Bid-Ask Spread

What Is the Bid-Ask Spread?

The bid-ask spread is the difference between the bid price (the highest price a buyer is currently willing to pay) and the ask price (the lowest price a seller is currently willing to accept). It is one of the most fundamental concepts in market microstructure and represents an implicit transaction cost that all market participants pay.

For example, if a stock has a bid of $49.98 and an ask of $50.02, the spread is $0.04. A trader buying at the ask ($50.02) and immediately selling at the bid ($49.98) would lose $0.04 per share, or 0.08% of the transaction value. This cost, while seemingly small, compounds over many trades and can significantly impact returns, especially for active traders.

What Determines the Spread

Liquidity is the primary driver. Heavily traded securities like Apple, SPY, or major forex pairs have penny-wide spreads because intense competition among market makers drives the spread to a minimum. Thinly traded small-cap stocks or exotic options may have spreads of several percent.

Volatility widens spreads because market makers face greater uncertainty about fair value and demand higher compensation (wider spreads) for the risk of inventory held during volatile periods. During market panics, even liquid securities see spread widening.

Information asymmetry affects spreads. If market makers believe that informed traders (insiders, hedge funds with superior information) are active in a security, they widen spreads to protect against trading with someone who knows more than they do.

Spread as a Trading Cost

For active traders, the bid-ask spread is often a larger cost than commissions. A day trader executing 20 round trips per day in a stock with a $0.05 spread pays $2 in spread costs per round trip (100 shares), or $40 per day. This is $800 per month that must be overcome before reaching profitability.

Understanding and minimizing spread costs is essential for trading strategies with small profit targets. Scalping and high-frequency strategies are only viable in securities with extremely tight spreads. Swing and position traders are less affected because the spread is a smaller percentage of their larger profit targets.