ETF Bid-Ask Spread: How to Calculate the Cost

An ETF bid-ask spread is the gap between the highest quoted bid and the lowest quoted ask. If the best bid is $49.95 and the best ask is $50.05, the spread is 10 cents per share, or 0.20% of the $50 midpoint. Buying 100 shares at the ask and immediately selling them at the bid would cost $10 in this example, assuming both fills happen at those prices and no commissions apply.

How to calculate an ETF bid-ask spread

Use a bid and ask from the same quote. The bid is the price on the buying side of the quote; the ask, also called the offer, is the price on the selling side. To express the gap as a percentage, divide by the midpoint of those two prices. Dividing by the last traded price can give you a different result.

Measure	Calculation	Example
Spread per share	Ask minus bid	$50.05 − $49.95 = $0.10
Midpoint	(Bid plus ask) ÷ 2	($49.95 + $50.05) ÷ 2 = $50.00
Spread percentage	Spread ÷ midpoint × 100	0.20%
Spread in basis points	Spread ÷ midpoint × 10,000	20 basis points
Full spread on 100 shares	Spread × shares	$10.00

The percentage helps when share prices differ. A 10-cent spread on an ETF with a $200 midpoint is 0.05%, one-quarter of the spread percentage in the $50 example. The spread on 100 shares is $10 in both cases, but 100 shares represents four times as much money at the higher midpoint.

Does the full spread apply to one trade?

At the example ask, a 100-share purchase costs $5,005. At the $50 midpoint, those same shares would cost $5,000. The difference is $5, or half the quoted spread. A sale at the $49.95 bid would bring in $4,995, also $5 below the midpoint. The $10 round-trip figure adds both differences while holding the quote fixed.

The midpoint is a reference for the calculation. You might receive a better price than the displayed bid or ask, or the quote might move before the trade reaches the market. A quote also covers a particular number of shares, so a larger order can fill at several prices. A last trade of $50 tells you where a transaction happened; it does not establish a price available now. Investor.gov's guide to trade execution explains how quote size and order routing affect the price you receive.

What does the 30-day median bid-ask spread mean?

The 30-day median bid-ask spread describes the middle of a set of recent percentage spreads. Under SEC Rule 6c-11, ETFs relying on the rule use national best bids and offers sampled at the end of each 10-second interval during trading days in the past 30 calendar days. They calculate the spread as a percentage of the midpoint for each sample, then publish the median rounded to the nearest 0.01 percentage point.

Look under trading information on the fund sponsor's website. For example, Vanguard's VT page labels the figure “30-day median bid/ask spread.” The SEC staff's disclosure guide explains where ETFs may report the figure under different disclosure requirements.

If a fund reports a median of 0.04% but you calculate 0.20% from a current quote, the current gap is five times the reported figure. Both can be right. The median summarizes the window; the quote reflects one moment. The median cannot tell you how wide the largest gaps were, how many shares were available, or what any specific trade cost.

Spread, expense ratio, and NAV answer different questions

Number	What it measures	Time basis
Bid-ask spread	Gap between the two sides of the quote	One observation, or a summary across observations
Expense ratio	Annual operating expenses relative to fund assets	Annual rate
Premium or discount to NAV	Difference between a stated market price and the fund's net asset value per share	Matched valuation point

A 0.20% spread and a 0.10% annual expense ratio do not add up to a 0.30% annual cost. The spread describes a quote around a possible transaction; the fund pays operating expenses out of its assets over time. The SEC's ETF investor bulletin explains both costs.

An ETF can also have a narrow spread and trade below NAV. If the bid is $49.94, the ask is $49.96, and NAV at the same valuation point is $50.10, the quote is two cents wide. Using the $49.95 midpoint as the market-price reference puts the fund 15 cents below NAV, about a 0.30% discount. The ETF NAV vs market price guide explains how to read the price reference in a fund report.

What a closing-price comparison can tell you

Use Compare Stocks to put two available ETFs on the same historical window. The chart shows how their closing prices moved from a common starting point. If the paths look similar, that tells you about price movement over those dates. To compare the funds' quoted spreads, look up their disclosures separately and record each figure's date. A daily close provides no bid and ask pair from which to reconstruct a spread or estimate your fill.

Equity Replay is an educational historical simulator. It is not investment advice, a broker, or a record of trades. Its historical replay uses closing prices and does not model execution, spreads, dividends, fees, taxes, cash flows, or rebalancing. All quotes and costs in the examples are hypothetical.