ETFs & Funds · Intermediate · 4 min

Cost averaging: the maths, not the myth

You can calculate the average cost of regular investments, explain why it is below the average price, and say what cost averaging can and cannot do.

Same amount, every month

Savings plans for ETFs are often sold with a promise: invest the same amount every month, and falling prices work in your favor. Some go further and claim you cannot go wrong this way.

The mechanism behind it is real and easy to calculate. But it is smaller than the marketing suggests, and it comes with a trade-off that is rarely mentioned. This lesson does the maths.

More units when cheap

Cost averaging means investing a fixed amount at regular intervals, for example 100 every month, whatever the price. Investor.gov describes it as investing in equal portions at regular intervals, regardless of the ups and downs of the market.

Because the amount is fixed, the number of units you buy changes with the price:

When the price is high, 100 buys few units.
When the price is low, 100 buys many units.

So more of your units are bought at low prices. As a result, your average cost per unit is lower than the simple average of the prices on the purchase dates. That is the maths, and it always holds when prices fluctuate.

What cost averaging does not do:

It does not prevent losses. If prices fall and stay low, everything you bought is worth less.
It does not beat lump-sum investing in rising markets. Investing everything at once puts all the money to work from day one. FINRA notes that spreading purchases out lowers risk, but often produces lower returns than a lump sum, especially over longer periods, because markets have tended to rise over time.

The strongest argument for regular investing is often behavioral. Most people earn money monthly, not in one lump. A fixed plan keeps them investing through good and bad times, without trying to guess the right moment.

Cost 7.50, price 8.33

A fictional investor puts 100 into a fund every month for three months.

Total invested: 300. Units bought: 10 + 20 + 10 = 40.
Average cost per unit: 300 ÷ 40 = 7.50.
Average price on the three dates: (10 + 5 + 10) ÷ 3 ≈ 8.33.

The investor paid less per unit than the average price. But if the price had stayed at 5, the 300 would be worth 200. The lower average cost is only an advantage if the price recovers.

Common mistakes

"Cost averaging means you can't lose." You can, if prices stay low.
Holding a lump sum in cash to drip it in. In rising markets, that often costs return.
Stopping the plan when prices fall. That is when the fixed amount buys the most.
Ignoring fees per purchase. A fixed fee on small monthly amounts can be a large percentage.

The takeaway

Investing the same amount regularly buys more shares when prices are low. That lowers the average cost, but it is no magic and often trails a lump sum.

Check yourself

Quick quiz

4 questions. Instant feedback, nothing is saved.

1Tap the month in which the fixed amount bought the most units.

Tap the chart.

A fixed amount buys more units when the price is low. That is the whole mechanism behind cost averaging.
2300 invested in the three months buys 40 units. What is the average cost per unit?

300 ÷ 40 = 7.50. The average price was 8.33, but more units were bought at the low price.
3FINRA compares regular investing with investing a lump sum at once. What does it say?

In rising markets, money waiting in cash misses gains. Cost averaging trades some return for a smoother entry.
4Does cost averaging protect you from losses in a falling market?

A lower average cost helps only if prices recover. In a long decline, every purchase loses value.

of 4 correct.