Reading an option chain in practice: liquidity filters, strikes and expiries

What you will learn

• How to read every column of an option chain and what the bid-ask width really costs you.
• How to use open interest and volume as liquidity filters, with concrete threshold numbers.
• How to choose expiries by days to expiration (DTE) and strikes by delta rather than by gut feeling.
• How to estimate the round-trip cost of a position before you enter it.

Explanation

Calls, puts, strike, expiry and premium were covered in the course Derivatives: Futures, Options, CFDs and Certificates. This lesson starts in front of a real option chain, deciding which of the hundreds of listed contracts is actually tradeable.

An option chain is the broker's table for one underlying: one row per strike, calls left, puts right, one tab per expiry. The columns that matter most:

• Bid and ask: the price at which you can sell immediately and the price at which you can buy immediately. The mid price is the average of the two. The bid-ask width (ask minus bid) is your main hidden cost.
• Volume: contracts traded today. It resets every day.
• Open interest (OI): contracts open and not yet closed, updated overnight; it shows whether a standing population of traders exists in that strike.
• Delta: sensitivity to a one-unit move, also a rough proxy for the probability of finishing in the money (an approximation, not a law).
• Implied volatility (IV) per strike, which the next lesson treats in depth.

Remember the contract multiplier: a standard US equity option controls 100 shares, so 2.90 means $290 per contract. Eurex specifications are covered in lesson 12.

Why width matters more than commission

Buying at the ask and selling at the bid pays the full width once per round trip. Compare strikes by width as a percentage of the mid price: 0.20 on a 3.00 option is 6.7 percent; 0.20 on a 0.60 option is 33 percent. Far out-of-the-money options look cheap in dollars but are often the most expensive in percentage terms.

Practical liquidity filters

Starting-point rules of thumb (tighten them for larger accounts):

• Width at most 10 percent of the mid for options above 1.00, or at most 0.05 to 0.10 absolute for cheaper options.
• Open interest of at least 500 contracts in the strike you trade.
• Daily volume of at least 100 contracts in that strike on a normal day.

Choosing the expiry

For premium selling, a common window is 30 to 45 DTE: decay is meaningful but gamma risk (how fast delta changes) is still moderate. For directional debit trades, many traders buy 60 to 90 DTE and exit before the last three weeks. For a known event, choose the first expiry after it.

Choosing the strike

Choose strikes by delta, because delta adjusts for volatility and time. Anchors: 0.50 delta at the money, 0.30 moderately out of the money, 0.16 roughly at the one-standard-deviation level. "Sell the 16-delta put" is a precise plan; "sell a put a bit below the price" is not.

Worked example

Hypothetical example: a stock trades at $100.00. The 30-DTE call chain shows the following rows (all numbers invented for teaching):

Step 1, width in percent: 100 strike 0.20 / 3.00 = 6.7 percent; 105 strike 0.30 / 1.15 = 26.1 percent. The 105 call fails the width filter and the OI filter (180 is below 500).

Step 2, cost of crossing the spread: you buy 5 contracts of the 100 call at the ask (3.10) and later sell at the bid, paying 0.10 per share each way: 0.10 x 100 shares x 5 contracts x 2 = $100 round trip, or 6.7 percent of the $1,500 premium before commission.

Step 3, decision: trade the 100 strike with a limit order at the mid (3.00), moving it a cent at a time only if it does not fill. Exclusions: commissions, exchange fees and taxes.

Common mistake

Practice activity

On paper, evaluate these three 45-DTE puts on a hypothetical $60 stock and decide which passes all filters. Put A: strike 57, bid 1.10, ask 1.20, volume 300, OI 2,100. Put B: strike 55, bid 0.55, ask 0.80, volume 25, OI 410. Put C: strike 58, bid 1.45, ask 1.70, volume 900, OI 3,000. Then estimate the round-trip spread cost for 3 contracts of the one you choose if you are filled 0.03 from the mid each way.

Key takeaways

• Judge an option's liquidity by width as a percentage of the mid price, open interest and volume, in that order.
• Cheap-looking far-out-of-the-money options often carry the widest percentage spreads.
• Choose expiries by DTE windows that match the strategy, and choose strikes by delta, not by round numbers.
• Estimate the round-trip spread cost in dollars before entering; it is often larger than the commission.

Next step

In the next lesson you learn to read implied volatility in context, using IV rank, IV percentile, term structure and skew to decide which type of strategy fits the current regime.