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Market Microstructure

Bid-Ask Spread

The difference between the highest price a buyer is willing to pay (bid) and the lowest price a seller is willing to accept (ask).

Detailed Financial & Mathematical Context

The bid-ask spread is a direct measure of market liquidity and transaction friction. Highly liquid assets (like BTC/USDT or EUR/USD) feature tight spreads measured in fractions of a basis point, whereas illiquid markets suffer wide spreads that increase market impact cost.

\text{Spread} = P_{ask} - P_{bid}, \quad \text{Spread \%} = \frac{P_{ask} - P_{bid}}{P_{mid}} \times 100

Video Explainer & Key Moments

Understand how bid-ask spreads, order book depth, and market maker incentives dictate liquidity and algorithmic execution slippage.

0:00 - Introduction to Limit Order Books
How bids and asks are matched in modern electronic markets
1:50 - Mathematical Spread Calculations
Absolute spread vs Percentage of mid-price
3:40 - The Role of Designated Market Makers
Inventory risk, adverse selection, and spread capture
5:10 - Impact of High-Frequency Trading on Spreads
How latency arbitrage tightens top-of-book quotes
6:30 - Measuring Effective vs Quoted Spread
Quantifying institutional execution slippage

Frequently Asked Questions

What is a bid-ask spread and why does it exist?

The bid-ask spread is the price difference between the highest price a buyer offers (bid) and the lowest price a seller demands (ask). It represents the compensation demanded by liquidity providers and market makers for taking inventory risk and immediacy cost.

How do algorithmic trading bots account for the bid-ask spread?

Algorithmic bots incorporate spread costs into their backtests and live execution engines. Crossing the spread with market orders incurs direct transaction costs, so bots frequently use passive limit orders to earn the spread or minimize execution friction.

What factors cause the bid-ask spread to widen?

Bid-ask spreads widen during periods of low market liquidity, high volatility, major macroeconomic news releases, market dislocations, or when trading illiquid small-cap assets where market makers require higher compensation for inventory risk.

What is the difference between quoted spread and effective spread?

The quoted spread is the difference between best bid and best ask at any moment. The effective spread measures the actual execution price relative to the mid-price at the time of trade execution, accounting for market impact and price improvement.

How does high-frequency trading (HFT) affect the bid-ask spread?

HFT firms acting as market makers generally narrow spreads by competing aggressively on quote updates, thereby reducing transaction costs for retail and institutional participants during normal market conditions.

Related Concepts

Order Book
Liquidity
Slippage
Market Maker

Comprehension Check

What happens to the bid-ask spread when market liquidity increases significantly?

[A]The spread widens because volatility increases.
[B]The spread narrows (tightens) due to increased competition among market makers.(Correct Answer)
[C]The spread remains unchanged because exchange fees fix the spread.
[D]The bid price falls while the ask price rises.
Explanation: Higher market liquidity and more market makers create tighter price competition, reducing the spread between best bid and best ask.
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