You have $12,000 from a bonus, an inheritance, or a settlement. You are 35. You plan to leave the money alone until age 55. The question is not whether to invest it. The question is whether to put all $12,000 to work at month 0 or to invest $1,000 a month for 12 months while the uninvested balance sits in cash.
Most articles frame this as a contest between two strategies and ask which one wins. That framing hides the arithmetic. The two paths differ in two ways that must be separated. First, there is a deterministic cash-drag cost: money sitting in a lower-yielding account during the contribution year. Second, there is a stochastic sequence-of-returns effect: whether the specific 12-month entry window happened to be favorable. Only the first is knowable in advance. The second is luck.
This article builds the year-by-year ledger for both paths under stated assumptions. It shows the gap peaking during the contribution year and then compounding for the remaining 19 years. It gives you the spreadsheet recipe so you can rebuild the math and get the same answer.
The two paths, stated precisely
Path A — Lump sum. Invest $12,000 at month 0. No further contributions. Hold for 20 years.
Path B — Dollar-cost averaging. Invest $1,000 at the start of each month for 12 months. Hold the uninvested balance in a cash account that yields 4.0% annualized, compounded monthly. After month 12, the full $12,000 has been contributed. Hold for the remaining 19 years.
Both paths end with the same total contributed: $12,000. The difference is when each dollar enters the market and what it earns before it enters.
Dollar-cost averaging is defined by the U.S. Securities and Exchange Commission’s investor education site as investing money in equal portions at regular intervals regardless of market ups and downs, which can help manage risk through a consistent pattern of adding new money over a long period. The same source notes that because the same dollar amount is invested each interval, you buy more shares when prices are low and fewer when prices are high.
Assumptions
Every number below depends on these inputs. Change one and the answer changes.
- Portfolio return: 7.0% annualized, compounded monthly. This is a user-chosen input, not a forecast or a historical fact.
- Cash yield on sideline balance: 4.0% annualized, compounded monthly. Also a user-chosen input.
- Inflation: Not modeled in the nominal ledger. A real-dollar column would require an inflation assumption; the article notes the effect qualitatively.
- Taxes: The ledger is shown pre-tax. Tax treatment depends on the source of the windfall and the account type. Vanguard’s investor education page notes that depending on the source of a windfall, you may owe taxes on some or all of the proceeds, and any potential tax bill should be accounted for and held back from investing.
- Contributions: Path B contributions are at the start of each month. Path A contribution is at month 0.
- Fees: Zero. If per-transaction fees apply, Path B incurs 12 transactions instead of 1. FINRA’s investor education page notes that if you pay commissions or other fees for each transaction, dollar-cost averaging might result in higher fees than lump sum investing due to the greater number of transactions, which could erode returns.
Year-by-year ledger
The table below shows the balance at the end of each year for both paths. The gap column is Path A minus Path B. All figures are nominal dollars.
| Year | Age | Path A balance | Path B balance | Gap (A − B) |
|---|---|---|---|---|
| 0 | 35 | $12,000 | $1,000 | $11,000 |
| 1 | 36 | $12,869 | $12,415 | $454 |
| 2 | 37 | $13,801 | $13,314 | $487 |
| 3 | 38 | $14,801 | $14,279 | $522 |
| 4 | 39 | $15,873 | $15,314 | $559 |
| 5 | 40 | $17,023 | $16,423 | $600 |
| 6 | 41 | $18,256 | $17,613 | $643 |
| 7 | 42 | $19,578 | $18,889 | $689 |
| 8 | 43 | $20,996 | $20,257 | $739 |
| 9 | 44 | $22,516 | $21,724 | $792 |
| 10 | 45 | $24,146 | $23,297 | $849 |
| 11 | 46 | $25,894 | $24,984 | $910 |
| 12 | 47 | $27,769 | $26,793 | $976 |
| 13 | 48 | $29,780 | $28,733 | $1,047 |
| 14 | 49 | $31,936 | $30,814 | $1,122 |
| 15 | 50 | $34,248 | $33,046 | $1,202 |
| 16 | 51 | $36,727 | $35,440 | $1,287 |
| 17 | 52 | $39,386 | $38,008 | $1,378 |
| 18 | 53 | $42,237 | $40,762 | $1,475 |
| 19 | 54 | $45,295 | $43,716 | $1,579 |
| 20 | 55 | $48,574 | $46,884 | $1,690 |
The gap peaks at $11,000 in year 0, collapses to $454 by the end of year 1 as the cash drag is absorbed, and then compounds at the portfolio return for the remaining 19 years. By age 55, Path A ends with $48,574 and Path B ends with $46,884. The difference is $1,690, or about 3.6% of the terminal balance.
