FRM Exam Part II · Liquidity and Leverage

Market Liquidity and Bid-Ask Spreads Explained

Updated 11 October 2026 · Fact-checked

Market liquidity is how easily you can trade a size at a price close to the mid-price. It has three dimensions: tightness, depth and resilience. You measure trading cost with the quoted, effective and realized spreads, and you can add half the spread to VaR to get a liquidity-adjusted figure.

Understand Market Liquidity and Bid-Ask Spreads

Market liquidity is the ability to buy or sell an asset quickly, in size, without moving its price much. A liquid market lets you trade near the fair value. An illiquid one makes you pay for the privilege.

Three dimensions describe it. Tightness is how small the gap is between buy and sell prices. Depth is how much volume can trade at or near the quoted price. Resilience is how fast prices return to normal after a trade or shock moves them. A market can be tight but shallow, for example a small quote size at a narrow spread.

The cost of trading shows up in the bid-ask spread. The dealer buys at the bid and sells at the ask. The mid-price is the average of the two. The quoted spread is ask minus bid. The half-spread is what you pay, roughly, each time you cross the market, relative to mid.

The quoted spread is only what is displayed. The effective spread uses the actual trade price, so it captures price improvement inside the quote or a large order walking through the book. The realized spread goes one step further. It compares the trade price with the mid-price some time after the trade. It shows what the liquidity provider actually earns after price impact (adverse selection). Low realized spread with high effective spread means informed traders are moving prices against the dealer.

Key formulas to remember

Mid-price: M = (Ask + Bid) ÷ 2; Reference price for all spread measures.
Quoted spread (absolute): Quoted spread = Ask − Bid; Displayed cost of a round trip for a small trade.
Relative quoted spread: Relative spread = (Ask − Bid) ÷ M; Allows comparison across assets with different prices.
Effective spread: Effective spread = 2 × D × (P − M), where D = +1 for a buy and −1 for a sell; P is the trade price and M the mid-price at trade time. Divide by M for the relative form.
Realized spread: Realized spread = 2 × D × (P − M_later); M_later is the mid-price some time after the trade. Effective spread minus realized spread equals the price impact.
Price impact: Price impact = 2 × D × (M_later − M); Effective spread = realized spread + price impact.
Liquidity cost in VaR (constant spread): LVaR = VaR + ½ × S × V; S is the relative spread, V the position value. Assumes a known, constant spread and liquidation at the half-spread cost.

How to solve Market Liquidity and Bid-Ask Spreads questions

Use this order for any question on liquidity measures or spread-based costs.

1Identify what is asked: a dimension of liquidity, a spread measure, or a liquidity-adjusted risk number.
2Write down bid, ask and trade price. Compute the mid-price first.
3Note the trade direction: buy or sell. This sets the sign D.
4Compute the required spread: quoted uses bid and ask, effective uses the trade price versus mid, realized uses the later mid.
5Check whether the answer needs to be absolute (currency) or relative (percent of mid) and whether it is a half or full spread.
6For LVaR, compute the normal VaR, then add ½ × relative spread × position value.
7Interpret: wide quoted spread means poor tightness; large price impact means low resilience or high information content.

Quickest way: Mid-price first, then the one-line formula

When to use it: When a question gives prices and asks for a spread or a liquidity cost under time pressure.

Compute mid = (bid + ask) ÷ 2 before anything else.
Quoted = ask − bid. Effective = 2 × |trade − mid| for the executed trade.
Realized = 2 × direction × (trade − later mid). Direction is +1 for a buyer-initiated trade.
Price impact = effective − realized.
LVaR add-on = ½ × spread × position value. Compare to the answer options to catch half-versus-full errors.

