Bid-Ask Spread¶
OVERVIEW¶
The bid-ask spread is the difference between the highest price a buyer is willing to pay (the bid) and the lowest price a seller will accept (the ask) for a security at a given moment. It represents the implicit transaction cost of crossing the market and serves as the primary measure of a market's liquidity.
Note
Tight spreads indicate liquid, competitive markets with many participants. Wide spreads signal illiquidity, uncertainty, or elevated risk that market makers are pricing in.
FEATURES¶
Spread Calculations¶
| Metric | Formula |
|---|---|
| Absolute Spread | Ask − Bid |
| Midpoint | (Ask + Bid) / 2 |
| Percentage Spread | (Ask − Bid) / Midpoint × 100 |
Tip
Always compare percentage spread — not absolute spread — when evaluating cost across securities at different price levels. A $0.05 spread on a $5 stock (100 bps) is far more expensive than a $0.05 spread on a $500 stock (1 bp).
Spread Reference by Instrument¶
| Instrument | Bid | Ask | Absolute Spread | Percentage Spread |
|---|---|---|---|---|
| SPY (large-cap ETF) | $558.42 | $558.43 | $0.01 | 0.18 bps |
| AAPL (large-cap equity) | $241.12 | $241.14 | $0.02 | 0.83 bps |
| Small-cap biotech | $8.50 | $8.75 | $0.25 | 290 bps |
| AAPL weekly call (options) | $2.10 | $2.20 | $0.10 | 4.7% vs. midpoint |
Note
A small-cap biotech quoted at $8.50 × $8.75 requires a ~3% move just to break even on a round-trip trade, before commissions.
HOW TO USE¶
Execution Strategy¶
- Limit orders inside the spread — Place limit orders between the bid and ask when liquidity is sufficient to pay only a portion of the spread rather than crossing it entirely.
- Effective spread monitoring — Track the difference between your execution price and the midpoint at order arrival as your primary execution-quality metric.
- Position sizing — Compare your strategy's edge in basis points against the round-trip spread cost before entering. If edge ≤ spread cost, the strategy is unprofitable by construction.
When Spreads Widen¶
Spreads widen in predictable conditions. Adjust execution strategy accordingly:
| Condition | Spread Behavior |
|---|---|
| Market open / close | Significantly wider |
| Pending news or earnings | Wider — market makers repricing risk |
| Pre/post-market sessions | Much wider due to low volume |
| Large order size | Price walks beyond displayed ask (market impact) |
| Rising volatility | Wider — market makers hedging inventory risk |
Tip
A ticker quoting a 1-cent spread at noon may show 20-cent spreads at 4:15 PM. Always check spread conditions relative to the session before executing.
Limitations to Know¶
- The NBBO (National Best Bid and Offer) does not reflect total available liquidity. Dark pools, hidden orders, and midpoint venues can offer execution inside the displayed spread.
- Payment for order flow arrangements mean retail brokers may route orders to wholesalers who provide price improvement beyond the displayed NBBO.
- Market impact is distinct from spread. Sweeping the book for large size can walk the price several cents beyond the displayed ask regardless of the quoted spread.
KEYBOARD SHORTCUTS¶
| Action | Shortcut |
|---|---|
| View live bid-ask for selected ticker | S |
| Toggle spread overlay on chart | Shift + S |
| Switch spread display: absolute / percentage | Alt + S |
| Copy midpoint price to clipboard | Ctrl + M |
| Open order entry at midpoint | M |
Related Terms: VWAP · Dark Pool · Payment for Order Flow · Open Interest · Implied Volatility