It comes down to brokerage. On our numbers, investing $500 a month beats $1,500 a quarter only if each trade costs less than about $5. At $3 a trade, monthly comes out about $680 ahead after 20 years. At $10 a trade, quarterly wins by about $2,000, and at $20 by almost $5,800. If your waiting money sits in a savings account earning 4%, the break-even falls to about $2.50 a trade. A simple rule of thumb: keep brokerage under about 0.5% of each trade. With $10 brokerage, that means investing at least $2,000 at a time.
The trade-off
Say you can put aside $500 a month. You have two choices:
- Monthly: buy $500 of an ETF every month, 12 trades a year.
- Quarterly: save it up and buy $1,500 every three months, 4 trades a year.
Monthly gets each $500 into the market sooner, so on average it has more time to grow. Quarterly pays brokerage on 8 fewer trades a year. The question is which effect is bigger.
Brokerage as a share of each purchase
The maths: 10 and 20 years
We modelled someone saving $500 at the end of every month, invested in a shares ETF returning 8% a year (the long-run assumption used in our financial goals calculator). The monthly investor buys every month. The quarterly investor keeps the cash in a transaction account and buys every third month. Brokerage is taken out of each purchase.
| Brokerage per trade | Monthly after 20 years | Quarterly after 20 years | Monthly ahead by | Brokerage paid (monthly vs quarterly) |
|---|---|---|---|---|
| $0 | $284,500 | $282,677 | +$1,823 | $0 vs $0 |
| $3 | $282,793 | $282,112 | +$681 | $720 vs $240 |
| $5 | $281,655 | $281,735 | −$80 | $1,200 vs $400 |
| $10 | $278,810 | $280,792 | −$1,983 | $2,400 vs $800 |
| $20 | $273,120 | $278,908 | −$5,788 | $4,800 vs $1,600 |
$500 a month for 20 years ($120,000 in total), 8% a year compounded monthly, cash earning nothing while it waits. Illustration only. Actual returns vary, and markets don't rise in a straight line.
How far ahead monthly investing ends up, by brokerage
Why the break-even is about $5
There's a simple way to see it. Investing quarterly means one month's $500 waits an extra month and another waits two extra months: three "$500-months" of waiting each quarter. At 8% a year, a month of growth on $500 is worth about $3.30, so the waiting costs about $10 a quarter, or $40 a year.
Investing quarterly saves 8 trades a year. If each trade costs $5, that's $40 saved, the same as the growth given up. Below $5 a trade, monthly wins. Above it, quarterly wins. If your waiting cash earns 4% in a savings account, the cost of waiting halves to about $20 a year, and the break-even halves to about $2.50 a trade.
What you pay each year in brokerage on $6,000 of investing
What this means in practice
- $0 to $3 brokerage: invest monthly, or whenever you have the money. Several brokers now offer low or no brokerage on ETFs, sometimes only for small trades or automatic investment plans. Check whether they hold your ETFs in your own name (CHESS-sponsored) or through a custodian. See how to buy ETFs in Australia.
- $5 to $10 brokerage: quarterly, or whenever you have $2,000 or more, is usually better.
- $15 to $20 brokerage: invest every six months, or put larger lump sums in less often. Better still, compare brokers.
The rule of thumb that falls out of the maths: keep brokerage under about 0.5% of each purchase. Divide your brokerage by 0.005 to find your minimum trade size: $5 means $1,000 or more, $10 means $2,000, and $20 means $4,000.
Things that matter more than monthly vs quarterly
- The ETF's fee. Over 20 years, a 0.5% a year difference in fees costs far more than any brokerage choice. Check with our fee cost calculator.
- Actually doing it. The biggest risk to a regular investing plan is stopping when markets fall. Automatic monthly plans help many people stay the course, and that can be worth a few dollars of brokerage.
- Lumps of money. If you receive a bonus or inheritance, the same logic suggests investing it sooner rather than drip-feeding, although some people choose to phase in for peace of mind. See the market correction playbook.
- One fund instead of three. Splitting $500 across three ETFs means three brokerage fees. An all-in-one ETF such as DHHF or VDHG needs just one trade.
- Bid-ask spread. Each trade also costs the spread between buy and sell prices. It's small for large funds like VAS or IVV, but wider for small ETFs.
Common questions
Is it better to invest monthly or quarterly?
If brokerage is under about $5 a trade, monthly usually comes out ahead because your money is invested sooner. Above $5, quarterly usually wins because you save on fees.
What is the minimum amount worth investing in an ETF?
A rule of thumb is to keep brokerage below 0.5% of the trade. That's $1,000 at $5 brokerage, $2,000 at $10 and $4,000 at $20.
Does dollar-cost averaging beat investing a lump sum?
Historically, investing a lump sum straight away has usually done better, because markets rise more often than they fall. Dollar-cost averaging reduces the regret of investing just before a fall, which helps some investors stick to their plan.
Should I hold cash in a savings account while I wait to invest?
Yes, if you're saving up for a quarterly purchase. Interest on the waiting money reduces the cost of investing less often.
General information only. Calculations assume 8% a year and are for illustration. Past performance is not a reliable indicator of future performance.
Joshua Stega is a financial adviser with 20 years' experience and the founder of ETF Adviser. He built ReviewETF to make every Australian ETF easy to find and compare.
General information only. It does not take into account your objectives, financial situation or needs. Consider the PDS and TMD before investing. Past performance is not a reliable indicator of future performance. Data as at 30 September 2026.