The question, stated precisely
You have a sum of money — an inheritance, a bonus, the proceeds of a sale — and a place you intend to put it. Do you invest all of it now, or spread it over the next six or twelve months? That is the lump sum versus dollar-cost averaging question, and it is different from the one people usually answer. Investing a salary every month as it arrives is not DCA against a lump sum; it is simply investing money as you get it, and there is no alternative to compare it with. The real choice only exists when the money is already in your hand.
What the arithmetic says
Spreading purchases means holding cash for part of the period. If the market rises more often than it falls — which, over long horizons, it historically has — then cash held back misses those rises. Studies of long-run index data consistently find lump sum ahead of twelve-month averaging roughly two thirds of the time, by a couple of percentage points on average. The other third of the time, when prices fall during the period, averaging wins, sometimes by much more.
A worked example with $12,000 over six months, buying $2,000 a month:
| Month | Price | Shares bought with $2,000 |
|---|---|---|
| 1 | $100 | 20.0 |
| 2 | $80 | 25.0 |
| 3 | $70 | 28.6 |
| 4 | $90 | 22.2 |
| 5 | $110 | 18.2 |
| 6 | $120 | 16.7 |
Total: 130.7 shares, average cost $91.81. The average of the six prices is $95.00 — the fixed-dollar method came out $3.19 lower because each payment bought more shares when the price was low. That gap is the entire mathematical content of dollar-cost averaging, and it exists whenever prices vary.
The lump sum in the same scenario bought 120 shares at $100 in month one and finished with 120 shares worth $14,400 against the DCA investor's 130.7 worth $15,684. Averaging won because the price dipped before it rose. Reverse the sequence — start at $120 and end at $100 — and the lump sum wins instead. Which one happens is unknowable in advance, which is why the argument is not settled by arithmetic.
Average cost vs average price
Two numbers that sound identical and are not. Average price is the plain mean of the prices you paid. Average cost is total spent ÷ total shares. Fixed-dollar purchases always make the average cost lower than or equal to the average price, because more of your money lands at lower prices. Fixed-share purchases (buying 10 shares a month regardless of price) make the two identical, and lose this small edge. The DCA Calculator shows both figures side by side for the prices you enter.
The argument that is not about returns
The strongest case for averaging has nothing to do with expected value. Putting a year's savings into the market on a single day and watching it fall 20% the following week is an experience that makes many people sell at the bottom, and a strategy you abandon at the worst moment performs far worse than either option on paper. Averaging spreads the regret, caps the size of the worst possible first impression, and keeps people invested. That is a real benefit, and it is paid for with a small expected return. Whether the trade is worth making is a question about you, not about markets.
- Choose lump sum if the money is destined for a long horizon, you have invested through a fall before without selling, and the amount is not life-changing relative to your net worth.
- Choose averaging if the sum is large relative to everything else you own, if you have never held through a drawdown, or if the alternative is leaving it in cash indefinitely because the decision feels too big.
- Keep it short. If you do average, three to six months is the usual compromise. Beyond twelve, you are mostly holding cash, and the strategy stops being a compromise and becomes a market-timing view.
Things that change the answer
- Commissions. Six purchases cost six commissions. On a $12,000 sum with $5 fees this is trivial; on a $1,200 sum it is 2.5% and eats the advantage.
- Interest on the uninvested cash. When short-term rates are high, cash waiting its turn earns something, which narrows the gap. When rates are near zero, it does not.
- Tax wrappers with annual limits. If contributions are capped per year, delaying may waste allowance you cannot reclaim.
- What you are buying. The historical edge for lump sum comes from broad indices that trend upwards over decades. It is a weaker argument for a single company, where the long-run drift is not a given.