Liquidity Analysis of U.S.-Listed ETFs: How the Bid-Ask Spread Affects Your Trading Costs

TL;DR： The bid-ask spread on U.S.-listed exchange-traded funds (ETFs) is a hidden transaction cost that directly affects your actual investment returns. Understanding how ETF liquidity works and mastering strategies to narrow trading costs can help you make more cost-effective decisions in the U.S. market.

When investing in U.S. ETFs (exchange-traded funds), many investors focus only on the expense ratio and overlook an equally important cost—the bid-ask spread. Whether it’s a large ETF tracking the S&P 500 or a smaller ETF tracking a specific sector, ETF liquidity directly impacts your trading costs. Grasping this concept is an important step toward improving investment efficiency. Below, we analyze how U.S. ETF liquidity works, unpack how the bid-ask spread affects your returns, and offer practical strategies to help you reduce trading costs.

什麼是 ETF 流動性？

ETF liquidity refers to the ability to quickly buy or sell an ETF in the market at a fair price. The higher an ETF’s liquidity, the narrower the gap between bid and ask, and the lower the trading cost.

兩層流動性結構

Liquidity in U.S. ETFs operates on two levels. The first is secondary-market liquidity, meaning how actively ETF shares trade on exchanges, measured by average daily volume. The second is primary-market liquidity, maintained by Authorized Participants (APs) through the creation and redemption of ETF shares, which helps keep the ETF’s market price close to its net asset value (NAV).

This two-tier structure is a key distinction between ETFs and single stocks. Even if an ETF’s secondary-market trading volume is relatively low, as long as its underlying assets are sufficiently liquid, Authorized Participants can step in to create new shares and help maintain reasonable bid-ask spreads.

ETF 流動性的衡量指標

When evaluating ETF liquidity, consider the following indicators:

• Average Daily Volume (ADV): Higher volume generally indicates more market participation and better liquidity
• Assets Under Management (AUM): Larger ETFs tend to be more liquid
• Liquidity of the underlying assets: The liquidity of the stocks or bonds held by the ETF often reflects its true liquidity better than the ETF’s own trading volume

注意： Judging ETF liquidity by trading volume alone can be misleading. Some ETFs with lower trading volume may still have tighter spreads if they track highly liquid large-cap U.S. equities.

買賣差價的運作原理

The bid-ask spread is the gap between the market bid (the highest price a buyer is willing to pay) and the ask (the lowest price a seller is willing to accept). When you buy an ETF, you pay the higher ask; when you sell, you receive the lower bid. That gap is the implicit cost you “pay” to the market on each trade.

如何計算買賣差價

Bid-ask spreads are commonly quoted in basis points (bps) and calculated as follows:

Bid-ask spread (bps) = (Ask − Bid) ÷ Ask × 10,000

Here’s a hypothetical example:

• Suppose an ETF’s bid is USD 100.00 and its ask is USD 100.10
• The spread is USD 0.10; in basis points: 0.10 ÷ 100.10 × 10,000 ≈ 10 bps (0.10%)
• If you buy 100 shares, your additional implicit cost is USD 10

Expressing spreads in basis points is more accurate than looking only at absolute dollar amounts. Even if a higher-priced ETF shows a larger absolute spread, the proportional cost in basis points may actually be lower.

做市商的角色

Liquidity in the U.S. ETF market relies heavily on the participation of market makers. They continuously quote bids and offers, earning the spread as compensation for providing liquidity. When market volatility rises, market makers take on greater risk and tend to widen their quotes, leading to larger bid-ask spreads.

Authorized Participants operate at the creation/redemption level. Through arbitrage, they help bring the ETF’s market price back to its NAV, preventing persistent, large premiums or discounts.

影響買賣差價的主要因素

ETF liquidity and bid-ask spreads are influenced by multiple factors. Understanding these can help you select more suitable ETFs and better time your trades.

基礎資產的流動性

An ETF’s holdings fundamentally determine its liquidity. ETFs that track large-cap U.S. blue-chip indices (such as S&P 500 trackers) typically have very tight spreads because the underlying stocks are highly liquid.

By contrast, ETFs tracking emerging-market equities or high-yield bonds often face lower underlying liquidity, plus additional cross-market costs and time-zone frictions, resulting in generally wider spreads. According to Invesco, assessing the liquidity of an ETF’s underlying holdings provides a more accurate guide to its likely spread behavior. (Source: Invesco Hong Kong ETF Investor Guide)

市場波動性

Volatility and bid-ask spreads are positively correlated. When markets are turbulent—such as around major economic data releases or geopolitical events—uncertainty rises. To compensate for added risk, market makers widen their quotes. This is one reason even large ETFs can exhibit meaningfully wider spreads during sharp market sell-offs.

交易時段

Spreads vary throughout the trading day. Invesco generally advises avoiding trades in the first 30 minutes after the open and the last 30 minutes before the close, when spreads are typically wider and price swings are larger.

U.S. trading hours are 9:30 a.m.–4:00 p.m. U.S. Eastern Time, corresponding to 9:30 p.m.–4:00 a.m. Hong Kong time during Daylight Saving Time and 10:30 p.m.–5:00 a.m. during Standard Time.

ETF 規模

An ETF’s AUM is also related to spreads. Larger funds attract more investors and market makers, and competition tends to narrow spreads. Smaller funds with fewer participants generally have weaker liquidity and wider spreads.

