ABC Tools

Dollar-Cost Averaging, Explained

After reading this you can project what a fixed, repeated investment grows to at an assumed return, and separate the money you put in from the growth on top.

What dollar-cost averaging is

Dollar-cost averaging (DCA) means investing the same amount of money on a fixed schedule, no matter what the price is that day. You might put in 500 every month into a broad index fund. Some months the price is high and your 500 buys fewer shares. Some months the price is low and it buys more. Over time your average cost per share settles somewhere in the middle.

Here is the hook. Suppose you invest 500 per month for 20 years and your investments earn an average of 7% per year. You will have contributed 500 × 12 × 20 = 120000 of your own money. At the end the account is worth about 260463. The extra 140463 is growth: it came from returns compounding on both your contributions and on earlier returns.

The calculator does this projection. You give it the contribution, the frequency, the number of years and an assumed annual return. It reports the future value and shows how much of that is contributions versus growth.

When to use this, and when not to

Use this projection when you invest on a regular schedule and want a rough sense of scale over years or decades. It answers questions like "if I keep putting in 200 a week, roughly where could I be in 15 years?"

Do not read the result as a forecast. The single assumed return is a smoothed average. Real markets move in jagged steps: up 22% one year, down 18% the next. The final number in a real account depends heavily on the exact sequence of returns, especially near the end when the balance is largest.

A steady 7% line is a teaching device, not a promise. Two investors with the same average return over 20 years can end with balances that differ by tens of thousands, purely because the good and bad years landed in a different order.

The tool also assumes you never miss a contribution and never withdraw. If your real plan has gaps, treat the output as an optimistic ceiling.

The formula and why it works

Each contribution is a fixed payment made at regular intervals, so this is the future value of an ordinary annuity. Let P be the amount per contribution, i the return per period, and n the total number of contributions.

FV = P \cdot \frac{(1 + i)^n - 1}{i}

Here P is your contribution (for example 500), i is the periodic return, and n is how many contributions you make in total. The whole fraction is a multiplier: it tells you how many "contribution-sized" dollars you end up with per dollar you put in each period.

The periodic return comes from the annual return. If the annual return is r as a decimal and there are m periods per year, then:

i = (1 + r)^{1/m} - 1

This compounds correctly: growing at rate i for m periods returns you to (1 + r) over one year. For monthly contributions m = 12, for weekly m = 52, for quarterly m = 4. The total number of contributions is n = m \times \text{years}.

The intuition behind the annuity formula: the first payment compounds for the full term, the second for one period less, and so on. Summing that geometric series gives the fraction above. Contributions total P \times n, and everything beyond that is growth.

A worked example with the demo numbers

500 per month, 20 years, 7% return

These are the tool's default values, so you can press the demo button and match every figure.

  1. Convert the annual return to a monthly rate. With r = 0.07 and m = 12: i = 1.07^{1/12} - 1 \approx 0.005654, about 0.5654% per month.
  2. Count the contributions: n = 12 \times 20 = 240.
  3. Build the multiplier: (1.005654)^{240} \approx 3.8697, so the fraction is (3.8697 - 1) / 0.005654 \approx 520.93.
  4. Multiply by the contribution: 500 \times 520.93 \approx 260463.
  5. Split it: contributions are 500 \times 240 = 120000; growth is 260463 - 120000 = 140463.

So the final value is about 260463, of which 120000 is your own money and 140463 is compounding. Growth is 54 percent of the total here, because early contributions had two decades to compound.

Of the roughly 260,463 final value, about 120,000 is money you paid in and 140,463 is growth.

Watch the balance build over time

The balance does not climb in a straight line. Early on, contributions dominate because there is little to compound. Later, growth pulls ahead. In the default plan the balance reaches about 86542 at year 10 (contributions 60000, growth 26542) and about 260463 at year 20. The second decade adds far more than the first, even though you paid in the same 60000 each decade.

The curve steepens over time. The same monthly deposit produces a much larger annual gain in year 20 than in year 2.

At a 5% annual return the default plan ends near 205,517; at 7% near 260,463; at 9% near 334,181. Contributions stay fixed at 120,000, so a higher assumed return changes only the growth portion, and it changes it steeply because the effect compounds over 240 months.

Reading and interpreting the result

Look at three numbers together: final value, total contributions, and growth. The split tells you how much of the outcome depends on your assumed return versus on your own saving.

In the default plan, contributions are 46 percent of the total and growth is 54 percent. Push the years out and growth's share rises fast. At 30 years the same 500 per month at 7% reaches about 609858, with contributions of 180000 (30 percent) and growth of 429858 (70 percent). Time does the heavy lifting.

Because growth depends on the assumed return raised to a large power, small changes in the return matter a lot. Dropping from 7% to 6% in the default plan cuts the final value from about 260463 to about 231020, a difference of 29443 that all comes out of growth.

Common mistakes

Treating the return as guaranteed
The output is one smoothed path. Try several returns (for example 4%, 7%, 10%) to see a realistic range instead of a single point.
Ignoring inflation
A projected 260463 in 20 years buys less than 260463 today. To see purchasing power, discount the result or use the Inflation-Adjusted Return Calculator.
Forgetting fees
A fund charging 1% per year lowers your effective return by roughly that amount. Enter 6% instead of 7%, or check the Investment Fee Impact Calculator to see the long-run cost.
Mixing up frequency and contribution size
Weekly 115 is not the same as monthly 500. Weekly at 115 is 5980 per year; monthly at 500 is 6000 per year. Match your real cash flow.

Related tools

Once you have a projection, these tools answer neighboring questions. For a general portfolio projection with a starting balance and monthly deposits, use the Investment Growth Calculator. To measure a return you actually achieved rather than assumed, use the ROI Calculator or the CAGR Calculator. If your buys happened at different prices and you want the blended cost, use the Stock Average Price Calculator.

For cash-flow analysis with irregular timing, the NPV Calculator and IRR Calculator handle uneven series. When dividends are reinvested, the Dividend Reinvestment (DRIP) Calculator shows share growth, and the Dividend Yield Calculator covers income. For position sizing on individual trades see the Position Size Calculator, and for tax on a sale see the Capital Gains Tax Calculator or, for digital assets, the Crypto Profit Calculator.

Frequently asked questions

Does dollar-cost averaging beat investing a lump sum?

Not usually, on average. Because markets rise more often than they fall, money invested earlier tends to compound longer. DCA is mainly a way to invest steadily from income you receive over time, and to avoid the risk of putting everything in right before a drop.

Why do weekly and monthly plans give slightly different results?

More frequent contributions compound very slightly more often and put money to work a little sooner. The gap is small: 500 monthly and 115.38 weekly at 7% over 20 years land within a fraction of a percent of each other. The dominant driver is total dollars invested per year.

Should I use a real historical return like 10%?

You can, but be careful. Long-run stock index averages before inflation and fees have often been near 9% to 10%, yet any 20-year window can differ widely. Running a lower figure such as 6% or 7% gives a more conservative planning number.

How do I account for rising contributions over time?

This tool assumes a fixed contribution. If you plan to increase your deposits, project each phase separately, or use the growth calculator and revise the amount as your income changes.

Try it yourself in the tool

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