When most investors compare exchange-traded funds, they look at the expense ratio. It is the number plastered on every advertisement, every comparison site, and every fund fact sheet. A low fee feels like a win, and for many products, it is. But the expense ratio is only part of the cost. Every time you buy or sell an ETF, you pay another cost: the bid-ask spread. It is rarely quoted in the same breath as the fee, and it can quietly eat a meaningful slice of your returns. This article explains how the spread works, why it is missing from most fee comparisons, and what you can do about it.

The Expense Ratio Is Not the Whole Bill

The expense ratio covers the fund's operating costs: management fees, administrative expenses, and sometimes marketing. It is deducted from the fund's assets daily, so the net asset value you see already reflects it. For a typical index ETF, that ratio might be around 0.03% to 0.10% per year. That is a small number, and it is the one most people focus on.

But the expense ratio ignores the cost of actually trading the fund itself. When you buy an ETF, you pay the ask price. When you sell, you receive the bid price. The difference between those two prices is the spread, and it is a real cost you incur on every transaction. Unlike the expense ratio, which is spread out over a year, the spread hits you all at once, every time you trade.

Consider a simple example. An ETF trades with a bid of $50.00 and an ask of $50.05. The spread is $0.05, or 0.1% of the price. If you buy and then sell later, you pay that spread twice, so the round-trip cost is 0.2%. If you trade once a month, that adds up to roughly 2.4% per year in spread costs alone, far exceeding the expense ratio. Even a buy-and-hold investor who trades once a year still pays 0.2% annually, which can be comparable to or greater than the fee.

This gap between quoted fee and actual cost is the central problem. A fund with a 0.03% expense ratio and a wide spread can be more expensive in practice than a fund with a 0.10% fee and a tight spread. But because the spread is not part of the fee disclosure, most investors never see it.

How the Spread Sneaks Into Your Returns

To understand the spread, you need to know how ETF pricing works. The fund itself holds a basket of securities, and its net asset value (NAV) is calculated from those holdings. But the ETF trades on an exchange, where prices are set by supply and demand. Market makers, often large banks or specialist firms, stand ready to buy and sell the ETF at two prices: the bid and the ask.

The bid is the highest price a buyer is willing to pay. The ask is the lowest price a seller will accept. The spread is the difference. Market makers earn their living from this difference, and it compensates them for the risk of holding inventory and the cost of hedging. For liquid ETFs tracking major indices, the spread might be just a penny or two. For niche or thinly traded funds, it can be much wider.

When you place a market order, you buy at the ask and sell at the bid. You are, in effect, paying the market maker's spread. This is not a hidden fee in the regulatory sense, but it is a cost you bear directly. If the spread is 0.10% and you trade twice a year, that is 0.20% per year, which is often more than the expense ratio.

Frequent trading amplifies the hit. An active trader who buys and sells an ETF several times a month can lose several percent of their returns to spreads annually. Even long-term investors who rebalance quarterly face a recurring drag. The spread is not a one-time cost; it is a toll on every crossing.

Compounding makes it worse. Over a 20-year period, an extra 0.5% in annual costs can reduce your final portfolio by roughly 10%, according to some estimates. That is not trivial. Yet because the spread is not itemized in any statement, most investors never notice it.

The Prospectus Fine Print You Never Read

The expense ratio comes straight from the prospectus, the legal document that describes the fund's fees and risks. The prospectus is required to disclose the expense ratio prominently, usually in a fee table. But the spread is not part of that table. Instead, it is mentioned, if at all, in a separate section about trading and secondary market pricing.

For example, the prospectus might state that shares may trade at a premium or discount to NAV, and that the bid-ask spread is a cost of trading. But it does not quantify the spread. You will not find a number like "0.07% per trade" in the fee table. The spread is determined by market conditions and is not fixed, so it cannot be disclosed as a simple percentage.

Regulators do require brokers to disclose order execution quality, but this data is buried in reports that most investors never see. The SEC's Rule 605 and 606 reports provide some information, but they are dense and not designed for retail comparison. As a result, the spread remains invisible to the typical investor.

The source document, the prospectus, does tell the truth if you read carefully. It will say that the expense ratio does not include brokerage commissions or other trading costs. It will mention that the fund may incur costs when buying and selling portfolio securities. But it will not tell you what the spread on your trade will be, because it depends on when and how you trade.

This is not a conspiracy. It is a structural feature of how ETF costs are reported. The expense ratio is a standardized, comparable number. The spread is variable and personal. But that does not make it less real.

Why the 'Low Fee' Mantra Misses the Point

Financial advisors and commentators often repeat the mantra: choose low-fee index funds. This advice is solid for the expense ratio. But it ignores the spread. A fund with a 0.03% expense ratio can have a spread of 0.10% or more, especially if it is new, small, or tracks an obscure index.

I have seen advisors compare two ETFs tracking the same index and recommend the one with the lower fee, without ever checking the spread. In one case, the lower-fee fund had a spread that was twice as wide as the competitor's, making it more expensive for any investor who trades more than once a year. The advice was not wrong, just incomplete.

