According to Morningstar data, the average bid-ask spread among 41 ETFs with over $10 billion in assets over a 30-trading-day period was 0.02%. However, the spread drops off in smaller funds, with an average spread of 1.72% for the 680 ETFs with less than $50 million in assets.

Looking at the underlying assets, ETFs that track very liquid markets, like U.S. stocks, show tighter spreads than those of less-liquid securities, like the bond market, and international exchanges that have little overlap with normal U.S. hours. For instance, 339 international equity ETFs showed a median bid-asks spread of 0.39%, compared to 256 U.S. equity ETFs with a median 0.19% spread. [The Underlying Value of an ETF’s Portfolio]

“A fair comparison of the costs of ETP ownership should include an analysis of trading costs such as the bid-ask spread,” Rawson added. “Both the liquidity of the ETP and that of its underlying holdings influence trading costs.”

For more information on ETFs, visit our ETF 101 category.

Max Chen contributed to this article.