Dollar-Cost Averaging (DCA)

How Fixed Recurring Investments Average Out

Dollar-cost averaging means investing the same fixed dollar amount at regular intervals, regardless of price — so a fixed amount automatically buys more shares when the price is low and fewer when it’s high. Enter the amount invested each period, the share price for each period, and optionally a current price, to see the average cost per share this actually produced, the total shares bought, and the current gain or loss.

This is distinct from the Return on Investment (ROI) Calculator calculator, which compares a single starting value to a single ending value — one lump sum in, one lump sum out. This calculator instead models a series of fixed, recurring investments made at different prices over time, which is what dollar-cost averaging actually is.

The Formula

For each period, shares purchased equal the fixed investment divided by that period’s price:

$$\text{Shares}_i = \frac{\text{Investment}}{\text{Price}_i}$$

The average cost per share is total dollars invested divided by total shares accumulated — not a simple average of the prices:

$$\text{Average Cost} = \frac{\sum \text{Investment}}{\sum \text{Shares}_i}$$

Worked Example

Investing $500 per period at prices of $100, $80, $120, $90:

1. Shares bought each period: 5, 6.25, 4.17, 5.56 — totaling about 20.97 shares.
2. Total invested: $2,000.
3. Average cost per share: $95.36 — lower than the simple average of the four prices ($97.50), because more shares were bought during the cheaper $80 period.
4. At a current price of $110, the position is worth about $2,306.94, a gain of about $306.94 (15.35%).

Key Factors to Consider

• Dollar-cost averaging works especially well specifically when prices are volatile, not just falling. The math that pulls average cost below the simple price average depends on genuine price VARIATION, not on prices generally falling — a volatile but flat-trending price still produces this effect, while a smoothly and steadily rising price produces very little benefit from it.
• Transaction costs or fees per investment period can offset some of the benefit. If each recurring investment carries a flat transaction fee, more frequent, smaller investments accumulate more total fees than fewer, larger ones — check whether your specific brokerage or platform charges per-transaction fees before choosing an investment frequency.
• This calculator handles a known, fixed set of past prices — it doesn’t project future prices. The average cost and gain/loss figures here are a look back at what a real (or hypothetical) recurring investment actually produced, not a forecast of what a future dollar-cost averaging plan will achieve.
• Automating the recurring investment is what makes dollar-cost averaging actually work in practice. The strategy’s real-world value often comes as much from removing the temptation to time the market as from the math itself — many investors set up automatic recurring purchases specifically to enforce this discipline.

Common Mistakes

• Assuming dollar-cost averaging always beats investing a lump sum. It doesn’t — it trades timing risk for consistency. If the price trends mostly upward, investing everything upfront usually outperforms spreading it out.
• Comparing the average cost per share to a simple average of the prices and expecting them to match. They only match when the price never changes — any real price variation pulls the true (investment-weighted) average cost below the simple average.
• Forgetting that a lower average cost per share isn’t the same as a profit. The position still needs the current price to exceed the average cost per share to be in the green.

Useful to Know

• Dollar-cost averaging is a strategy about when to invest a given amount of money, not what to invest in — it says nothing about picking a good stock, fund, or asset in the first place, which is a separate decision entirely.
• The interval between investments (weekly, monthly, quarterly) doesn’t change the underlying math this calculator shows — what matters is that each period’s fixed amount is divided by that period’s own price, regardless of how much time passes between periods.
• Many employer-sponsored retirement plans already apply dollar-cost averaging automatically: a fixed percentage of each paycheck buys shares at whatever price prevails on that pay date, which is exactly the same mechanism this calculator models.
• This calculator’s average cost figure is unrelated to a brokerage’s own “average cost basis” reporting for tax purposes, which may use a different accounting method (e.g. FIFO or specific lot identification) — check your brokerage statements directly for the figure that actually applies to your tax situation.