Dollar Cost Averaging Calculator

How to Use

Enter the fixed amount you invest each period and the price at each of up to four periods (leave a period at 0 to skip it). Enter the current price to value your position. The calculator buys more units in periods with a lower price and fewer in periods with a higher price, then shows your average cost, total units, current value, and gain or loss, updating live as you type. Because the investment amount is fixed rather than the number of units purchased, this tool automatically models the exact mechanic that gives dollar cost averaging its name.

The Core Mechanic: Fixed Currency, Variable Units

The entire effect of dollar cost averaging comes from one simple rule: invest the same currency amount every period, and let the number of units purchased vary with price. When price is low, that fixed amount buys more units, when price is high, it buys fewer. This isn't a prediction or a strategy about where prices will go, it's a purely mechanical consequence of the fixed-amount rule, and it's why average cost per unit tends to land below the simple, unweighted average of the prices paid across all periods, cheaper periods contributed disproportionately more units to the final total.

Worked Example: Four Periods of Fluctuating Prices

Using this tool's own defaults, investing 5,000 per period across four periods priced at 100, 90, 110, and 95, with a current price of 120. Each period buys a different number of units: 5,000 ÷ 100 = 50 units, 5,000 ÷ 90 = 55.56 units, 5,000 ÷ 110 = 45.45 units, and 5,000 ÷ 95 = 52.63 units, totaling 203.64 units for a total investment of 20,000. Average cost per unit is 20,000 ÷ 203.64 = 98.21, notably below the simple average of the four prices, which is (100+90+110+95) ÷ 4 = 98.75. At the current price of 120, this position is worth 203.64 × 120 = 24,437, a gain of 24,437 − 20,000 = 4,437 over what was invested.

Why the Average Cost Beats the Simple Price Average

This isn't a coincidence or a special property of this particular example, it's a mathematical certainty whenever prices vary across periods and the invested amount stays fixed. The formal reason is that average cost per unit under dollar cost averaging is a harmonic mean of the period prices, weighted toward lower prices, while a plain average of the prices is an arithmetic mean, and the harmonic mean of a set of varying positive numbers is always less than or equal to their arithmetic mean. In plain terms: buying more shares when they're cheap and fewer when they're expensive mechanically pulls your blended cost down below what a naive price average would suggest, regardless of whether prices are trending up, down, or sideways overall.

What Dollar Cost Averaging Doesn't Do

It's easy to mistake dollar cost averaging's cost-lowering mechanic for a guarantee of profit, it isn't one. If the price trends steadily downward across every period, average cost still falls each period, but so does the actual value of the position, a lower average cost doesn't protect against a genuinely declining asset. What dollar cost averaging actually protects against is a specific, narrower risk: committing an entire lump sum at a single bad moment, right before a price drop. Spreading purchases across time reduces that particular timing risk without eliminating the underlying market risk of the asset itself declining in value over the whole period.

Frequently Asked Questions

What is dollar cost averaging?

Dollar cost averaging is investing a fixed amount at regular intervals regardless of price, instead of one lump sum. Because the fixed amount buys more units when the price is low and fewer when it's high, your average cost per unit tends to land below the simple average of the prices you bought at.

Does dollar cost averaging guarantee a profit?

No. It reduces the risk of investing everything right before a price drop, but if the price trends downward across every period, your average cost still falls and you can still show a loss. It's a risk-management approach to timing, not a guarantee of returns.

Why is average cost per unit usually lower than the simple average of the prices?

Because a fixed investment amount buys more units when the price is low and fewer units when the price is high, so low-price periods are automatically weighted more heavily in the final average, purely as a mechanical side effect of investing a fixed currency amount rather than a fixed number of units each period. This effect is sometimes called the mathematics of dollar cost averaging, and it holds whenever prices fluctuate, even without any price trend in either direction.

Is dollar cost averaging better than investing a lump sum all at once?

Historical studies of broad market indices generally find lump-sum investing outperforms dollar cost averaging more often than not over long horizons, since markets trend upward over time and money invested earlier has more time to grow. Dollar cost averaging's real advantage isn't higher average returns, it's reduced regret risk and smoother emotional experience, spreading out entry points so a single badly-timed lump sum right before a downturn doesn't do as much damage. Which approach fits better depends on risk tolerance and whether the lump sum is even available upfront.

How many periods should I use when comparing dollar cost averaging scenarios?

This calculator supports up to four periods, useful for a quick comparison, real dollar cost averaging plans typically run monthly or quarterly over a much longer stretch, a year or more. For a longer-running plan, group your actual purchase history into representative periods (average price per quarter, for instance) to fit within this tool's four-period layout, or track a longer running average manually using the same underlying method this tool applies.

What happens if I skip a period by leaving its price at 0?

A price of 0 is treated as a skipped period, no investment amount or units are added for that period, so it doesn't affect the average cost, total invested, or total units at all. This lets you model plans with fewer than four actual periods without the empty periods distorting the result.