What is the Bid-Ask Spread?

By EC Assets Research Team · Published 19 August 2026 · Updated 19 August 2026

Bid-Ask Spread: The gap between the best price a buyer will pay and the best price a seller will accept. Crossing it is the price of immediate execution: half the spread on entry, half on exit, paid on every round trip and multiplied by portfolio turnover.

What the Spread Actually Measures

The bid is the highest price a buyer is currently willing to pay; the ask is the lowest price a seller will accept. The difference between them is the bid-ask spread, and it is the price of immediacy — what a participant pays for the right to transact now rather than wait for a counterparty on their own terms.

It is the most reliably underestimated cost in portfolio management. Management fees are quoted, negotiated and reported; the spread is paid silently on every transaction, scales with turnover, and appears in no fee schedule.

How It Works

The quoted spread and its relative form:

$$\text{spread} = P_{\text{ask}} - P_{\text{bid}} \qquad \text{relative spread} = \frac{P_{\text{ask}} - P_{\text{bid}}}{P_{\text{mid}}}$$

A buyer paying the ask acquires an asset immediately worth the mid, so the immediate cost is half the spread. Selling later at the bid costs the other half. The round trip therefore costs approximately the full spread, before any market impact.

The effective spread measures what was actually paid rather than what was displayed:

$$\text{effective spread} = 2 \times |P_{\text{execution}} - P_{\text{mid at arrival}}|$$

Three forces set the width. Order processing covers the mechanics of making a market. Inventory risk compensates the market maker for holding a position they did not choose. Adverse selection compensates them for the possibility that whoever is trading knows something they do not — the component that widens fastest when information is moving.

Indicative round-trip costs

Orders of magnitude for normal conditions, not quotes. Actual spreads vary by name, size, venue and time of day, and widen sharply in stress.

Worked Example

A stock is quoted 99.95 bid, 100.05 ask. The mid is 100.00 and the spread is 0.10, or 10 basis points.

A manager buying 10 million of that stock pays 5 basis points — 5,000 — to transact immediately. Selling the position later costs another 5,000. The round trip is 10,000, or 10 basis points, before market impact and commission.

Now apply turnover. A strategy running 100 percent annual turnover replaces its portfolio once a year, so it pays approximately one full spread on the whole book:

Spread cost only. Market impact, commission and financing sit on top, and impact grows with order size relative to available liquidity.

A strategy with a 40 basis point expected edge and 300 percent turnover in 10 basis point instruments spends three-quarters of its edge on spreads alone.

When It Applies (and Limitations)

The quoted spread is not the achieved cost. Displayed quotes apply to displayed size. A large order consumes the top of the book and executes progressively worse, so effective spread routinely exceeds quoted spread for institutional sizes. Measuring one and budgeting the other understates cost.

Spreads widen when they matter. Liquidity is a fair-weather quantity. The adverse selection component expands during information events and market stress, which is when portfolios most often need to trade. Cost assumptions calibrated on calm markets are wrong in the situations that determine outcomes.

The spread is not the whole cost. Market impact — the price movement caused by the order itself — usually exceeds the spread for institutional sizes. Delay cost and opportunity cost from unfilled orders complete the picture. Implementation shortfall measures all of it against the price when the decision was made.

Different venues, different spreads. Crossing networks and dark pools execute at or inside the mid, avoiding part of the spread in exchange for uncertainty about whether the order fills at all.

Quote-driven markets differ from order-driven ones. In corporate credit and much of the OTC world there is no continuous public book, and the spread is what a dealer quotes in response to an inquiry — a function of relationship, size and inventory, not a posted price.

Why It Matters for Institutional Investors

Turnover is a cost decision. Every rebalancing rule, signal-refresh frequency and stop-loss policy has a spread bill attached. A portfolio construction choice that raises turnover must clear the spread it adds before it improves anything.

Capacity is set by liquidity. A strategy that works in size in large-cap equity may be uninvestable in small caps at the same turnover, because the spread scales while the edge does not. This is a common reason live results fall short of backtests, which frequently assume execution at the mid.

Index rules carry the cost too. Equal-weighted and risk-based index schemes require systematic rebalancing trades, and those trades pay the spread. Part of the paper advantage of such schemes over cap weighting is consumed at execution.

Liquidity is priced. Amihud and Mendelson showed that expected returns rise with the spread: less liquid assets must offer more return to compensate holders for the cost of exit. Part of what looks like a return premium in less liquid segments is payment for that illiquidity rather than a free reward.

It is a monitorable metric. Rising average spreads in a portfolio's holdings signal deteriorating liquidity before it shows up anywhere else. For books with redemption obligations, that is an early-warning indicator worth reporting.

References

1. Demsetz, H. (1968). The Cost of Transacting. Quarterly Journal of Economics, 82(1), 33–53.
2. Glosten, L. R., & Milgrom, P. R. (1985). Bid, Ask and Transaction Prices in a Specialist Market. Journal of Financial Economics, 14(1), 71–100.
3. Amihud, Y., & Mendelson, H. (1986). Asset Pricing and the Bid-Ask Spread. Journal of Financial Economics, 17(2), 223–249.
4. Perold, A. F. (1988). The Implementation Shortfall: Paper versus Reality. Journal of Portfolio Management, 14(3), 4–9.