Dollar-Cost Averaging (DCA) Basics: How Fixed-Amount Buying Works and Where It Falls Short

What dollar-cost averaging is

Dollar-cost averaging means buying stocks, ETFs or coins with the same amount of money on a fixed schedule, such as the 25th of every month or every Monday. The key is buying the same amount of money, not the same number of units. It is used in two situations. One is investing money as it arrives, such as from a salary; there is no lump sum to invest at once, so DCA is the natural choice. The other is deliberately splitting a lump sum you already have over several months. The two have different comparisons, so mixing them causes confusion. This guide uses examples to show what DCA does and does not do in numbers.

Why average cost ends up below the average price

When you buy a fixed amount, you get more units when the price is low and fewer when it is high. So the average cost (total invested ÷ units held) can never be higher than the simple average of the prices over the same period, and the gap grows as prices swing more. Mathematically the average cost is the harmonic mean of the prices, which is always at or below the arithmetic mean, and the two are equal only when every price is the same. The figure below is an example of buying 100 each month for 12 months while the price halved and then came back. The bars in the lower panel show the units bought each month; they are tallest in the months when the price was low.

The opposite case: a steadily rising period

The same method does not always come out ahead. If the price rises steadily, later purchases are more expensive, and money invested later spends less time in the market. The example below is 12 months in which the price rose by 5 each month from 100 to 155. The DCA average cost is 125.13 and the holding is worth 1,486, up about 24%, but the same 1,200 invested at 100 in the first month would have grown to 1,860, up 55%. In short, DCA tends to beat a lump sum over periods that fall and then recover, and to trail it over periods that rise steadily. Since you cannot know in advance which kind of period lies ahead, DCA is better understood as a way to spread timing risk than as a way to increase returns.

Risks DCA spreads and risks it does not

What DCA reduces is the risk of buying everything at one moment. It avoids putting a lump sum in right near a peak and eases the stress and regret of picking a time. But the risk of the asset itself remains. An asset that falls for a long time, or never recovers, loses money even when bought in pieces. Also, as contributions build up the balance grows, so the same percentage drop late in the plan costs far more money than early on. That is why a crash close to retirement or to when you need the money hurts more. So when reading DCA results, look not only at the final return but also at the largest decline in value during the accumulation period (maximum drawdown).

• Reduced: the risk of buying all at once, the stress and regret of timing
• Unchanged: the risk that the asset itself falls for a long time or never recovers
• Increased: the money impact of declines late in the plan

Interval, amount and costs

The difference between weekly and monthly intervals is often smaller than people expect. Shorter intervals spread purchase prices more finely, but more purchases mean higher costs where there is a minimum fee per trade. For foreign stocks, currency conversion costs and exchange rates also enter the result; if you contribute in your home currency, each purchase converts at that day's rate, so you are averaging into the exchange rate too. Whether dividends are reinvested also changes long-run results a great deal. Where fractional units are not available, the amount does not divide evenly, so the money actually spent varies slightly each time. If you do not set rules in advance for when to pause and restart, it is easy to stop in a falling market and restart in a rising one, which works against the very idea of DCA.

How to read the results

Because money goes in over many dates, a simple return (value ÷ total invested − 1) does not tell you how long the money was invested: the first contribution may have been in for years and last month's for one month. So also look at the annualized internal rate of return (IRR), which reflects the date and amount of each purchase. When comparing with a lump sum, it is fair only if the same total goes in on the same first day. Repeating the calculation with different start dates shows that the same asset can give very different results depending on the starting year. The important thing is not to generalize one calculation into a verdict on the method.

• Simple return: value ÷ total invested − 1
• Annualized IRR: yearly return that reflects each purchase date and amount
• Maximum drawdown: largest fall in value from a previous high during accumulation
• Lump-sum comparison: the same total bought at once on the first purchase date

Using the tools on this site

This site's Stock DCA Calculator replays monthly, weekly or every-trading-day purchases with real Yahoo Finance prices, calculating total invested, value and return, annualized return (IRR) and maximum drawdown, and comparing with investing the same money at once on the first day. You can also choose whether dividends are reinvested and convert to Korean won at each purchase date's exchange rate. The Crypto DCA Simulator calculates daily, weekly or monthly purchases at real Binance daily opening prices and draws your average cost line on the price chart. Both tools have a table of results by start date, so you can see how differently the same method performed across periods. For the illusions average cost can create, see the guide on the average purchase price trap.

Limits and disclaimer

The figures in this guide use example prices made to show the principle; they are not records of real assets. Actual results depend on the asset, period, interval, costs, taxes and exchange rates, and past DCA results do not guarantee the same in the future. DCA does not remove risk; it spreads out when you buy. This guide explains the principle and limits of dollar-cost averaging, does not recommend buying any asset and is not investment advice.