The bid-ask spread is the gap between the highest price a buyer is currently willing to pay for an option (the bid) and the lowest price a seller is currently willing to accept (the ask). It’s a real, immediate cost of trading, separate from any commission: an order that crosses the spread to guarantee an instant fill pays that gap as a real cost, and an order that tries to fill closer to the middle has to wait for the market to actually meet it there, which doesn’t always happen.
Why the spread exists at all
Market makers quote both a bid and an ask, and the gap between them is partly their compensation for standing ready to trade either side at any moment, and partly a reflection of risk: the less certain a market maker is about where an option should actually be priced, the wider they’ll quote it to protect themselves. A liquid, heavily traded option has market makers competing against each other for order flow, which pushes spreads tighter. A thinly traded option has less competition and more uncertainty, and spreads widen accordingly.
Volume and open interest drive spread width
Spread width isn’t random from one option to the next; it tracks how much real trading activity is happening in that specific contract. SPX and SPY options, among the most actively traded in the world, tend to have tight spreads even on size, because there’s constant two-sided flow for market makers to work with. A far-dated expiration, an obscure strike, or a less popular underlying tends to show a noticeably wider spread on the exact same size, simply because there’s less flow for a market maker to lean on when quoting a fair price.
A real example: working an order through five reprices
This isn’t abstract for this account. On Tuesday, September 8, 2026, a same-day SPX call credit spread took five separate order submissions before it actually filled: the strikes and price walked in gradually over several minutes, from an initial attempt that didn’t get taken, through several reprices, before finally landing at 7710/7730 for a 0.90 credit, covered in that week’s real recap. Every other trading day that same week filled inside one or two tries. That kind of gradual walk toward a fillable price, not a single instant hit at the first number entered, is a normal, real part of trading a multi-leg spread against a live, moving bid-ask spread, not a sign anything was wrong with the order or the underlying that day.
Why this account trades more than one underlying, partly because of this
Bid-ask spread cost is one real, concrete reason this account hasn’t treated SPX, SPY, and XSP as fully interchangeable. SPY’s much larger overall trading volume tends to produce tighter bid-ask spreads even on small size, which can make it easier to get filled at a fair price than a lower-volume alternative at the exact same strike distance. The fuller comparison, including the tax and settlement differences that matter just as much as spread width, is covered in XSP vs. SPY options. None of that makes one underlying simply better than another; it means the real, practical cost of getting filled is a genuine input into which product a given trade actually uses, not just an afterthought once the strategy is already decided.
Frequently asked questions
What is the bid-ask spread in options trading?
The bid is the highest price a buyer is currently willing to pay for an option; the ask is the lowest price a seller is currently willing to accept. The difference between the two is the bid-ask spread, and it's a real, immediate cost of trading: an order placed at the midpoint or better has to actually get filled there, and an order that just crosses the spread to guarantee a fill pays that spread as a real cost, separate from any commission.
Why are some options' bid-ask spreads wider than others?
Spread width mostly comes down to volume and open interest. Heavily traded options on liquid underlyings, SPY and SPX among them, tend to have tight spreads because market makers compete to fill orders and there's constant two-sided flow. Thinly traded strikes, far-dated expirations, or less popular underlyings tend to have wider spreads, since there's less competition and more risk for whoever is quoting the price.
How does the bid-ask spread affect a multi-leg spread like a credit spread?
Each leg has its own bid-ask spread, and a multi-leg order effectively has to cross a combined version of both when it's placed as a single spread order. This is one real reason limit orders on a spread often don't fill instantly at the price first entered, and why working an order gradually toward the market, a few cents at a time, is a normal, real part of getting a fair fill rather than a sign something is wrong.
Does this account's underlying choice affect bid-ask spread costs?
Yes, directly. SPY's much larger overall volume tends to produce tighter bid-ask spreads even on small size, covered in the fuller comparison in XSP vs. SPY options, which is one real reason this account has traded both rather than treating them as interchangeable.