ETF Bid-Ask Spread: The Hidden Transaction Cost
Measure the gap between best bid and ask in dollars and percentage before trading an ETF.
Published 2026-07-21
Two ETFs on the same index, at the same management fees, can cost very differently to buy — and the difference appears on no prospectus: the bid-ask spread, the distance between the best buying and selling price, is paid at every transaction. Converted into dollars on a real amount, it runs from negligible — a one-cent spread — to substantial, ten or twelve cents rivalling years of management fees. The liquidity behind the number has two storeys, the fund's own and its underlying assets', a fund of thinly traded securities carrying their spreads into its own. Execution has its rules: mid-session rather than open or close, hours when a foreign fund's underlying market is open, and a limit order as soon as the spread is visible. This article shows the dollar-cost calculation, the twin-fund comparison and the execution habits that eliminate a cost invisible yet perfectly avoidable.
Read both prices before placing the order
The gap between an exchange-traded fund's best bid and best ask is a transaction cost, paid on purchase as on sale, and invisible on every statement. It is read before the order, directly in the book: two displayed prices, their difference in cents. That figure varies through the day and by security, which makes any general rule useless. Checking it takes five seconds and it alone determines whether the order should go now, later, or with a limit attached.
Express the spread as a percentage of price
Five cents of spread says nothing until it is related to the price: five cents on a fifty-dollar security is a tenth of a percent, five cents on a five-dollar security is a full one percent. Applied to the round trip, that second case costs two percent, more than many index funds' annual management fees. That conversion to a percentage is what allows comparing two similar funds: at equal management fees, the one with the thinner spread genuinely costs less to hold.
Distinguish the fund's liquidity from its assets'
A lightly traded exchange-traded fund is not necessarily illiquid: its real liquidity comes from that of the securities it holds, market makers being able to create units on demand. A fund of large Canadian equities, even thinly traded, therefore keeps a narrow spread. Conversely, a fund holding illiquid bonds or emerging-market equities shows a wide spread despite respectable volume. The right indicator is therefore the nature of the underlying assets, not the fund's own daily volume, contrary to the most widespread intuition.
Trade while the underlying market is open
Spreads widen when market makers cannot price the held assets precisely: at the session's open and close, and above all when the underlying market is shut. A European equity fund traded in Toronto in the afternoon, once European exchanges have closed, shows a distinctly wider spread than in the morning. The resulting practice is simple: trade these funds mid-session, when the underlying markets are open, and use a limit order rather than a market order. The same rule applies to bond funds during a fixed income market's quiet hours, and to any fund holding assets that trade in a different time zone from the exchange where the fund itself is listed. Placing the order at ten in the morning rather than at nine or at four is the entire technique, and it costs nothing to follow. Avoid the open and the close.
Quebec scenario: compare before confirming
Two ETFs track the same Canadian index at identical management fees, and an accountant in Sherbrooke is about to choose at random when a column on the exchange's site catches his eye: the bid-ask spread. The first fund shows $24.50 bid against $24.51 ask; the second, less traded, $24.42 against $24.54. The translation into dollars speaks better than percentages: on his $15,000, crossing the spread costs about $3 in the first case, $37 in the second — at purchase and again at sale, a real transaction cost no expense ratio mentions. The liquidity behind the number has two storeys, the brokerage desk explains: that of the ETF itself, maintained by market makers, and that of its underlying assets, because a fund of rarely traded stocks carries its holdings' spreads into its own. The hour matters too: spreads widen at the open, at the close and during announcements; for a fund of foreign assets, the spread tightens when the underlying market is itself open. His execution rules take three lines in his notebook: prefer the thin-spread fund at equal index, place orders mid-session, and for any fund with a visible spread, a limit order rather than market. Cost of the lesson: zero dollars, for once — chance having been intercepted in time.
Checklist
- Record the compared funds' bid-ask spreads
- Convert the spread into dollars on your amount
- Count the round trip, purchase and sale
- Choose the thin-spread fund at equal index
- Check the underlying assets' liquidity
- Place orders mid-session
- Wait for foreign funds' underlying market to open
- Use limits on any visible spread
- Add spread and fees together in comparisons
Frequently asked questions
How do I measure the bid-ask spread's cost?
Convert it into dollars on your trade: half the spread, multiplied by the amount, at purchase and again at sale. On a one-cent-spread fund, the cost is negligible; at twelve cents, it rivals years of management fees. No expense ratio mentions this cost.
Why do two identical funds show different spreads?
Liquidity has two storeys: that of the ETF itself, maintained by market makers according to volumes, and that of its underlying assets, whose spreads climb into its own. At equal index, the most traded fund almost always offers the thinnest spread.
What moment should an order be placed?
Mid-session — never the open or the close, where spreads widen. For a fund of foreign assets, favour the hours when the underlying market is itself open: market makers then tighten their prices. A limit order completes the precaution on any visible spread.