Lump Sum Or Dollar Cost Averaging: Which Wins?

A bonus landed, larger than I expected, and I sat on it in cash for four months while I read about whether to put it in all at once. The reading was good. The four months were not, and the cost of them was not zero.

This is the question people mean when they say they are waiting for a better entry point, and it has a clean arithmetic answer and a messier real one.

The arithmetic answer

If you spread $60,000 over twelve monthly instalments of $5,000, then averaged across the year roughly half of your money is sitting in cash rather than invested. That is not a subtle effect. It is the whole effect.

So the question collapses into something simpler: over the average twelve month stretch, does being invested beat holding cash? Historically the answer has been yes more often than no, because markets have risen in most twelve month windows. That is the same sentence as saying lump sum wins most of the time.

Put numbers on it. $60,000 invested at once, with the market returning 7% over the year, finishes at about $64,200. The same $60,000 fed in at $5,000 a month, with the uninvested balance earning nothing, finishes at about $62,250.

$1,950.

That is 3.25% of the amount, given up in exchange for nothing except a smoother path. If the cash had been parked in a savings account earning something, the gap narrows, and it does not close.

Notice the shape rather than the endpoint. The instalment line barely moves for the first four months, because barely any of the money is exposed yet. That flatness is the thing people are buying.

What the flatness is worth

Turn the market around and the picture inverts. If the market falls 20% over the year instead, the lump sum ends at about $48,000 and the instalment approach ends higher, because most of the money went in at lower prices.

So spreading it out is insurance. It costs you about $1,950 in the ordinary case, which is the premium, and it pays out in the bad case. That is a coherent trade and nobody should be talked out of it with a chart.

What is not coherent is doing it because you think you know which case is coming. If you knew that, you would not be splitting the difference.

Where the money came from matters

There is a version of this question that is not a question. If the money is already invested and you are moving it from one fund to another, there is no timing decision to make, because you are exposed the whole time either way. Selling on Monday and holding cash for six months while you average into a different fund is a market call wearing a portfolio-cleanup costume.

Genuinely new cash is the only case this applies to. A bonus, an inheritance, a house sale, a settlement, a redundancy payment.

And notice that nearly everyone reading this is already averaging in. A 401k contribution is $500 going in twenty-four times a year at whatever price the market happens to be that Friday. Nobody agonises over it, because it never arrives as a decision.

The honest behavioural question

Here is the one that actually decides it. Imagine you put the whole $60,000 in on a Monday and by the following month the account shows $48,000.

Not how you would feel. What you would do.

If the honest answer is that you would sell, then the lump sum was the wrong move regardless of the arithmetic, because the arithmetic assumes you stay invested and you have just told yourself you would not. Selling at $48,000 turns a paper move into $12,000 you no longer have.

If the honest answer is that you would leave it alone and probably not look, put it in on Monday. You are paying $1,950 for protection against a thing you have decided you would not do.

The failure nobody warns about

Spreading it out over twelve months is a plan. Spreading it out over an unspecified period while you watch the news is not a plan, and the second one is what usually happens.

I have watched friends decide to average in, get spooked by month three, pause, and still be holding the cash a year later. The cost of that is not $1,950. It is the entire return on the entire amount for the entire year, and it never appears on a statement because it is money that was never at risk.

The fix is to make the schedule mechanical. Pick the amount, pick the dates, set the automatic transfers so the decision is made once and executed by software.

Waiting for a dip

The third option people actually have in mind is neither of these. It is holding the cash until the market falls, then buying.

The problem is that the plan has no second half. If the market drops 10%, is that the dip, or the first third of a 30% one? Everyone I know who has tried this was right about the fall and wrong about the buying, because the moment that makes prices attractive is the same moment that makes buying feel insane.

A twelve month schedule solves that by taking you out of the decision. You still buy more shares when prices are low, which is the thing waiting for a dip is supposed to achieve, and you do it without having to identify a bottom in real time.

If you want the dip trade anyway, write down the level and the amount you will buy at it before anything happens. Written down, in advance, is the only version of that plan that has ever survived contact with a falling market.

Decide before the money arrives

Whatever the sum is, bonus, inheritance, house sale, settlement, write down what you are going to do with it while it is still someone else's problem. Cash that lands in your account without a plan attached tends to stay cash.

One sentence is enough. All of it on the day it clears, or $5,000 on the first of every month for twelve months starting the month it clears. Then go check that the transfers are actually scheduled, because a plan nobody automated is a plan you get to reconsider every thirty days.