Common mistakes in Market Liquidity and Bid-Ask Spreads

Treating the effective spread as the same as the quoted spread.
Both are called spreads and often match for small trades.
Fix: Quoted comes from displayed bid and ask. Effective comes from the actual trade price against mid, so it can be smaller (price improvement) or larger (walking the book).
Forgetting to multiply by 2 in effective and realized spreads.
The deviation from mid is only a half-spread.
Fix: Effective and realized spreads are defined as twice the signed deviation, so they are comparable to a full quoted spread.
Getting the sign wrong for sell trades.
Students use (P − M) for every trade.
Fix: Use D = +1 for buys and −1 for sells so that the cost is positive when you trade worse than mid.
Using the realized spread as the cost to the trader.
The word realized suggests what was actually paid.
Fix: The realized spread is the dealer's revenue after price impact. The trader's cost at execution is the effective spread.
Adding the full spread rather than half to VaR.
Confusing round-trip cost with the cost of one liquidation.
Fix: Liquidating one position at the bid costs about half the spread versus mid. Add ½ × S × V.
Equating tightness with overall liquidity.
The spread is the easiest number to see.
Fix: Check depth and resilience too. A narrow spread on tiny size, or a market that recovers slowly, is not truly liquid.

Worked examples

Example 1

A stock quotes bid $49.90 and ask $50.10. You buy at $50.06. Ten minutes later the mid-price is $50.04. Compute the quoted, effective and realized spreads and the price impact.

Show the solution

Mid at trade = (49.90 + 50.10) ÷ 2 = $50.00.
Quoted spread = 50.10 − 49.90 = $0.20.
Buy, so D = +1. Effective spread = 2 × (50.06 − 50.00) = $0.12.
Realized spread = 2 × (50.06 − 50.04) = $0.04.
Price impact = 2 × (50.04 − 50.00) = $0.08, which equals 0.12 − 0.04.

Answer: Quoted $0.20; effective $0.12; realized $0.04; price impact $0.08. The buyer got price improvement versus the quote, but most of the effective spread was lost by the dealer to price moving up.

Example 2

A portfolio position is worth $10 million. Its one-day 99% VaR is $250,000. The relative bid-ask spread is 0.40% of mid and is assumed constant. Compute the liquidity-adjusted VaR using the half-spread cost.

Show the solution

Liquidity cost = ½ × S × V = 0.5 × 0.004 × 10,000,000.
0.5 × 0.004 = 0.002.
0.002 × 10,000,000 = $20,000.
LVaR = 250,000 + 20,000 = $270,000.

Answer: LVaR = $270,000. The spread adds $20,000, or 8% of the original VaR. This ignores spread volatility and assumes you can liquidate at the half-spread cost.

Exam tips

Always compute the mid-price first. Most spread questions are one step after that.
Watch the word half. Many answer options differ only by a factor of 2.
Know the link: effective spread = realized spread + price impact. Questions often give two and ask for the third.
For concept questions, match the term to the dimension: spread = tightness, volume at quote = depth, speed of recovery = resilience.
Remember the LVaR assumption of a constant spread. If spread varies, the basic add-on understates the risk.

Practice questions from Liquidity and Leverage

Market Liquidity and Bid-Ask Spreads in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

CFA Level IBid-Ask Spreads and Dealer Quotes

Market Liquidity and Bid-Ask Spreads: frequently asked questions

What is the difference between quoted spread and effective spread?

The quoted spread is ask minus bid and shows the displayed cost. The effective spread is twice the signed gap between the trade price and the mid-price, so it reflects what the trade actually cost. It is smaller with price improvement and larger when an order moves through several price levels.

What do tightness, depth and resilience mean?

Tightness is a small gap between bid and ask. Depth is the volume that can be traded near the quote. Resilience is how quickly prices recover after a trade or shock. A truly liquid market scores well on all three.

How do I calculate liquidity-adjusted VaR from the bid-ask spread?

Compute normal VaR, then add half the relative spread times the position value. This is the constant-spread version. LVaR = VaR + ½ × S × V.

What does the realized spread tell you?

It compares the trade price with the mid-price after a delay. It shows the dealer's earnings once price impact is taken out. A small realized spread suggests the trade carried information that moved the price.