The mechanism, in one paragraph
Investing all at once puts every dollar in the market for the whole period. Spreading it out leaves part of the money in cash for part of the period, earning the cash rate instead of the market rate. If the market rises over that window, the money held back missed the rise, and investing at once wins. If the market falls and then recovers inside the window, the money held back buys at lower prices, and spreading wins. There is nothing subtler than that going on, and everything else is a question of how often each case happens.
That framing also explains a result people find counterintuitive: spreading loses more often than it wins, historically, because markets have risen more often than they have fallen over any given twelve-month stretch. Vanguard's 2012 study of US, UK and Australian data going back to 1926 found investing immediately ahead of a twelve-month spread in roughly two thirds of the periods it examined, by an average of a couple of percentage points. That is a statement about the past frequency of rising markets, not a rule about your particular twelve months.
The three paths, and why they end at the same place
Each scenario moves prices differently through the spreading window but lands on the same closing level, the one your assumed return implies. Fixing the endpoint is what makes the comparison readable. Because the all-at-once side buys everything at the starting price and holds to the same endpoint, its result is identical in all three rows. Only the spread side moves, and it moves purely on the average price it paid along the way.
| Path | What the spread side pays on average | Effect |
|---|---|---|
| Steady rise | Above the starting price | Spreading loses |
| Fall, then recovery | Below the starting price | Spreading wins |
| Flat, then a late rise | Near the starting price | Roughly a draw, tilting to whichever side the cash rate favours |
The cash rate matters more than people expect when it is high. Money waiting to be invested at 4 or 5 percent is not doing nothing, and that return narrows the gap in the rising case considerably. Set it to zero and the case for investing at once gets stronger on every row.
What the arithmetic does not decide
The comparison above measures expected money. It does not measure the experience of putting a large amount in on a Monday and watching it fall 15 percent by Friday, which is the actual reason most people spread a windfall. That is not irrational. If a bad start would cause you to sell at the bottom, then a strategy with a worse expected value and a better chance of being stuck with is the one that produces more money in your account, and the arithmetic on this page cannot see that.
The same holds in reverse. If the money is already invested somewhere and you are moving it between funds, spreading it out is not a risk reduction at all, since you are out of one market and into cash rather than staying invested. Spreading only reduces exposure when the starting point is cash.
Assumptions worth changing before you read the answer
The return figure drives everything, including the shape of the paths, since they all end where the return says they end. A 10 percent assumption makes investing at once look strong in every row. A 2 percent assumption makes it nearly a draw. Neither is a forecast, and the number to use is whatever you would defend to yourself, not whatever produces the answer you wanted.
The drawdown figure is the other lever. A 20 percent fall halfway through a twelve-month window is a serious market event; a 40 percent one is a generational one. Setting it larger makes the case for spreading look stronger, and it is worth noticing that you are then comparing an ordinary rising year against an exceptional falling one. If the horizon rather than the entry point is what you actually want to model, the compound interest calculator handles regular contributions over long periods more directly.
Questions people ask
Does dollar cost averaging reduce risk?
It reduces exposure, which is not the same thing. During the spreading months part of your money is in cash, so a market fall hits a smaller balance and the worst case is milder. That is genuine, and the price of it is that the average case is worse, because the same cash also misses the rises that happen more often than the falls. Once the last contribution goes in, the two approaches hold identical portfolios and are exposed to identical risk from then on. Spreading changes the entry, not the investment.
What if I am contributing from a salary rather than a windfall?
Then this page is not the right comparison, because there is no lump sum to choose between. Investing each paycheck as it arrives is simply investing as soon as you have the money, which is the all-at-once strategy applied repeatedly. The question this page answers only exists when you are already holding cash and deciding when to deploy it. For salary contributions over a long horizon, the compound interest calculator is the more useful page.
Why does the all-at-once column show the same number in every row?
Because all three paths are constructed to end the spreading window at the same price level, and the all-at-once side buys at the start and holds through to that same endpoint. What happens in between never touches its result, since it makes no further purchases. The spread side buys throughout, so the route determines its average purchase price and therefore its result. Holding the endpoint fixed is what isolates the effect of the route from the effect of the return assumption.
Is a shorter or longer spreading period better?
Shorter periods behave more like investing at once and longer ones amplify whatever the market does in the meantime, in both directions. Twelve months is the conventional choice and is what most published comparisons use, which makes it a reasonable default rather than an optimum. Extending it to three years leaves a large share of the money in cash for a long stretch, which historically has cost more than it saved. There is no period that is right independent of what prices do, which is the honest and unsatisfying answer.