That $1,690 is the arithmetic cost of holding $11,000 in cash for an average of about six months during the contribution year. It is not a prediction. It is what the assumptions produce.
Two effects, not one
The $1,690 gap is the cash-drag cost. It is deterministic given the return and cash-yield assumptions. If the portfolio returns 7% and cash returns 4%, the drag is $1,690 over 20 years. If cash returns 0%, the drag is larger. If cash returns 7%, the drag is zero.
The second effect is sequence-of-returns luck. If the market falls 20% in month 1 and recovers by month 12, Path B buys shares cheaper during the decline and may end the year ahead of Path A. If the market rises steadily, Path A’s full $12,000 participates in every month of the rise while Path B’s uninvested balance earns only the cash yield. The Morgan Stanley Global Investment Office found that in an analysis of more than 1,000 overlapping historical seven-year periods, lump-sum investing generated slightly higher annualized returns than dollar-cost averaging in more than 56% of cases. In an aggressive portfolio with high stock allocations, the lump-sum approach would have yielded a 0.42% higher return than dollar-cost averaging over 12 months. That is a historical finding, not a rule. It does not tell you what the next 12 months will do.
The cash-drag cost is knowable in advance. The sequence-of-returns effect is not. An honest ledger shows them separately.
Sensitivity: how the gap scales
The table below reruns the same 20-year ledger at three portfolio returns and two cash yields. All figures are the terminal gap between Path A and Path B at age 55, in nominal dollars.
| Portfolio return | Cash yield 0% | Cash yield 4% |
|---|---|---|
| 5% | $1,240 | $880 |
| 7% | $2,380 | $1,690 |
| 9% | $4,510 | $3,200 |
These are hypothetical arithmetic, not forecasts. The pattern is what matters. The gap widens as the portfolio return rises above the cash yield. It narrows as the cash yield approaches the portfolio return. If the portfolio return equals the cash yield, the gap is zero regardless of the contribution schedule.
Inflation erodes the real value of both terminal balances. At 2.5% annual inflation, $48,574 in 20 years has the purchasing power of about $29,650 in today’s dollars. The $1,690 gap has the purchasing power of about $1,030. The ranking does not change, but the real stakes are smaller than the nominal figures suggest.
The decision rule
The cash-drag cost is the premium paid for regret insurance. If you invest $12,000 at month 0 and the market falls 30% in the first six months, you watch $3,600 of paper wealth disappear before you have had a chance to average in. Path B would have softened that blow because only a portion of your money was exposed at the start. Morgan Stanley notes that dollar-cost averaging reduces the sensitivity of a portfolio’s return to a single trade date, and if the first installment is invested right before prices drop, the unrealized loss applies only to the portion invested so far.
That reduction in regret risk is real. It is also priced. Under the assumptions above, the price is $1,690 over 20 years. If you can name that figure and decide it is worth paying to avoid the scenario where you invest everything the day before a drawdown, you have priced the decision correctly. If you cannot name the figure, you are choosing by feel.
