Lump sum vs. dollar-cost averaging

Is easing in actually safer?

You have a lump of cash to invest. Putting it all in at once feels risky, so spreading it out feels careful. I'd push back: easing in means sitting in cash while the market usually rises. Here's what it costs, and when it pays off.

The reasoning

Why the holding period doesn't appear here

You might expect a slider for how long you hold. There isn't one, and that's on purpose. The moment your last slice is invested, both strategies own the same thing and ride the same returns from then on. So the percentage gap between them is locked at the end of the deployment window and never moves again. Everything that decides the outcome happens while you're still easing in.

That's also why the answer is a percentage, not a dollar figure tied to a horizon. The dollars grow with whatever the market does afterward, but the share of your wealth that the choice gave up (or gained) is fixed the day you finish deploying.

What this page can't tell you

I want to be upfront about one limit. This page shows you the mechanic: why easing in wins or loses, and by how much, for a market path you choose. It doesn't tell you how often each outcome happens in real markets. That's a question for actual history, and it's the next layer I plan to build. The short version from the historical record: because markets rise more often than they fall, investing at once comes out ahead most of the time. Easing in earns its keep as insurance, and as a way to stay invested when one big entry would be too much to stomach.

Easing in doesn't remove risk. It swaps the risk of a bad entry for the near-certain cost of sitting in cash. Sometimes that's worth it for peace of mind; just know what the peace costs.

About this model

Lump sum vs. dollar-cost averaging: the short answer

Say you invest $60,000 all at once instead of easing it in over 12 monthly buys, and the market rises 12% during those 12 months. The lump sum comes out $2,088 ahead, or 3.1%: $67,200 against $65,112. Easing in only wins when prices spend the deployment window below where they started. In a market that dips and recovers to the same level, averaging in finishes 17.1% ahead; in one that peaks and fades back, it trails by 13.9%.

Figures use the model's default inputs, stated in the text. Estimates only, not financial advice.

How the model works

The Outflow lump sum vs. dollar-cost averaging model compares investing a lump sum today with splitting it into equal monthly purchases. You pick a market path (it rises, falls, dips and recovers, or peaks and fades), and the model places each purchase on it. Easing in beats the lump sum whenever its average cost per share ends up below the lump sum's entry price.

Cash that's waiting to be invested earns the short-term rate you set, 4.5% by default, instead of being treated as earning nothing. Assuming idle cash earns zero is a common way to overstate the cost of easing in, and I don't want to stack the deck.

There's no holding-period input, on purpose. Once the last purchase is made, both strategies own the same investment and earn the same returns, so the percentage gap between them is locked in at the end of the deployment window.

Questions

Frequently asked questions

Is dollar-cost averaging safer than investing a lump sum?

It lowers the risk of a badly timed entry, but it replaces it with the near-certain cost of holding cash while prices usually rise. Ease $60,000 in over 12 months in a market that rises 12%, and the cost is $2,088, or 3.1%.

When does dollar-cost averaging win?

When prices spend the deployment window below your starting point, so your later purchases buy more shares at lower prices. If the market dips and then recovers to where it started, averaging in finishes 17.1% ahead of the lump sum in the model.

Why doesn't the holding period change the result?

Once the last slice is invested, both strategies own the same thing and ride the same returns. The percentage gap between them is locked in at the end of the deployment window and never moves again.

Does the model use historical market data?

No. The market paths are illustrative shapes, not forecasts or history. The model shows the mechanic and the size of the effect for a path you choose. Historically, investing at once has come out ahead most of the time, because markets rise more often than they fall.