Bid-Ask Spread Explained

The spread is the least discussed and most consistently paid cost in trading, because it does not appear on a statement as a fee.

What the two numbers mean

Every quote has two sides. The bid is what someone will pay right now; the ask, sometimes called the offer, is what someone will sell for right now.

Both come with a size — how many shares or contracts are available at that price. A tight spread with almost no size behind it is not the liquidity it appears to be, and reading only the prices while ignoring the sizes is a common oversight.

The midpoint between bid and ask is often treated as the fair value of the instrument, and it is a convention rather than a price anyone is obliged to trade at.

The spread is a cost you always pay

A market buy fills at the ask. A market sell fills at the bid. So a position opened and closed immediately, with no price movement at all, loses the spread.

This makes the spread a fixed toll on every round trip, and it compounds with frequency. A strategy trading many times a day pays it many times a day, and the arithmetic can consume an edge entirely without any single trade looking like a loser.

The cost is proportional to the instrument's price. A five-cent spread on a contract worth ten dollars is half a percent; the same five cents on a contract worth fifty cents is ten percent. Cheap instruments are frequently expensive to trade.

What makes spreads wide or narrow

Participation. More active buyers and sellers means more competition to quote, which narrows the gap. The most heavily traded instruments carry the tightest spreads.

Volatility. When prices move fast, quoting becomes riskier for whoever is providing liquidity, and spreads widen to compensate. This is why spreads are worst exactly when you most want to trade.

Time of day. Spreads are typically wider at the open, before the market settles, and around the close. For futures, the overnight session carries wider spreads than the regular session because participation is thinner.

Instrument specificity. In options, spreads vary enormously across a single chain. Near-the-money strikes on major index products can trade penny-wide, while far out-of-the-money strikes on the same underlying and expiry are far wider.

Why options spreads deserve special attention

The general rules above apply, and options add a pattern that catches people out.

Out-of-the-money short-dated strikes deteriorate through the session. A spread of a few cents at the open can widen substantially by mid-afternoon, and the final half hour is worst as market makers unwind hedges ahead of expiration.

So the same contract has a different execution cost at different hours, and the deterioration is largest on the strikes that look cheapest. A blanket claim that options are illiquid is wrong — the deterioration is specific, and knowing where it concentrates is what makes the knowledge useful.

Crossing versus resting

Two ways to interact with a spread, and the trade between them recurs throughout trading.

Crossing the spread with a market order or an aggressive limit fills immediately and pays the spread. You get certainty of execution at an uncertain price.

Resting inside the spread with a passive limit order may fill at a better price and may not fill at all. You get certainty of price at uncertain execution.

Neither is correct in general. On an exit that must happen, crossing is usually right and the spread is the cost of certainty. On a discretionary entry, resting is often worth attempting.

Some execution systems work an order across the spread progressively, starting near the midpoint and stepping toward the far side until filled — automating what a trader would otherwise do by repeatedly cancelling and replacing.

Why it is the best live liquidity measure

Other indicators are lagging or indirect. Open interest updates once daily after clearing, so the figure on your chain during the session is yesterday's. Volume describes the day so far, not the current book.

The spread and its associated size are current. They describe what you can actually do right now, which makes the spread the right final check before submitting an order, whatever screening happened earlier.

What this means for automation

Make the spread a gate, not a statistic. A check that declines a trade when the quote has widened beyond a configured distance converts a bad fill into a missed trade, which is usually the better failure.

Express the threshold proportionally. A fixed cent threshold means very different things on a fifty-cent contract and a ten-dollar one. A percentage of the midpoint travels better across instruments and price levels.

Model it in backtests. A backtest filling at a modelled midpoint has crossed no spread and consumed no liquidity, which systematically overstates results. The overstatement is largest for high-frequency strategies, because the cost applies per trade while the edge per trade is small.

Keep the check on the fast path. A spread gate only helps if it runs before submission rather than alongside it, which means heavier analysis belongs on worker thread pools while the gate itself stays in the order path.

The honest limits

A narrow spread is not a guarantee of a good fill. Size behind the quote matters, and a large order consumes the visible book and fills progressively worse.

Quoted spreads can move between the moment you read them and the moment your order arrives, which is exactly what happens in the fast conditions where the reading mattered.

And minimising spread cost does not create an edge. It reduces a drag on whatever edge exists. Position sizing bounds what a bad fill costs — capital divided by twenty as the ceiling on any single position, under the divide-by-20 rule.

Frequently asked questions

What is the bid-ask spread? The gap between the highest price a buyer will pay and the lowest a seller will accept. You pay it on every round trip.

Why do spreads widen? Lower participation, higher volatility, and specific times of day including the open, the close, and overnight sessions in futures.

Is a narrow spread always better? Narrower is cheaper, and size behind the quote matters too. A tight spread with no depth is not the liquidity it appears to be.

Should I use market or limit orders? Crossing the spread gives execution certainty at an uncertain price; resting inside it gives price certainty at uncertain execution. On exits that must happen, crossing is usually right.

Why is my live performance worse than my backtest? Backtests typically fill at a modelled price having crossed no spread, which systematically overstates results.

Disclaimer: This article is educational content about options mechanics. It is not investment advice, financial advice, tax advice, legal advice, or a recommendation to buy or sell any security, nor a recommendation of any strategy or position structure. Any instruments, figures, or examples are used solely to illustrate mechanics. Options trading involves substantial risk of loss and is not suitable for all investors; selling options can produce losses substantially greater than the premium received. Please read Characteristics and Risks of Standardized Options before trading options. Automated trading carries additional risks including software defects, connectivity failures, broker API changes, and outages that may prevent orders from being placed, modified, or cancelled. Past performance does not indicate future results, and no configuration, position-sizing rule, or risk setting can guarantee a profit or prevent a loss.

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