Dollar-Cost Averaging, and the Arithmetic Behind Why It Lowers Your Average Cost

Dollar-cost averaging means investing the same amount of money at regular intervals, regardless of what the market is doing. The SEC describes it as investing in equal portions at regular intervals through the ups and downs, and notes the consequence: a fixed sum buys more units when prices are low and fewer when prices are high.

That consequence has a measurable effect, and it is worth seeing in numbers.

The worked example

Invest $300 a month for four months in a fund whose price moves around:

• Month 1, price $30: your $300 buys 10.00 shares
• Month 2, price $25: your $300 buys 12.00 shares
• Month 3, price $20: your $300 buys 15.00 shares
• Month 4, price $25: your $300 buys 12.00 shares

You invested $1,200 and acquired 49 shares.

• Your average cost per share is $1,200 divided by 49, which is $24.49
• The average price across the four months is $25.00

Your average cost came in below the average price, without you predicting anything.

Why that happens every time

This is not luck, and it is not specific to those numbers. Buying a fixed dollar amount means the quantity you buy varies inversely with price. More shares at $20 than at $30. Those larger purchases at lower prices carry more weight in your average cost.

Mathematically, your average cost is the harmonic mean of the prices, and the harmonic mean is always less than or equal to the arithmetic mean unless every price is identical. So the effect is guaranteed by the structure, not by market conditions.

What is not guaranteed is that you make money. Your average cost being $24.49 helps only if the price eventually exceeds it. In a market that falls and stays down, dollar-cost averaging gives you a lower average cost on an investment that is still worth less than you put in. It manages how you enter; it does not determine the outcome.

What it does and does not do

What it does:

• Removes timing from the decision. You are not trying to identify a good moment, which is a problem few people solve reliably
• Reduces the impact of a single bad entry point. Investing everything the day before a sharp fall is a specific risk that spreading purchases dilutes
• Makes the behaviour automatic. A standing instruction continues during exactly the periods when discretion tends to fail

What it does not do:

• Guarantee a better result than investing all at once. Markets rise more often than they fall over long periods, so a lump sum invested earlier has often finished ahead. Dollar-cost averaging trades some of that expected return for a narrower range of outcomes
• Protect against loss. Spreading purchases across a sustained decline still leaves you holding a declining asset
• Justify a bad investment. Averaging into something you should not own simply buys more of it

You may already be doing it

If you contribute to a workplace retirement plan from each paycheque, you are dollar-cost averaging by default. The same fixed amount goes in every period at whatever the price happens to be. That is the mechanism working without anyone deciding to use it, which is much of its appeal.

The comparison that actually matters

There is a genuine choice when you have a lump sum available now, say an inheritance or a bonus. Investing it immediately puts more money to work sooner, which has historically tended to produce more. Spreading it over some months reduces the consequence of being unlucky with the date.

Both are defensible. The deciding factor is usually not the expected return but whether a sharp fall immediately after investing would cause you to sell. An approach you hold through a bad quarter beats a theoretically superior one you abandon.

What is not defensible is holding the money in cash indefinitely while waiting for a better entry point. That is timing the market with extra steps, and the cost of the delay compounds, as shown in compound interest, worked out by hand.

Model different contribution schedules with our investment calculator to see what regular contributions accumulate to over long periods.

Sources

• SEC Investor.gov: Dollar cost averaging
• SEC Investor.gov: Introduction to investing
• SEC Investor.gov: Compound interest calculator

Current as of August 2026. Prices in the example are illustrative and chosen to demonstrate the arithmetic. Investments can lose value, and past performance does not indicate future results.