溢價與折價：另一個隱性成本

In addition to the bid-ask spread, ETF premiums (Premium) and discounts (Discount) are also important factors affecting trading costs.

什麼是溢價與折價？

An ETF’s market trading price may deviate from the NAV of its underlying assets:

• Premium: The ETF trades above NAV—i.e., you buy at a higher price
• Discount: The ETF trades below NAV—i.e., you buy at a cheaper price, but you may face losses when selling

These deviations arise from timing gaps between market supply/demand and the ETF share creation/redemption mechanism. Brief deviations are normal, but large and persistent ones warrant special attention.

如何監察溢折價

Before placing a trade, check the ETF issuer’s official website to confirm the current premium/discount level. In general, large, highly liquid U.S. ETFs typically exhibit minimal premiums/discounts with limited impact. You can track real-time quotes and related data using the Longbridge Market Quotes tool.

提示： If you plan to trade in large size, check the ETF’s real-time premium/discount before executing to avoid entering when the premium is elevated.

降低買賣差價成本的實用策略

With the drivers in mind, the following strategies can help you reduce the trading costs arising from spreads.

善用限價盤（Limit Order）

The advantage of market orders (Market Order) is immediate execution, but in low-liquidity or highly volatile conditions, you may be filled at a much worse price than expected. By using limit orders (Limit Order), you can set the maximum buy price or minimum sell price you’re willing to accept, effectively controlling costs.

Longbridge Securities offers U.S. ETF trading, allowing users to flexibly choose order types to fit different strategies. For more analysis of order types, see the comparison guide to limit orders vs. market orders.

選擇合適的交易時段

As noted above, avoiding the first 30 minutes after the open and the last 30 minutes before the close can help you steer clear of periods with wider spreads. The U.S. market’s midday session (approximately 11:00 a.m.–1:00 p.m. ET) generally offers better liquidity and tighter spreads.

優先選擇高流動性 ETF

Large U.S. ETFs that track broad market indices (such as the S&P 500 or Nasdaq-100) typically have substantial average daily volume and very tight spreads. If your objectives can be met via broad-market ETFs, prioritizing these highly liquid products can significantly reduce implicit trading costs.

以基點計算真實成本

Don’t look only at absolute dollar spreads—translate them into basis points to assess proportional costs. When comparing ETFs, using basis points as a common yardstick provides a more accurate view of true trading costs.

香港投資者的實際考量

For individual investors in Hong Kong, keep the following in mind when trading U.S. ETFs.

交易時間的差異

Due to time-zone differences, Hong Kong investors typically trade U.S. markets in the evening. During the initial period after the U.S. open (i.e., 9:30–10:00 p.m. Hong Kong time), spreads may be wider, so consider entering after conditions stabilize. U.S. trading holidays also differ from Hong Kong’s; refer to the U.S. stock market holiday schedule and trading calendar for details.

整體交易成本的全面評估

When assessing the true trading cost of U.S. ETFs, consider the following together:

For long-term investors, the cumulative impact of the expense ratio may be more significant than a single spread; for more frequent traders, the cumulative effect of spreads should not be overlooked.

常見問題解答

買賣差價與管理費哪個更重要？

Both matter, but on different timelines. Expense ratios accrue annually and have a greater impact on long-term holdings; spreads are incurred on each trade and thus matter more for frequent traders. Long-term, low-turnover investors should focus more on expense ratios, while more active traders should pay close attention to both.

為何部分 ETF 的成交量很低，但差價仍然很窄？

Because ETF liquidity comes not only from secondary-market trading volume but also from the liquidity of the underlying assets. If an ETF holds highly liquid large-cap U.S. equities, Authorized Participants can create new shares at any time to balance supply and demand, keeping spreads tight even when secondary-market volume isn’t high.

在市場大跌時買賣 ETF，差價會顯著擴闊嗎？

During periods of sharp market moves, market makers face higher risk exposure and typically widen quotes to compensate, which increases spreads. In rapid sell-offs or liquidity stress, even large ETFs can experience temporarily wider spreads. In highly volatile markets, consider using limit orders instead of market orders to control execution costs.

如何判斷一隻 ETF 的買賣差價是否合理？

As a rule of thumb, ETFs tracking major U.S. indices tend to have the tightest spreads; those tracking specific sectors or small caps are somewhat wider; and those tracking emerging markets or alternative assets often have the widest spreads due to lower underlying liquidity. Use real-time data tools to observe spreads across time and product types, and compare within peer groups to judge reasonableness.

總結

Analyzing U.S. ETF liquidity is an important part of investment decision-making, and the bid-ask spread is the key indicator with the most direct impact on trading costs. Understanding the two-tier liquidity structure, the core factors that influence spreads, and adopting appropriate trading strategies can help you avoid unnecessary hidden costs when investing in U.S. ETFs.

Remember these core principles: evaluate spreads in basis points rather than absolute dollars; choose appropriate trading windows; use limit orders to control execution prices; and consider expense ratios, spreads, and premiums/discounts together.

Which investment instrument to choose ultimately depends on your objectives, risk tolerance, market views, and experience. Whatever ETF you select, make sure you fully understand its mechanics, risk characteristics, and trading rules, and establish a robust risk management plan. You can learn more through Longbridge Academy or Download the Longbridge App to deepen your investing knowledge and make more informed decisions.