Réponse rapide

Every option quote has two prices. The bid is what buyers are currently willing to pay; the ask is what sellers are currently willing to accept. The gap between them is the bid/ask spread.

The spread is a real cost of trading. An order that buys at the ask and later sells at the bid gives up the full spread even if the option's value never changes in between.

Liquidity, meaning how easily a contract trades in size without moving the price, varies enormously across the options market. Heavily traded stocks quote penny-wide spreads near the money; thinly traded names and distant strikes can quote spreads that are a large fraction of the premium itself.

Un regard plus attentif

A worked example. An option is quoted $2.30 bid, $2.50 ask. The midpoint is $2.40, a reasonable estimate of fair value at that moment. A buyer who pays the ask spends $250 per contract; if nothing changes and the position is closed at the bid, the sale brings $230. The round trip cost $20 per contract, roughly 8 percent of the option's value, purely from the spread.

Compare a penny-wide market of $2.39 bid, $2.41 ask on a heavily traded name. The same round trip costs $2 per contract. Both quotes describe an option worth about $2.40; the difference is the toll for getting in and out.

Three readings from the quote itself help gauge liquidity:

• Spread width relative to premium. A $0.20 spread means something different on a $2.40 option than on a $12.00 one.
• Displayed size. The number of contracts bid for and offered at those prices shows how much can trade before the quote moves.
• Activity measures. Volume counts contracts traded today; open interest counts contracts outstanding. Both tend to concentrate in near-dated, near-the-money strikes.

Spreads are not fixed. They widen when uncertainty jumps, around news and earnings, at the market open, and in fast markets, and they tighten in calm conditions on busy products.

Le détail formel

Options quotes are supplied largely by market makers, firms that stand ready to buy at the bid and sell at the ask across thousands of series while hedging their inventory in the underlying. The spread compensates them for hedging costs, inventory risk, and the risk of trading against better-informed order flow; those costs rise for volatile stocks, distant expirations, and strikes far from the money, which is why spreads widen there.

The displayed national best bid and offer aggregates the best prices across all options exchanges. Orders placed between the bid and the ask frequently do trade, so the quoted spread overstates the effective cost for patient orders; equally, the midpoint is an estimate rather than an entitlement, and no particular order is assured a fill there.

A common misconception equates high open interest with liquidity right now. Open interest measures contracts outstanding, accumulated over time; the ability to trade at a fair price this minute is better read from the current spread and displayed size on the chain.