Direct answer
A bid-ask spread is the difference between the bid price (the price a market participant is willing to buy at) and the ask price (the price a market participant is willing to sell at). A worked example shows how that difference creates an immediate cost when you buy at the ask and later sell at the bid.
Mechanism and definition (with explicit assumptions)
Assumption A1 (no real-time data): The numbers below are hypothetical and used only to show the mechanics.
Definition:
- Bid price: price at which you can sell.
- Ask price: price at which you can buy.
- Spread:
- In price terms: Ask − Bid.
- Often quoted as a “number of pips” in forex contexts, but here we keep it in simple price units.
Assumption A2 (single spread for each step): In the example, the bid and ask you observe at entry remain the ones used in the calculations for that moment. Later, you will explicitly choose what new bid/ask values are.
How the spread creates cost: If you buy at the ask and later sell at the bid, you do not capture the full mid-price change; you pay the spread at the start (and possibly again if conditions change).
Evidence or example (worked numbers)
Scenario 1: Enter and exit immediately
Assumption A3 (lot sizing kept simple): Use a notional position size of 1 unit and compute profit/loss per unit.
At entry (time t0):
- Bid = 1.2000
- Ask = 1.2005
- Spread = 1.2005 − 1.2000 = 0.0005
Step 1 (buy): You buy at the ask price 1.2005.
- Cost per unit = 1.2005
Step 2 (sell right away): You sell at the bid price 1.2000.
- Sale proceeds per unit = 1.2000
P/L per unit = Sell − Buy = 1.2000 − 1.2005 = −0.0005.
Interpretation: In this simplified scenario, the immediate loss equals the spread because you bought at ask and sold at bid without any favorable price movement.
Scenario 2: Break-even after a mid-price move (explicitly stating what must happen)
To avoid guessing, define what changes and what stays constant.
Assumption A4: The “directional movement” you care about is captured through new bid/ask values at exit time t1. You will treat bid and ask at t1 as given.
Suppose at exit (time t1):
- Bid = 1.2010
- Ask = 1.2015
You still bought at entry ask = 1.2005. You sell at exit bid = 1.2010.
P/L per unit = 1.2010 − 1.2005 = +0.0005.
So in this numerical setup, the position breaks even when the exit bid is high enough that:
- exit_bid − entry_ask = 0
- i.e., exit_bid must equal entry_ask.
Note the “asymmetry”: you compare exit bid to entry ask, not bid-to-bid or ask-to-ask.
Limitations and risks (what can go wrong, and what you can verify)
Material limitation 1: Spread can change between entry and exit
Assumption breakdown risk: Real markets may widen or narrow spreads. If the spread widens after entry, the exit bid might not move as favorably as expected.
Verification you can do: When reviewing any execution, compare the actual bid/ask (or executed prices) at entry and exit and compute (exit sell price − entry buy price) for your specific trade.
Material limitation 2: Execution and additional costs can dominate
A worked bid-ask example isolates spread, but real outcomes can include other frictions such as commissions, financing, and execution differences.
Verification you can do: Check your platform statement for executed prices and separate spread-related differences from other listed fees.
Material limitation 3: “Historical relationships” don’t guarantee future results
Even if spreads have behaved a certain way in past conditions, future liquidity and volatility can differ.
Verification you can do: Recalculate the same math using the latest observed bid/ask around actual trade timestamps instead of relying on past averages.
Verification or next question
If you want to verify your understanding independently, take any two timestamps where you can observe bid and ask (or executed entry/exit prices), then compute:
- Spread at entry = ask − bid
- P/L per unit = exit_bid − entry_ask for a buy-then-sell example
- How sensitive results are when you alter the assumed exit bid/ask.