Bid-Ask Spread Explained

The bid-ask spread is the gap between the highest price a buyer is willing to pay and the lowest price a seller is willing to accept for the same security. This gap exists because market makers profit by buying at the bid and selling at the ask—and the wider the spread, the higher your hidden transaction cost.

Why the bid-ask spread exists

Every price you see on a stock exchange represents the most recent trade—not a fixed price at which you can instantly trade. When you want to buy or sell, you interact with the market maker or dealers who are standing ready to take the other side.

A market maker buys shares from sellers at the bid price and immediately turns around to sell them to buyers at the higher ask price. That gap—the spread—is their compensation for holding inventory risk. If the stock price suddenly falls after they buy, they lose money. If it rises after they sell, they also lose money. The spread is their cushion against that risk.

In thinly traded securities, spreads widen because the market maker is taking on more uncertainty. In highly liquid stocks like Apple or Microsoft, spreads narrow because the market maker can quickly lay off the inventory to another buyer or seller at nearly the same price.

How market makers profit and hedge

Market makers do not rely on the spread alone. They actively manage inventory using algorithmic price discovery and hedging. If a market maker accumulates too much of a stock (because more buyers than sellers arrived), they will widen their ask price to discourage new buyers and attract sellers. If they run short of inventory, they widen their bid to attract sellers and discourage buyers.

Professional market makers also trade on information. They observe order flow—which side is hitting them more often—and adjust their prices accordingly. If they see more sell orders approaching, they may lower both the bid and ask slightly to avoid getting stuck holding falling stock. This is why slippage can occur even on a quiet day if a large order floods the market.

Market makers also hedge using derivatives, short selling, or offsetting trades in related securities. A market maker in one stock might lay off risk by trading an ETF that contains that stock, or by adjusting their book across many symbols.

Measuring the spread’s cost to you

The bid-ask spread is a real, but often invisible, cost. Suppose you want to buy 100 shares of a stock priced at $50.00 bid / $50.05 ask. You submit a market order to buy. You fill at the ask: $50.05 × 100 = $5,005 total. A moment later, if you sell those same 100 shares, you fill at the bid: $50.00 × 100 = $5,000. You have lost $5—the full spread—without any change in the stock’s intrinsic value.

Over a year of active trading, spread costs accumulate. A trader making 50 round trips (buy and sell) in a stock with a $0.10 spread pays 50 × $0.10 × shares = effectively $5 per 100 shares traded, or 10 basis points (0.1%) per round trip. That compounds quickly.

Large institutional traders sometimes negotiate better prices. They may place block trades directly with market makers, who offer a lower spread if the trade is large enough to offset the risk. Retail investors pay the posted spread—the market spread visible to all.

Spreads widen during stress

During market turbulence—earnings announcements, central bank policy decisions, or flash crashes—spreads blow out. Market makers reduce the size they are willing to buy or sell at a given price because the risk of adverse price movement rises. In extreme cases, the spread can widen to dollars, not cents, and liquidity evaporates entirely.

A stock that normally trades with a $0.01 spread might widen to $1.00 or more during a market crisis. This is why large trades executed during volatile periods often experience significant slippage. The market maker is not trying to punish you; they are protecting themselves against the increased chance of a gap move.

How to minimize your spread cost

Trade the most liquid securities. Stocks in the S&P 500 index, especially mega-cap names, typically have single-cent spreads. Micro-cap stocks, bonds, or options often have much wider spreads.

Use limit orders. A limit order lets you specify the maximum price you will pay to buy (or the minimum to sell). You may not fill immediately, but you avoid the worst-case slippage and sometimes benefit from price improvement.

Avoid trading at market open or just before close. Volatility rises around these windows, spreads widen, and you are more likely to get a bad fill. Mid-session trading usually offers tighter spreads.

Trade round-lot sizes. Market makers often quote tighter spreads for standard order sizes (usually 100-share blocks for stocks). Odd-lot orders sometimes trigger wider spreads or manual handling fees.

Know your venue. Some brokers route to market makers or alternative trading systems that offer better execution than others. Checking execution quality over time—comparing your fill prices to the best bid-ask available when you submitted the order—reveals whether your broker is adding or improving on the spread.

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