The bid-ask spread is the difference between quoted selling and buying prices for a stated instrument and quantity. If the bid is $50 and ask $50.02, the quoted spread is two cents. Investor.gov defines the bid and ask.

Relate the Quote to an Actual Order

In a simplified unchanged market with sufficient quantity, buying 100 shares at $50.02 and immediately selling at $50 gives a $2 gross loss before other charges. You have crossed the same two-cent spread on that round trip, not paid two separate two-cent fees.

A resting limit order can fill at a different relation to the spread, or not fill at all. A marketable order may receive price improvement or reach worse price levels when the displayed quantity is insufficient. You do not always buy at exactly the displayed ask and sell at exactly the displayed bid.

Convert the Units

Share spreads are often quoted in currency per share; futures can be described in ticks or points; FX quotes may use pips or fractional pips. Convert to money using the correct quantity, contract multiplier and account-currency rate where needed.

A narrow quoted spread does not establish a low total cost if commissions, financing, other charges or limited depth are material. Compare the full fee and execution basis of the actual product.

Avoid Double-Counting in Performance

When entry and exit use actual fills, their P&L already reflects the executed price difference. Do not subtract a full spread again automatically. A backtest using midpoint prices needs an execution-cost model because those midpoints may not be available for your orders.

Similarly, a target measured from an actual buy fill differs from a target measured from the pre-entry midpoint. You cannot always subtract two pips from a ten-pip target without stating the reference.

Use Observations Rather Than a Universal Normal Range

Measure quotes and fills at the times and sizes you intend to trade. Product, venue, account, session and market conditions can change the spread. A familiar symbol or high daily volume does not guarantee a narrow executable quote when you need it.

Our fill-quality guide explains how to compare price improvements with missed and partial fills.

Risk Disclaimer: Trading involves substantial risk of loss. Past performance is not indicative of future results. See our full risk disclaimer.