The problem is that the spread is easy to miss. It is not included in the fee table, not in the marketing materials, and not in most comparison tools. A study by the Investment Company Institute, as of a few years ago, estimated that trading costs, including spreads, could add several basis points to the total cost of owning an ETF. For active traders, the impact is even larger.

This does not mean the low-fee mantra is useless. It means it is incomplete. The full cost of owning an ETF is the expense ratio plus the spread plus any commissions or taxes. Ignoring the spread can lead you to choose a fund that is more expensive in practice, and it can lead you to trade more often than is wise.

Comparing Funds: The Real Cost Gap

To see the real cost gap, compare two real ETFs that track the same index. For instance, consider two S&P 500 ETFs: one from a major provider with a low fee and high liquidity, and another from a smaller provider with a similar fee but lower trading volume. As of early 2025, the first fund might have an expense ratio of 0.03% and an average spread of 0.01%, while the second might have an expense ratio of 0.05% and an average spread of 0.05%. For a buy-and-hold investor who trades once a year, the first fund costs 0.04% per year (fee plus spread), while the second costs 0.10%. The first is cheaper by 0.06% per year.

But if you trade monthly, the difference widens. The first fund costs 0.03% plus 0.12% (12 trades times 0.01%) for a total of 0.15%. The second costs 0.05% plus 0.60% for a total of 0.65%. The spread dominates, and the fund with the lower expense ratio is not necessarily the cheaper one.

Liquidity is the main driver of spread size. Funds that trade heavily and have large assets under management tend to have tighter spreads because market makers can hedge more easily. Funds that are thinly traded or track niche sectors have wider spreads. Size and volume matter more than the expense ratio in determining the spread.

Some brokers now offer "spread" data on their platforms, and some comparison websites include an estimated cost of trading. But these tools are not universal. The onus is on the investor to check the spread before buying, and to consider how often they plan to trade.

How to Actually Minimize Your Costs

The first step is to check the spread before you trade. Most broker platforms show the bid and ask prices in real time. You can calculate the spread as a percentage of the price. If the spread is more than a few basis points for a large-cap index ETF, you might be looking at a less liquid fund.

Use limit orders instead of market orders. A limit order lets you specify the maximum price you are willing to pay or the minimum price you are willing to accept. This protects you from paying a wider spread than necessary, especially during volatile periods. Market orders are convenient, but they execute at the current ask or bid, which can be worse than you expect.

Avoid trading during the first and last minutes of the trading day, when spreads tend to be widest. The middle of the day usually offers tighter spreads. Also avoid trading around major news events or earnings releases, when volatility can cause spreads to widen dramatically.

Hold longer to reduce trade frequency. Each trade incurs a spread, so the fewer trades you make, the less you pay. For long-term investors, this is a strong argument for a buy-and-hold strategy. Rebalancing once or twice a year is usually enough to keep your portfolio on track.

Finally, consider the total cost: fee plus spread plus taxes. For taxable accounts, capital gains taxes are another cost that is often overlooked. The informed investor looks at the whole picture, not just the expense ratio.

The Bottom Line for the Careful Investor

The expense ratio is a useful starting point, but it is not the whole cost. The bid-ask spread is a hidden tax on every trade, and it can be larger than the fee. The careful investor should check the spread before buying, use limit orders, and trade less often.

There is a counter-argument: for many large, liquid ETFs, the spread is so small that it is negligible. A 0.01% spread on a $100 purchase is one cent. That is not going to change your retirement. But for smaller funds, or for active traders, the spread can be significant. The key is to know which situation you are in.

Some experts argue that the spread is not a cost at all, because you can always get a better price by using a limit order. That is true, but only if you are patient and willing to wait for your order to fill. In a fast-moving market, you may miss the price entirely.

The regulatory landscape is also shifting. The SEC's Regulation Best Interest, which applies to brokers, requires them to act in their clients' best interest and disclose conflicts of interest. This has led to more transparency about order routing, as discussed in a related article on SEC Reg BI and order routing. But even with better disclosure, the spread remains a cost you bear directly.

So, the most practical advice is to be aware. The expense ratio is a good starting point, but it is not the final word. Check the spread, use limit orders, and trade less. The informed investor pays less, not because they found a magic fund, but because they understand the full cost structure. However, it is important to recognize the limitations of this advice. The spread is only one component of total cost, and for some investors, the effort to minimize it may not be worth the potential savings. For instance, if you are investing small amounts infrequently, the spread may be negligible, and the time spent optimizing could be better used elsewhere. Additionally, the strategies suggested here—such as using limit orders and reducing trade frequency—are general guidelines, not a one-size-fits-all solution. Your specific circumstances, including your investment goals, time horizon, and risk tolerance, should guide your decisions. As with any financial decision, your mileage may vary, and it is always wise to consult a professional for personalized advice.

This article is for informational purposes only and does not constitute personalized investment advice. The strategies discussed are general in nature and may not be suitable for your individual situation. Always consider your own financial situation and consult a qualified advisor before making any investment decisions.