The bid-ask spread is the gap between an option's two quoted prices. The bid is what buyers are offering right now, so it is the price you get when you sell; the ask is what sellers are demanding, so it is the price you pay when you buy.
The single "price" your app shows is usually the last trade or the midpoint, and no button on your screen trades at it. Buy and you pay the ask; sell and you receive the bid. You cross that gap on the way in and again on the way out; the difference goes to the market maker quoting both sides. It never appears on your statement as a fee, which is exactly why beginners never count it.
Stock spreads on big names are a penny or two. Option spreads are far wider, for a structural reason: a stock has one order book, while its options are scattered across hundreds of contracts, every strike times every expiration. Slice liquidity that thin and each contract has few orders; remote strikes can be quoted so wide the market is barely there.
The numbers
The $105 call on a $100 stock is quoted 2.90 bid / 3.10 ask. The mid is $3.00, so the premium is worth about $300 on paper. Now trade it:
- You buy at the ask. $310 leaves your account.
- You change your mind a minute later and sell at the bid. $290 comes back.
The option never moved. You lost $20, about 6.5 percent of what you paid, purely for crossing the spread twice. To merely break even, the mid has to climb from $3.00 to $3.20, a 6.7 percent move in the option, before you make your first dollar.
Now take an illiquid strike quoted 2.50 bid / 3.50 ask. Same $3.00 mid, same value on paper. Buy at $3.50, sell at $2.50: the round trip costs $100, roughly 29 percent of what you paid. The option's mid must rise a full third, from $3.00 to $4.00, just to carry you back to zero.
Where it bites
Market orders sign a blank check. A market order says: fill me at whatever the quote is. On a liquid contract that costs pennies; on a wide one it fills at an ask nobody sane would pay. The defense is a limit order at or near the mid: you name your price and accept you might not get filled. On an ugly quote, that is often the cheapest outcome.
Illiquid strikes turn edge into noise. Far out-of-the-money and long-dated options routinely quote at 30, 50, even 100 percent spreads. When the round trip eats a third of the position, your idea has to be extraordinary just to pay the toll. Every payoff diagram is drawn at fair value; the spread drags the whole picture against you before the story starts.
Spreads blow out exactly when you need out. On calm days market makers quote tight. On the day everything gaps and you finally need out, they step back and the same contract quotes three times wider. The quote you saw yesterday is not the price you get today. Liquidity is a fair-weather friend, and it charges the most when you are desperate.
Go feel it
The fastest way to meet the spread: open a play-money trade and notice it starts slightly red before anything happens. Buy your first call in Lesson 1: What You're Actually Buying and watch the toll get collected, then see what the same friction does to a premium seller's thin margins in The Premium Trap.
Related concepts: option premium · payoff shapes · implied volatility