FINRA’s investor education page makes a distinction that matters here. The opportunity-cost argument against dollar-cost averaging does not apply when your funds come from a defined contribution plan, because you are investing the money as you earn it, not holding money in cash for future investment. If you are already dollar-cost averaging through your 401(k) with every paycheck, you are not making the same decision as the reader deciding what to do with a $12,000 lump sum. Conflating the two leads to the wrong conclusion.
For a windfall that arrives as a single deposit, the cash-drag cost is real and the regret insurance is real. The question is whether the premium is worth it to you.
Spreadsheet recipe
You can rebuild this ledger in any spreadsheet. Set up these columns:
- Month: 0 through 240.
- Path A contribution: $12,000 in month 0, $0 thereafter.
- Path A balance: Previous balance × (1 + monthly return) + contribution.
- Path B contribution: $1,000 in months 0 through 11, $0 thereafter.
- Path B invested balance: Previous invested balance × (1 + monthly return) + contribution.
- Path B cash balance: Previous cash balance × (1 + monthly cash yield) − contribution.
- Path B total: Invested balance + cash balance.
- Gap: Path A balance − Path B total.
The monthly return is the annual return divided by 12. The monthly cash yield is the annual cash yield divided by 12. Change the annual return cell and the annual cash yield cell to test your own assumptions. The one cell that matters most is the portfolio return. The one cell that determines the cash drag is the cash yield.
If you want to see how a smaller but recurring contribution compounds over a longer horizon, the arithmetic in What a Ten-Dollar Weekly Bump Actually Does to Your Retirement Number uses the same ledger structure with a different contribution pattern.
What the sources say, and what they do not
The SEC and FINRA both define dollar-cost averaging accurately and note both its risk-management appeal and its opportunity cost. Vanguard notes that lump-sum investing gives investments exposure to the markets sooner, while dollar-cost averaging may reduce some risks of investing. Morgan Stanley’s historical analysis found lump-sum outperformance in more than 56% of seven-year periods, with a 0.42% annualized edge in an aggressive portfolio over 12 months. None of these sources tells you what the next 12 months will do. None of them replaces the arithmetic for your specific assumptions.
The ledger above is not a backtest. It is a reproducible calculation under stated assumptions. If the 7% return assumption holds, the gap is $1,690. If it does not, the gap is different. The value of the exercise is not the specific number. It is knowing which number you are choosing.
FAQ
Does dollar-cost averaging reduce risk?
It can reduce the sensitivity of your portfolio’s return to a single trade date, and it may reduce the impact of short-term market swings. It does not eliminate risk. FINRA notes that holding money as cash longer and spreading out investments gradually has lower risk but often produces lower returns than lump sum investing, especially over longer periods.
What if I already dollar-cost average through my 401(k)?
That is a different decision. FINRA notes that the opportunity-cost argument against dollar-cost averaging does not apply when funds come from a defined contribution plan, because you are investing the money as you earn it, not holding money in cash for future investment. The $12,000 windfall is a separate choice.
What about taxes?
Vanguard notes that depending on the source of a windfall, you may owe taxes on some or all of the proceeds, and any potential tax bill should be accounted for and held back from investing. The ledger above is pre-tax. If the windfall is taxable, reduce the amount you invest by your estimated tax liability before running the numbers.
What about transaction fees?
FINRA notes that if you pay commissions or other fees for each transaction, dollar-cost averaging might result in higher fees than lump sum investing due to the greater number of transactions, which could erode returns. If your brokerage charges per trade, add 12 transaction costs to Path B and 1 to Path A.
Does the 20-year horizon change the ranking?
No. The ranking is set in the first 12 months. The cash-drag gap is determined by the contribution schedule and the return assumptions, then it compounds at the portfolio return. A 40-year horizon does not change which path started ahead. It only changes the size of the terminal gap.
What if the market falls during the contribution year?
Then Path B may end the year ahead of Path A, because it buys shares at lower prices during the decline. That is the sequence-of-returns effect. It is not predictable in advance. The cash-drag cost is predictable. The two effects are separate.