ETFs Made Easy: Understanding Spreads

Buying and selling ETFs wisely means understanding the difference between being a buyer and a seller.

Reviewed by: Dave Nadig
Edited by: Dave Nadig

If you’re coming to the ETF world from mutual funds, it can take a little time to get used to the vagaries of those first two letters—“ET,” as in “exchange-traded.”

Back in December I covered the very basics of ETF trading, but I’ll repeat the fundamentals here. Because ETFs trade on an exchange just like a stock, the price you pay to buy an ETF is generally going to be a bit less than the price you’ll fetch when you sell it. The difference between the two is called “the spread.”

Imagine you’re trading something like the PowerShares KBW Property & Casualty Insurance ETF (KBWP | C-58), a small, 24-stock ETF. As I’m writing this, it’s quoted in the market as $43.55 bid, $43.69 ask. That means you’d pay $43.69 if you wanted to buy it, but you’d only get $43.55 if you wanted to sell it. The 11-cent difference between those two prices is the spread.

In general, there are a few additional things you need to consider to know whether that spread is reasonable. First, it’s important to recognize that the spread is constantly changing. Using an intraday forensics tool like the (free!) Trillium Surveyor, we can look carefully at exactly how that spread changes and where trades actually get executed. Here’s yesterday spread action in KBWP:

Source: Trillium Surveyor

Shifting Spreads

The gray band here is the spread, and the crosses are actual trades.

You can see that the 11-cent spread being quoted right now isn’t atypical of the fund, and if you wanted to trade around lunchtime, you were actually looking at a spread of more than 20 cents. You can also see that quite a few trades happen not at the advertised bid or ask, but somewhere in the middle.

That’s not unusual, as people put in orders “inside the spread” that are immediately executed, or negotiated trades are reported to the tape.

A few patterns repeat regularly—some ETFs take a while to settle in—especially if they own a lot of stocks which themselves take time to get going in the morning. Here’s the first few minutes of bids and asks for the iShares MSCI USA Quality Factor ETF (QUAL | A-83) on April 22.

Source: Trillium Surveyor

Beware Trading At The Open

This is an extremely common pattern—the initial bids and asks here are almost 30 cents wide. That said, five minutes into the day, those spreads collapsed to about 6 cents wide, where they remain all day long. This is one of the reasons we always say, “don’t trade the open.” ETFs need their morning coffee just like everyone else.

The second question you need to ask is whether those spreads are “fair.”

The way you’d normally check that is by looking at the intraday net asset value (NAV)—a near-real-time calculation disseminated by the exchange on which the ETF is listed—to see how it maps to where the ETF is actually trading. For example, here’s how the actual trades of KBWP matched up against iNAV yesterday:

Source: FactSet Research Systems

Finding Fair Value

You can see that while KBWP (the blue line) only traded a handful of times, the underlying stocks that drive the intraday NAV (the yellow line) changed values all day. For a thinly traded ETF like KBWP, this looks “pretty good.”

To know with absolute certainty, you’d have to calculate a “bid fair value” and an “ask fair value” for every one of the underlying holdings, then compare that with the bid and ask of the ETF. In fact, that’s exactly what market makers do while they’re trying to figure out when to leap in and arbitrage out any price discrepancies.

But as an ETF investor, you can mostly ignore that level of detail and simply look at spreads and INAV to see whether they’re all in the ballpark.

One final note: It can be helpful to glance at how an ETF normally trades. When you go to the Screener at, or an individual fund page, you’ll see an “average spread” calculation. KBWP’s, for instance, is 18 cents wide, or 42 basis points. Calculating that is trickier than it looks.

In many places, you’ll find just take the last spread of the day—at the close—and average that out over some number of days. The problem is that closing spreads are just plain dumb. Most investors should never be trading at the close at 4 p.m. Eastern Time on the nose and on the open market.

A Better Idea

What we do here at FactSet is a little more complex.

We look at every millisecond of each day and calculate a time-weighted average spread. So if the spread is 20 cents wide all day, but blasts open to 25 cents right at the close, that 25 cents won’t matter much in the calculation. We also discard outliers outside a standard-deviation band each day to make sure we’re really representing the real-world spread an investor is experiencing.

We then take the median of 45 trading days of those time-weighted average spreads.

That may sound complicated, and in practice, it’s a lot of math. But luckily all those computations can give you a very good understanding of what “normal” is for an ETF. And in this case, “normal” for KBWP is 18 cents wide.

Dave Nadig is VP and director of exchange-traded funds at Factset Research Systems. At the time this article was written, the author held no positions in the securities mentioned. You can reach Dave Nadig at [email protected], or on Twitter @DaveNadig.

Prior to becoming chief investment officer and director of research at ETF Trends, Dave Nadig was managing director of Previously, he was director of ETFs at FactSet Research Systems. Before that, as managing director at BGI, Nadig helped design some of the first ETFs. As co-founder of Cerulli Associates, he conducted some of the earliest research on fee-only financial advisors and the rise of indexing.