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Bid-Ask Spread

Difficulty beginner

Definition

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

Bid: $150.02 (highest buy order)
Ask: $150.03 (lowest sell order)
Spread: $0.01 (1 cent)
Spread %: 0.01 / 150.025 × 100 = 0.0067%

Components of the Spread

1. Order Processing Costs

  • Exchange fees, clearing costs, technology costs
  • Typically 10-20% of spread

2. Inventory Holding Costs

  • Risk of holding position while waiting for offsetting trade
  • Larger for volatile securities

3. Adverse Selection Costs

  • Risk of trading against informed participants
  • Wider when information asymmetry is high

Spread Calculation

Absolute Spread

Spread = Ask - Bid

where: Ask lowest sell quote · Bid highest buy quote. does: raw dollar gap between best buy and sell quotes — the headline transaction cost for any market order.

Relative Spread (Percentage)

Relative Spread = (Ask - Bid) / Midpoint × 100
where Midpoint = (Ask + Bid) / 2

where: Ask, Bid best quotes · Midpoint mean of bid and ask · multiplication by 100 gives a percentage. does: spread scaled to price — the right comparison across assets at different price levels. Used for cross-instrument cost analysis.

Effective Spread

Effective Spread = 2 × |Trade Price - Midpoint|
Measures actual execution cost vs. quoted spread.

where: Trade Price actual fill price · Midpoint (Bid+Ask)/2 at the time of the trade · the factor of 2 makes the metric round-trip comparable to the quoted spread. does: the cost actually paid on a fill — frequently smaller than the quoted spread because internalizers, dark pools, and price-improving orders execute inside the NBBO.

Realized Spread

Realized Spread = 2 × Side × (Trade Price - Midpoint_t+k)
Measures price impact after trade (k = time interval).

where: Side +1 for a buy, −1 for a sell · Midpoint_t+k quote midpoint k seconds (or minutes) after the trade · k typically 5 minutes for equities. does: isolates the temporary (inventory) component of the spread from the permanent (information) component — the difference between effective and realized spread is market impact.

Typical Spreads by Asset

Asset Class Typical Spread Notes
Large-cap stocks $0.01 Tick-bound, rebate-driven
Small-cap stocks $0.05 - $0.50 Wider with lower liquidity
SPY ETF $0.01 Most liquid ETF
Treasury bonds 1/32 - 1/16 Very tight
Corporate bonds (IG) $0.10 - $0.50 OTC, dealer-driven
EUR/USD 0.5-1.0 pips Most liquid FX pair
Exotic FX pairs 10-50+ pips Wide spreads
Bitcoin $1-10 Varies by exchange
Options (ATM) $0.01 - $0.05 Tight for liquid underlyings
Options (OTM) $0.10 - $1.00+ Wide, low liquidity

Spread Dynamics

What Makes Spreads Widen

Factor Mechanism
Volatility Higher risk → wider spread
Low liquidity Fewer market makers → wider spread
News events Uncertainty → wider spread
Market open/close Uncertainty, order imbalance
Adverse selection Informed trading → wider spread
Inventory imbalance Market maker protection

What Makes Spreads Tighten

Factor Mechanism
High volume Competition among market makers
Low volatility Lower risk → tighter spread
Competition Multiple MM venues
Technology Faster processing → tighter spreads
Maker-taker rebates Incentive to quote tightly

Spread and Your Strategy

Break-Even Calculation

Required Move = Spread + Commission + Slippage

Example:
Bid: $100.00, Ask: $100.05, Commission: $0.005/share
Spread cost (round trip): $0.05
Commission (round trip): $0.01
Total cost: $0.06/share
Stock must move $0.06 (0.06%) just to break even

Spread Impact on Strategy Viability

Strategy Type Spread Tolerance Notes
Scalping Must be minimal Spread is primary cost
Day trading Low Multiple trades compound cost
Swing trading Moderate Fewer trades, spread less critical
Position trading Higher Spread diluted over large moves
HFT/Market making N/A Profit FROM the spread

Hidden Spread Costs

Implementation Shortfall

Implementation Shortfall = Paper Return - Actual Return
= (Decision Price - Execution Price) × Quantity + Commissions

Includes spread, market impact, opportunity cost of delay.

where: Decision Price quote at the moment the trade decision was made · Execution Price average realized fill price · Quantity signed share count · Commissions explicit fees. does: the honest scorecard for execution — measures total slippage from the theoretical "trade instantly at the decision price" benchmark. The gold-standard metric for execution-algo quality.

Price Improvement

Price Improvement = NBBO - Actual Execution Price
Positive when you execute better than NBBO (common with internalizers).

where: NBBO National Best Bid and Offer (best public quote on the relevant side) · Actual Execution Price realized fill. does: measures how much inside the public spread you executed. Internalizers, retail wholesalers, and midpoint dark pools routinely produce sub-penny price improvement on retail flow.

Measuring Spread Quality

Realized Spread Analysis

Measures how much of the spread is temporary impact vs. permanent information.

Market Maker Perspective

Profit from Spread

MM Revenue = Spread × Volume - Adverse Selection Losses - Costs

If MM quotes: Bid $100.00 / Ask $100.05
- Buys at $100.00, sells at $100.05
- Gross profit: $0.05 per round trip
- Minus: losses when trading against informed traders
- Minus: operational costs

where: Spread × Volume gross spread capture if every fill was uninformed · Adverse Selection Losses P&L bled to informed counterparties (the spread isn't kept on those fills) · Costs infrastructure, exchange fees, regulatory capital. does: the market maker's P&L identity. Explains why widening spreads is not pure profit — wider spreads attract less flow and informed traders still win on the fills they take, so the optimum spread balances volume against adverse selection.

Trading Around the Spread

Strategies

Strategy Approach Risk
Spread capture Place limit at bid, sell at ask Inventory risk
Rebate arbitrage Collect maker rebates Latency competition
Queue jumping Price ahead by $0.01 Adverse selection
Spread trading Trade widening/narrowing Regime change

Key Metrics Summary

Metric Formula Use
Quoted Spread Ask - Bid Visible cost
Effective Spread 2 × Trade - Mid
Realized Spread 2 × Side × (Trade - Mid_future) Information content
Volume-Weighted Spread Σ(Spread_i × Volume_i) / Total Volume Representative cost
Time-Weighted Spread Σ(Spread_i × Time_i) / Total Time Average market quality

Practical Tips

  1. Trade During Peak Hours — Tightest spreads mid-session
  2. Use Limit Orders — Avoid paying full spread
  3. Check Multiple Venues — NBBO may not be best for your size
  4. Avoid News Times — Spreads widen dramatically
  5. Size Appropriately — Large orders walk the book
  6. Track Implementation Shortfall — Real cost vs. decision price
  7. Consider Rebates — Maker orders can offset costs

Next Steps