What Is a Bid-Ask Spread, and How Does It Work in Trading?

What Is a Bid-Ask Spread?

A bid-ask spread is the amount by which the ask price exceeds the bid price for an asset in the market. The bid-ask spread is essentially the difference between the highest price that a buyer is willing to pay for an asset and the lowest price that a seller is willing to accept.

An individual looking to sell will receive the bid price while one looking to buy will pay the ask price.

Understanding Bid-Ask Spreads

A security's price is the market's perception of its value at any given point in time and is unique. To understand why there is a "bid" and an "ask," one must factor in the two major players in any market transaction, namely the price taker (trader) and the market maker (counterparty).

Market makers, many of which may be employed by brokerages, offer to sell securities at a given price (the ask price) and will also bid to purchase securities at a given price (the bid price). When an investor initiates a trade, they will accept one of these two prices depending on whether they wish to buy the security (ask price) or sell the security (bid price).

The difference between these two, the spread, is the principal transaction cost of trading (outside commissions), and it is collected by the market maker through the natural flow of processing orders at the bid and ask prices. This is what financial brokerages mean when they state that their revenues are derived from traders "crossing the spread."

The bid-ask spread can be considered a measure of the supply and demand for a particular asset. The bid can be said to represent the demand for an asset, and the ask represents the supply, so when these two prices move apart, the price action reflects a change in supply and demand.

The depth of the "bids" and the "asks" can have a significant impact on the bid-ask spread. The spread may widen significantly if fewer participants place limit orders to buy a security (thus generating fewer bid prices) or if fewer sellers place limit orders to sell. As such, it's critical to keep the bid-ask spread in mind when placing a buy-limit order to ensure it executes successfully.

Market makers and professional traders who recognize imminent risk in the markets may also widen the difference between the best bid and the best ask they are willing to offer at a given moment. If all market makers do this on a given security, then the quoted bid-ask spread will reflect a larger than usual size. Some high-frequency traders and market makers attempt to make money by exploiting changes in the bid-ask spread.

The Bid-Ask Spread's Relation to Liquidity

The size of the bid-ask spread from one asset to another differs mainly because of the difference in liquidity of each asset. The bid-ask spread is the de facto measure of market liquidity. Certain markets are more liquid than others, and that should be reflected in their lower spreads. Essentially, transaction initiators (price takers) demand liquidity while counterparties (market makers) supply liquidity.

For example, currency is considered the most liquid asset in the world, and the bid-ask spread in the currency market is one of the smallest (one-hundredth of a percent); in other words, the spread can be measured in fractions of pennies. On the other hand, less liquid assets, such as small-cap stocks, may have spreads that are equivalent to 1% to 2% of the asset's lowest ask price.

Bid-ask spreads can also reflect the market maker's perceived risk in offering a trade. For example, options or futures contracts may have bid-ask spreads that represent a much larger percentage of their price than a forex or equities trade. The width of the spread might be based not only on liquidity but also on how quickly the prices could change.

Bid-Ask Spread Example

If the bid price for a stock is $19 and the ask price for the same stock is $20, then the bid-ask spread for the stock in question is $1. The bid-ask spread can also be stated in percentage terms; it is customarily calculated as a percentage of the lowest sell price or ask price.

For the stock in the example above, the bid-ask spread in percentage terms would be calculated as $1 divided by $20 (the bid-ask spread divided by the lowest ask price) to yield a bid-ask spread of 5% ($1 / $20 x 100). This spread would close if a potential buyer offered to purchase the stock at a higher price or if a potential seller offered to sell the stock at a lower price.

Elements of the Bid-Ask Spread

Bid-ask spread trades can be done in most kinds of securities, as well as foreign exchange and commodities.

Traders use the bid-ask spread as an indicator of market liquidity. High friction between the supply and demand for that security will create a wider spread.

Most traders prefer to use limit orders instead of market orders; this allows them to choose their own entry points rather than accepting the current market price. There is a cost involved with the bid-ask spread, as two trades are being conducted simultaneously.

The Bottom Line

The bid-ask spread serves as an effective measure of liqudity, as more liquid securities will have small spreads while illiquid ones will have larger ones. Investors should keep an eye on the spread of any security they wish to buy or sell to get a sense for how frequently it trades and to decide on the type of order to use when making a transaction.

Correction—Dec. 4, 2022: This article’s question-and-answer segment was edited from a previous version that incorrectly defined bid-ask spread.