You have a pile of cash and you are afraid the market will drop the day after you buy, so you think about spreading the purchase over a few months. This course lets you run that on S&P 500 history first, then looks at what Vanguard and Schwab found.
Run it on S&P 500 history
Starting October 2025, after 1 year investing all at once is ahead by $1,857
$100,000 invested all at once is worth $115,739 after 1 year. Split over 3 months, it is worth $113,882.
All at onceOver 3 monthsMonth 12: all at once $115,739, split $113,882
Split over
Starting month
Once the split purchases are done, both sides hold the same shares and move together from then on. So whether you look after 1 year or 20, the winner does not change and neither does the percentage gap. Only the dollar gap grows and shrinks with the account.
Now every starting month
Out of 453 starting months, investing all at once beat a 3-month split 306 times
Investing all at once won 67.5% of the time
Value after 1 year
All at once
Over 3 months
A typical start (median)
$114,430
$112,887
A bad start (worst 5%)
$81,959
$82,909
Same starting month: all at once ahead by (median)
$1,407
The first two rows rank each approach on its own, so the two figures in a row may come from different starting months. The third row compares the two approaches from the same starting month.
This uses the S&P 500 Total Return index, which includes dividends, with starting months from February 1988 to October 2025: 453 in all. VOO tracks this index but charges 0.03% a year, which is not deducted here. For comparison, Vanguard found that investing all at once won 68% of the time, using global stocks from 1976 to 2022 and a 3-month split.
Chapter 1
Investing all at once wins more often
A 2023 Vanguard study compared two approaches. One invests the whole amount immediately. The other splits it into three equal parts and invests one part a month. After a year, which one has more money?
Using global stocks from 1976 to 2022, investing all at once came out ahead 68% of the time. For US stocks alone it was 66.4%.
The reason is not complicated. Markets go up more often than they go down, so cash that sits and waits usually misses gains.
Chapter 2
But the two end up close
Winning more often is not the same as winning by much. The same study gives dollar figures. Start with $100,000, and a year later:
Value after one year
All at once
Over 3 months
A typical year (historical median)Splitting made $2,360 less than all at once
$111,940
$109,580
A bad year (roughly 1 in 20)Splitting lost $2,959 less than all at once
$82,947
$85,906
The second row is the one to look at. Most people split a purchase because they fear a crash right after buying. Yet in that bad year, investing all at once lost $17,053 and splitting still lost $14,094. Splitting did not avoid the drop. It reduced the loss by $2,959.
In years worse than that the gap may be wider. The Vanguard study does not give figures for them.
Chapter 3
The big gap is with never buying
Schwab ran a different comparison. Five people each receive $2,000 a year for 20 years (2005 to 2024) and put it into the S&P 500. The only difference is when they buy. After 20 years they have:
Bought at each year's low$186,077
Invested the day the money arrived$170,555
Spread over 12 months$166,591
Bought at each year's high$151,343
Stayed in cash, waiting for a better moment$47,357
Over 20 years, investing immediately ended $3,964 ahead of spreading it out. That is not much.
The large gap is with the last person. The one who bought at the high every single year ended with 3.2 times as much as the one who stayed in cash.
Chapter 4
What I do: all at once
I invest all at once, and I am firm about it. The $2,360 that splitting gives up in a typical year buys a better night's sleep. If that is what keeps you in the market, it is money well spent. I choose not to spend it.
If you really are afraid of a drop right after buying, split it over three months. Fix the purchase dates in advance and buy on each date whatever the market did that day. The outcome to avoid is splitting that slowly turns into waiting.
One case needs no decision: buying every month when your paycheck arrives. That is not the splitting discussed here. Investing money as soon as you have it is already investing all at once.
Notes
The calculator and both studies use indexes, not VOO itself: the calculator uses the S&P 500 Total Return index, Vanguard uses MSCI World and Russell 3000, and Schwab uses the S&P 500. VOO tracks the S&P 500 and charges 0.03% a year.
The $100,000 is the example used in the Vanguard paper.
Vanguard's main comparison splits over 3 months. The longer the split, the more often investing all at once wins: for US stocks it is 73.7% against a 6-month split.
Schwab's figures do not include taxes or fees.
Whether to save for a child in a 529 or a UTMA account is not covered here.
All of this is historical data. It does not predict the future and is not investment advice.
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