Bid-Ask Spread: The Cost Hiding in Every Single Trade

Investors will obsess over a fund's expense ratio down to the hundredth of a percentage point, then completely ignore a trading cost that can be several times larger. The bid-ask spread is paid on every single buy and every single sell, silently, and it never once appears as a line item on any account statement.

The core principle

The bid-ask spread is the gap between the bid, the highest price a buyer is currently willing to pay, and the ask, the lowest price a seller is currently willing to accept. In practice, a buyer transacts near the ask and a seller transacts near the bid, so anyone who buys and later sells the same security pays this spread as a real, structural cost, entirely separate from any commission the broker charges, which for most retail stock and ETF trades today is zero. Even in a zero commission environment, the spread remains, quietly, the most common way an ordinary retail trade still generates revenue for the firms on the other side of it.

The spread exists because market makers, the firms and algorithms that continuously stand ready to buy and sell, need to earn something for providing that constant liquidity and for bearing the risk of holding inventory while prices move, a service sometimes described as compensating them for immediacy: the guarantee that a buyer or seller can transact right now rather than waiting for a matching counterparty to happen to show up. On a security that trades enormous volume every second, competition among market makers compresses that spread down to a fraction of a cent. On a security that trades rarely, competition is thin, inventory risk is higher, and the spread widens to compensate, sometimes dramatically, occasionally reaching several percent of the security's price on the thinnest, most obscure names.

Spread widths are not static; they widen and narrow with volume and volatility throughout a single trading day and across different market conditions. Spreads are typically at their tightest during the middle of the trading day, when volume is heaviest and the most market participants are actively quoting. They widen at the market open and close, when overnight news is being digested and volume can be thinner, and they widen dramatically during periods of extreme volatility, exactly when investors are most likely to feel the need to trade urgently, an unfortunate but consistent pattern.

How the math works

Example 1: spread as a percentage of a trade. On a heavily traded, large cap stock priced around $100, the spread might be a single penny, from a bid of $99.995 to an ask of $100.005, or roughly 0.01% of the price, essentially negligible for a typical trade. On a thinly traded, niche ETF also priced around $100 but with a bid of $99.85 and an ask of $100.15, the spread is $0.30, or 0.30% of the price, thirty times wider in percentage terms than the liquid stock example above. If that same niche ETF charges an annual expense ratio of 0.10%, a single round trip trade's spread cost of 0.30% is three times the entire year's stated fee, paid in one transaction rather than spread across twelve months.

Example 2: spread cost on a rebalancing trade. Suppose an investor rebalances a $300,000 portfolio annually, trading $30,000 through a niche international small cap ETF with a typical spread of 0.25%. Each round trip (one sale into the position, one eventual sale out) costs roughly $30,000 x 0.0025 = $75 per leg, or $150 for a full round trip, repeated at each rebalancing. Over 15 years of annual rebalancing, even holding the dollar amount traded constant, that is roughly 15 x $150 = $2,250 in spread costs alone, on top of the fund's stated expense ratio, and the true figure would typically be higher still since rebalancing trade sizes usually grow as the portfolio grows.

How it shows up in real portfolios

For an investor who buys a broad market ETF or mutual fund a few times a year and holds it for decades, the bid-ask spread is a minor, almost forgettable cost, since the most widely traded funds carry spreads of a penny or two on prices often well above $50 a share. The situation changes considerably for investors drawn to specialized, thematic, or leveraged ETFs, which frequently trade a fraction of the daily volume of a broad market fund and correspondingly carry much wider spreads, sometimes 0.20% to 0.50% or more, particularly at market open, at market close, or during volatile sessions when market makers widen spreads to protect themselves from rapid price moves.

Consider a high earning professional actively managing a self-directed brokerage account alongside a demanding full time job, a 47 year old executive who trades a niche sector ETF a dozen times a year chasing short term sector rotations. Even a modest 0.20% average spread, paid on both the buy and the eventual sell of each trade, applied to a typical $25,000 position size across twelve round trips a year, adds up to roughly 12 x 2 x ($25,000 x 0.002) = 12 x 2 x $50 = $1,200 a year in pure spread cost, a figure that would not appear anywhere on his statement but would show up, inevitably, as a persistent drag against whatever return the underlying trading strategy was actually generating.

New, recently launched ETFs deserve special caution on this point, since a fund can have an attractively low advertised expense ratio while still trading with a wide spread simply because it has not yet built up meaningful trading volume or a deep base of assets. Checking a fund's average daily dollar volume, not just its expense ratio and not just its assets under management, before committing a meaningful sum is a habit that catches this specific and easily overlooked risk.

Actionable breakdown

• Check the bid-ask spread before trading any ETF.
  • Look up both numbers directly, not just the last price.
  • Be especially careful with niche or thematic funds.
• Favor highly traded, high volume funds when spread matters.
  • Broad market ETFs typically carry the tightest spreads.
  • Compare average daily volume across similar fund choices.
• Use limit orders to avoid crossing a wide spread by accident.
  • Set the limit between the bid and the ask, not right at the ask.
  • Accept the tradeoff: a slower, less certain fill in exchange.
• Weigh spread cost heavily if you rebalance or trade often.
  • Trade during regular market hours, when spreads are tightest.
  • Consolidate frequent small trades into fewer larger ones.
  • Check a fund's average daily volume, not just its expense ratio.
  • Compare bid-ask spreads across two or three similar candidates.

Common pitfalls

• Comparing two similar funds only on their published expense ratio and ignoring the spread entirely, which can dwarf the fee difference for a low volume fund.
• Trading illiquid ETFs right at market open or market close, exactly when spreads tend to widen the most.
• Trading frequently in a low volume fund, letting a small per-trade spread cost compound into a real annual drag over dozens of transactions.
• Placing a market order in a wide-spread security when a limit order, even at a slightly worse price, would have avoided the widest part of the spread entirely.
• Buying a newly launched ETF for its attractive expense ratio without checking whether it has built up enough trading volume to keep its spread reasonably tight.

Related concepts

• Bid: the buy-side half of the spread, and what you actually receive when selling at market.
• Ask: the sell-side half of the spread, and what you actually pay when buying at market.
• Expense ratio: the visible fee that spread cost is so often mistakenly assumed to be the whole story.
• Basis point: the unit used to measure and compare spread cost across different securities.
• Funds and ETFs guide: broader guidance on evaluating the true, all-in cost of a fund.

The bottom line

For low volume investments and frequent traders, the bid-ask spread can matter more than the published expense ratio, so check both the spread and the fund's real trading volume before you trade, not after.

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