a century of real market history rather than a handful of hand-picked examples. Every single trading day in the S&P 500 from December 30, 1927 through July 10, 2026, 24,747 of them, was treated as a possible starting date, for windows ranging from six months to ten years.

The straightforward answer

Investing a lump sum all at once beat spreading it out gradually about 61% of the time over a one-year window. That number climbs the longer the window runs, reaching roughly 65% at five and ten years. This lines up closely with published academic and industry studies on the same question, which is a useful sanity check: our number isn't an outlier.

The average dollar difference is real but not enormous at shorter windows. On $10,000 over one year, investing it all at once produced about $200 more on average than spreading it out. Over ten years, that gap widens to roughly $3,200 on average, since a bigger head start compounds for longer.

Two assumptions matter a lot here, and we tested both directly rather than guessing. First, money that hasn't been invested yet has to sit somewhere: we assumed it earns 3.94% a year, the historical median short-term Treasury rate, a reasonable stand-in for a savings or money-market account. Assume 0% instead (money doing nothing while it waits) and the case for investing it all at once looks artificially stronger, especially over longer windows. Second, we tested spreading the money out weekly, monthly, and quarterly instead of just monthly. It barely matters. Monthly is already close to as smooth as gradual investing gets.

Why spreading it out isn't free insurance, but isn't pointless either

Investing all at once usually wins on the final number, but that's not the whole story. It also means being fully exposed to the market from day one: if a downturn hits early, the full amount feels it. Spreading the money out means less is ever at risk at any single moment.

That shows up clearly in the data. Looking at the worst dip experienced along the way, not just the final result, spreading the money out produced a smaller worst-case decline than investing it all at once in 99% of every one-year window tested, and in 100% of every five-year window tested. The average worst dip for a lump sum over a one-year window was about twice as deep as the average worst dip for the same money spread out. Investing all at once wins more often, but spreading it out almost always feels calmer while you're in it.

When spreading it out actually wins

The biggest factor in whether spreading it out comes out ahead isn't luck, it's where the market already was relative to its own recent trend at the moment you started. Grouped by how far the market was running above or below its typical level at that point:

• After the market had already climbed far above its usual trend (an extended, "euphoric" run-up), spreading the money out beat investing it all at once more often than not. Investing it all at once only won about a third of the time in this group over a one-year window.
• During an ongoing pullback, the two approaches were close to a coin flip.
• After a steady, unremarkable climb, or shortly after a dip had already started recovering, investing it all at once won clearly, more often than the 61% baseline.

In plain terms: the case for spreading money out gradually is strongest right when the market has already run hot for a while without a pause, and weakest during an ordinary, unexciting climb.

Beyond the S&P 500 index

The same one-year test run on 478 individual S&P 500 companies (filtered to companies with at least eight years of history, to avoid distortion from recent spinoffs and mergers, the same data-quality issue we flagged in What 517 Series Tell You) lines up closely with the index: a median win rate of 62% for investing all at once, right next to the index's own 61%. It does shift by company, though. More volatile individual stocks tended to show a somewhat lower win rate for investing all at once, meaning spreading it out did relatively better on the market's wilder names.

For context: cash

Being invested, either way, beat simply parking the money in a savings-style account earning 3.94% a year in a clear majority of cases, roughly 6 times out of 10 over a one-year window, rising as the window lengthens. Neither investing approach beats cash every single time. Both beat it more often than not.

Try it yourself

We built an interactive version of this study covering every scenario above: pick an amount, pick a window from six months to ten years, and see what actually happened, historically, across every one of the 24,747 starting days tested. Explore the calculator here.

This is a study of one real, already-happened price series, not a simulation of possible futures or a prediction. Past results, including every number above, do not guarantee or predict what markets will do next. This is a Phase 1 finding from an ongoing statistical research project. Data: Yahoo Finance (S&P 500, ^GSPC) and FRED (3-month Treasury rate). Methodology and full reproducibility details available on request.