ETF liquidity explained: bid-ask spreads, premium, and discount to NAV

ETF liquidity is not the same as the trading volume shown on an exchange screen. Because new ETF shares can be created or redeemed by authorised participants at any time, the true liquidity of an ETF depends on the liquidity of its underlying assets—not on how many shares change hands each day. This distinction matters when selecting ETFs and when assessing whether large trades can be executed efficiently.

What ETF liquidity is

Liquidity in the context of an ETF has two layers. The first is secondary market liquidity: how easily an investor can buy or sell ETF shares on the exchange at a price close to the current market price. This is reflected in the bid-ask spread—the difference between the highest price a buyer is willing to pay and the lowest price a seller is willing to accept. A narrow spread indicates abundant secondary market liquidity; a wide spread indicates the opposite.

The second layer is primary market liquidity: the capacity of authorised participants to create or redeem ETF shares by trading in the underlying assets. Because APs can create new shares whenever there is excess demand, the supply of ETF shares is effectively unlimited as long as the underlying assets can be bought. This means that an ETF can have very low secondary market volume—few shares trading hands each day—and still be highly liquid, provided its underlying assets are liquid. The creation and redemption mechanism ensures that supply expands to meet demand.

How bid-ask spreads work

The bid-ask spread on an ETF is set by market makers, who stand ready to buy and sell ETF shares throughout the trading day. Market makers manage their inventory risk by hedging in the underlying market. When the underlying assets are liquid and easy to trade, hedging is cheap, and spreads are narrow. When the underlying is illiquid—as with some high-yield bond ETFs or small-cap emerging market equity ETFs—hedging is expensive, and spreads widen to compensate the market maker for the risk they are taking.

For large-cap equity ETFs tracking liquid indexes, bid-ask spreads are typically one or two basis points. For ETFs tracking illiquid fixed income markets, spreads of 10–50 basis points are common. This matters: a round-trip trade (buy then sell) in a fund with a 0.20% spread costs the investor 0.20% in spread alone, before any expense ratio.

Premium and discount to NAV

The premium or discount is the percentage difference between the ETF's market price and its NAV. A premium means the ETF trades above NAV; a discount means it trades below. For most ETFs tracking liquid markets, premiums and discounts are very small—typically less than 0.10%—because authorised participants arbitrage them away through the creation and redemption mechanism.

Premiums and discounts can widen significantly during periods of market stress, when the ETF trades on an open exchange but the underlying market is temporarily closed or illiquid. Examples include international equity ETFs trading during off-hours for their underlying markets, bond ETFs during credit market dislocations, and commodity ETFs when the futures market is experiencing unusual term structure dynamics. In these cases, the ETF's market price may reflect more current information than the published NAV.

What the evidence shows

Research on ETF trading costs has found that the visible on-screen volume is a poor predictor of actual execution quality for larger trades. A study by the CFA Institute covering 2015–2019 found that ETFs with very low average daily volumes but highly liquid underlying assets—such as some fixed income ETFs tracking government bond markets—could absorb large trades efficiently because APs were readily able to create new shares. In contrast, some ETFs with high daily volumes but illiquid underlying assets—certain high-yield or bank loan ETFs—showed significant price impact on large block trades.

The implication for investors is that ETF selection on the basis of volume alone is misleading. A more relevant question is whether the underlying assets are liquid enough to support the size of trade being considered, and what the current bid-ask spread indicates about market-making activity in that fund.

Limitations and trade-offs

For retail investors making small, regular purchases of broad-market ETFs, liquidity is rarely a binding constraint. The bid-ask spread on a large equity ETF is negligible for positions of a few thousand dollars. Liquidity considerations become more important for larger positions, less liquid underlying markets, and for investors who may need to exit quickly in stressed conditions.

The timing of trades also matters. ETF bid-ask spreads are typically widest at market open and narrowest during peak trading hours when underlying market liquidity is deepest. For ETFs tracking overseas markets, the best execution window is when both the ETF's exchange and the underlying market are simultaneously open. Trades placed outside those windows may be executed at wider spreads.

ETF liquidity in pfolio

pfolio constructs portfolios using ETFs selected from a curated investable universe that accounts for liquidity alongside cost and tracking quality. Monthly rebalancing—rather than more frequent trading—limits the impact of bid-ask spreads on portfolio performance over time. ETFs in the pfolio universe are listed in the Assets section. Portfolio construction methodology, including how assets are selected and weighted, is described in how we build portfolios.

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