Bid-Ask Spread

The bid-ask spread is the difference between the bid price, the highest price a buyer is currently willing to pay for a security, and the ask price (also called the offer), the lowest price a seller is currently willing to accept. At any moment a security has both, and the small gap between them is the spread. It exists because buyers and sellers do not meet at a single price; there is a price to buy at and a slightly higher price to sell at, and the difference is what a round trip costs before any commission.

The spread is an implicit trading cost. Because you typically buy at the ask and sell at the bid, a security whose price has not moved at all will still show a small loss immediately after you buy it, equal to the spread, since you would have to sell back at the lower bid. The order types that interact with the spread, the market order and the limit order, are covered on their own pages; this page is about what the spread is, what makes it wide or narrow, and who earns it.

The spread is a cost you pay by crossing it. Suppose a stock is quoted at a bid of $19.98 and an ask of $20.00. A buyer using a market order pays $20.00; a seller using a market order at the same moment receives $19.98. The 2-cent gap is the spread, and a round trip, buying then immediately selling, would lose that 2 cents per share to nothing but the spread. On a heavily traded stock that gap is trivial, but it is always present and always paid by whoever crosses it to trade immediately.

Liquidity is what sets the width. A security that trades in large volume with many buyers and sellers has a narrow spread, often a penny or less, because competition among traders keeps the bid and ask close together. A thinly traded security, a small stock, an obscure bond, an option far from the money, has a wide spread, because there are fewer participants and more risk in quoting a price. This is a concrete way that illiquidity costs money: the same order costs more to fill in a market with few participants, and the extra cost is invisible because it is embedded in the price rather than charged as a fee.

The spread is the market maker's pay. In many markets a market maker or dealer stands ready to buy at the bid and sell at the ask continuously, providing the liquidity that lets others trade whenever they want. The spread is that firm's compensation for the service and for the risk of holding inventory that may move against it before it can offload. This is why the spread widens when markets are volatile or a security is hard to hedge: the risk of quoting a price has gone up, so the price of providing liquidity goes up with it.

A limit order lets you sit inside the spread instead of paying it. A market order accepts the current bid or ask and pays the spread for immediacy. A limit order can be placed between the bid and the ask, offering to buy a little above the bid or sell a little below the ask, which can capture part of the spread rather than pay it, at the cost of the trade possibly not filling. So the spread is not an unavoidable tax on every trade; it is the price of demanding immediate execution, and an investor willing to wait can often reduce it.

You see a security quoted with a bid and an ask. If you place a market order to buy, you pay the ask; if you place a market order to sell, you receive the bid. The difference between them is the spread, which you have effectively paid the moment you trade, because reversing the trade would cost you that gap again.

A hypothetical example of the round-trip cost. An investor buys 1,000 shares of a stock quoted $49.95 bid and $50.05 ask, a spread of $0.10. The purchase fills at the ask, $50.05, for $50,050. If the quote has not moved and the investor sells immediately at the bid, $49.95, they receive $49,950. The $100 difference (1,000 shares times the $0.10 spread) is lost entirely to the spread, with no price movement and before any commission. On a stock with a $0.01 spread the same round trip would cost $10, which is why the spread's width, driven by how actively the security trades, is the number that matters.

Pros

• The spread is the mechanism that lets you trade immediately, because a market maker is standing ready to take the other side at a quoted price.
• Its width is a quick, visible gauge of a security's liquidity: a narrow spread signals an active, competitive market.
• It can be reduced or captured by using a limit order rather than a market order, so an informed trader is not forced to pay it in full.

Cons

• It is a real cost paid on both entry and exit, yet it never appears as a fee, so investors routinely overlook it.
• It is wide on illiquid securities, which makes trading them more expensive exactly when a holder may most need to sell.
• It widens in volatile or stressed markets, raising the cost of trading at the moments an investor is most likely to want to act.