Bid-Ask Spread

The bid-ask spread is the fundamental unit of transaction cost in U.S. equity markets — the price of immediacy. When you view a stock quote and see a bid price of $99.98 and an ask price of $100.00, the $0.02 spread represents what it costs to immediately enter or exit a position: a purchase at the ask of $100.00 and an immediate disposition at the bid of $99.98 would result in a $0.02 per share loss before commissions. For large institutional trades, even tiny per-share spreads can translate to significant aggregate costs.

Before decimalization of U.S. stock prices in 2001, spreads were quoted in fractions — as large as 1/8 of a dollar ($0.125) per share. The SEC mandated the conversion to decimal pricing (Regulation NMS), which dramatically compressed spreads for most U.S.-listed securities, particularly large-cap stocks. Today, heavily traded S&P 500 components like Apple, Microsoft, and Amazon regularly have bid-ask spreads of just one cent ($0.01 per share), reflecting the enormous depth of their order books and the intense competition among market makers and high-frequency trading firms.

Spreads widen significantly during periods of uncertainty or reduced liquidity. At market open (9:30 a.m. Eastern Time), spreads are often temporarily wider as price discovery unfolds. During earnings announcements, when a company's value can shift dramatically depending on reported results, market makers widen their quotes to reflect heightened risk. During the March 2020 COVID-19 volatility, bid-ask spreads across U.S. equities widened materially — particularly for smaller stocks and ETFs with underlying assets whose values were uncertain — illustrating the direct relationship between market uncertainty and transaction costs.

For exchange-traded products like the SPDR S&P 500 ETF (SPY), which is among the most liquid securities in the world, the bid-ask spread is typically just one cent even during normal trading hours. For thinly traded penny stocks on the OTC market, spreads of 5%, 10%, or even wider are not uncommon, meaning investors face substantial embedded costs on every transaction. FINRA and the SEC monitor market maker conduct to ensure spreads reflect genuine market conditions rather than artificial price manipulation.

For educational purposes, the bid-ask spread should be considered alongside commission costs when assessing the total cost of trading. In the era of zero-commission retail brokerage (pioneered by Robinhood and subsequently adopted by major brokers including Charles Schwab and Fidelity), the bid-ask spread has become the primary remaining explicit trading cost for retail investors. The practice of 'payment for order flow' (PFOF) — in which brokers route retail orders to market makers in exchange for compensation — has been a subject of ongoing SEC review, as critics argue it can result in retail investors receiving prices slightly inferior to what competitive routing might achieve.