# What Is the Bid-Ask Spread?

Learn how the bid-ask spread reflects executable prices, liquidity, and transaction cost, with absolute and percentage calculations and order-depth risks.

Canonical: https://wiki.fcontext.com/stocks/bid-ask-spread/
Fact checked: 2026-07-13

> For educational purposes only; not investment advice.

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## Direct answer

The **bid** is the highest displayed price at which a buyer is currently willing to buy; the **ask** is the lowest displayed price at which a seller is willing to sell. The **bid-ask spread** is the difference between them:

`Spread = best ask - best bid`

The spread is both a market quote and a potential transaction cost. A trader who buys at the ask and immediately sells at the unchanged bid gives up the spread before fees. It is not a fixed charge: quotes and displayed quantities can change continuously.

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## Why a spread exists

Buyers generally prefer lower prices and sellers prefer higher prices. Their unexecuted limit orders form price levels in the order book. The best bid and best ask describe only the nearest opposing prices at that moment, not a guaranteed price for every share.

Competition among orders can narrow the spread. Uncertainty, volatile news, low participation, inventory risk for liquidity providers, and thin order-book depth can widen it. Spreads therefore often differ across securities and across trading sessions for the same security.

The midpoint, `(bid + ask) / 2`, is useful for comparison but is not necessarily executable. The last-sale price is also different: it records an earlier transaction and can lie at the bid, ask, between them, or outside a later quote.

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## Cost example and calculation

Suppose the quote is `$99.95 bid / $100.05 ask`.

- Absolute spread: `$100.05 - $99.95 = $0.10`
- Midpoint: `($99.95 + $100.05) / 2 = $100.00`
- Percentage spread: `$0.10 / $100.00 = 0.10%`

Buying 100 shares at $100.05 and immediately selling them at $99.95 would produce a $10 difference before fees, assuming both displayed quantities remain available. Measured against the midpoint, each marketable side pays roughly half the quoted spread in this simplified snapshot.

Size matters. If only 25 shares are offered at $100.05, a 100-share market buy may consume higher ask levels. Its average execution cost can then exceed the displayed spread. Conversely, a limit order may obtain a better boundary but remain partially or entirely unfilled.

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## Practical risks

- **Displayed size is limited:** the best quote may cover fewer shares than the order.
- **Quotes can disappear:** orders may be changed or canceled before another order reaches the venue.
- **Spreads can widen suddenly:** openings, extended hours, earnings releases, and market stress can reduce liquidity.
- **A narrow spread can mislead:** one small order at each best price says little about deeper levels.
- **A wide spread raises the break-even hurdle:** a price move must first overcome execution cost.
- **Zero commission is not zero cost:** spread, slippage, fees, and market impact remain relevant.

Compare spreads at similar times and express them as a percentage when comparing securities with very different prices. A $0.10 spread is small for a $1,000 stock but large for a $1 stock.

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## Common misconceptions

**"The quoted price is the price I will receive."** A quote includes a price and quantity; execution depends on what remains when the order arrives.

**"The spread is always a loss."** It is a potential execution cost. A resting limit order can receive a different result, but it also bears non-execution and adverse-selection risk.

**"High daily volume guarantees a tight spread."** Daily volume is an aggregate. Current depth, volatility, session, and order size still matter.

**"The midpoint is a fair and tradable price."** It is a calculation, not an order. No counterparty must trade there.

**"A one-cent spread means unlimited liquidity."** It reveals only the gap between the best displayed prices, not the quantity available beyond them.

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## Related topics

- [Market Orders vs. Limit Orders](/stocks/market-and-limit-orders/)
- [Trading Volume and Liquidity](/stocks/volume-and-liquidity/)
- [Pre-Market and After-Hours Trading](/stocks/pre-market-after-hours/)

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## Authoritative sources

- [Types of Orders](https://www.investor.gov/introduction-investing/investing-basics/how-stock-markets-work/types-orders) - SEC Investor.gov (accessed 2026-07-13)
- [Understanding Order Types](https://www.finra.org/investors/investing/investment-products/stocks/order-types) - FINRA (accessed 2026-07-13)