An options chain is the menu of every option contract available on a share, laid out by expiry date and strike price, with calls on the left and puts on the right. Reading one means decoding a handful of columns, chiefly bid, ask, volume, open interest and implied volatility, in order to judge what a contract really costs and whether it is liquid enough to be worth trading at all.

A chain looks intimidating on first sight, being a wall of numbers in small type. It is built from a few repeating ideas, and once you can read those you can size up any contract in seconds. This module walks every column in plain English, using one worked chain throughout.

The anatomy of a chain: calls, puts and strikes

A chain is organised in two levels. First by expiry date, then within each expiry by strike price.

For a single expiry, read it as three columns: calls on the left, the strike prices down the centre, and puts on the right. Each row is one strike, and that strike gives you both a call and a put. The layout is not decorative. It is built so that your eye can run down the centre column, find the price you care about, and then move left or right depending on which direction you are interested in.

Throughout this module we use a fictional London listed share, HLXB.L, trading at 100p, with strikes running from 90p to 110p. The strike nearest the current price, which here is the 100p row, is the at the money strike, and it is the busiest part of any chain.

That orientation rule is the one thing worth committing to memory before anything else, because every other column on the page is read differently depending on whether the contract is in or out of the money. If you find yourself confused halfway down a chain, it is almost always because you have lost track of which side you are on. Go back to the centre column and work outwards.

Bid and ask: the two prices that matter most

Every contract shows two live prices rather than one, and understanding why is the beginning of understanding what a chain costs you.

The bid is the highest price a buyer is currently willing to pay, so it is what you receive if you sell right now. The ask, also called the offer, is the lowest price a seller is currently willing to accept, so it is what you pay if you buy right now. The midpoint sits halfway between them, and it is the fairest single reference for what the contract is actually worth.

Now do the thought experiment that makes the point. Buy this call at the 4.4p ask, change your mind a second later, and sell it back at the 4.0p bid. You are down 0.4p and the share has not moved at all.

Nothing went wrong there. You simply paid the gap, which is what dealers earn for standing ready to trade with you at any moment. That gap is the cost of doing business in options, and it is the next thing to understand properly.

The bid-ask spread: your hidden cost

The spread is the difference between the ask and the bid, and it is the single best gauge of how expensive a contract is to trade.

Be precise about how it costs you, because this is commonly stated loosely. You cross the spread twice, once entering and once exiting. Against the midpoint, each crossing costs you roughly half the spread. So a complete round trip costs you about one full spread, which is exactly what the thought experiment above showed: a 0.4p spread produced a 0.4p loss on the round trip. Two crossings, one spread.

Because a spread of 0.4p means something entirely different on a 4p option and on a 40p option, the useful figure is the spread as a percentage of the ask.

• Below 10%. Excellent. A liquid, efficiently priced contract.
• Between 10% and 20%. Acceptable, but check open interest before committing.
• Above 20%. Caution. The spread alone is consuming a large slice of any gain you might make.

Take the 105p call, with a 1.9p bid and a 2.3p ask. The spread is 0.4p, and 0.4 ÷ 2.3 gives 17% of the ask. Borderline, so tradable but worth checking liquidity first.

Now compare a thinly traded strike sitting on a 36% spread. For every £100 of premium you commit, roughly £36 is lost to spread friction across the round trip. Think about what that demands of you. The share has to move enough to cover 36% before you break even, which means a correct call on direction is nowhere near sufficient. You need to be correct by a wide margin. This is how experienced traders lose money on trades they got right, and it happens before the Greeks from module 3 have done anything at all.

Volume and open interest: reading liquidity

Two columns measure how actively a contract trades, and they are easy to confuse because they sound alike.

Volume is the number of contracts traded so far today. It resets to zero at the start of every session. Open interest is the total number of contracts currently outstanding, meaning positions that have been opened and not yet closed, and it accumulates across many sessions.

Open interest is the more reliable liquidity signal, and the reason is in those definitions. Volume can be high because one institution put through a single large trade this morning, which tells you nothing about whether anyone will be there tomorrow when you want out. Open interest reflects standing commitment by many participants over time, and standing commitment is what produces a counterparty when you need one.

Here is the call side of our chain.

Read down the open interest column and notice the shape. The 100p at the money strike is deepest at 3,890, and interest thins as you move away in either direction. That pattern holds on almost every chain you will ever look at, because the at the money strike is the one that serves both audiences at once. Hedgers want protection near the current price, and speculators want the strike most responsive to a move.

Notice too that spread and open interest move together. The deepest strike has the tightest spread at 9%, and the thinnest strike has the widest at 30%. That is not a coincidence to be memorised, it is cause and effect: where many participants are willing to trade, competition narrows the gap.

As a working rule, treat open interest of at least 500 as the floor for a contract you intend to trade actively. Below about 100 you may struggle to get a fair fill at all, and the quoted price becomes close to theoretical.

Implied volatility: the price of uncertainty

Implied volatility is the market’s forecast of how much the share will move, expressed as an annualised percentage. It is the single biggest driver of an option’s time value, because a wider range of expected outcomes makes the right to choose more valuable.

It is worth being clear about the direction of the logic, since it runs backwards from what people expect. Implied volatility is not an input someone types in. It is derived from the price the option is actually trading at, by asking what volatility figure would justify that price. So implied volatility does not cause the premium. It is a way of reading the premium, translated into a unit you can compare across strikes, across expiries and across companies. When you hear that an option is expensive, this is the number that says so.

Which brings the trap. You can be entirely right on direction and still lose money. After a scheduled event such as earnings or a regulatory decision, the uncertainty resolves and implied volatility can collapse within minutes. This is IV crush: the share moves your way, but the premium deflates faster than the move adds value, and the position ends up down.

The volatility smile: why far options can cost more

Look down the IV column in the table above and you will see it is not flat. The at the money 100p call sits at 27%, but the out of the money 110p call is up at 32%. Plot implied volatility across all the strikes and you get a curve rather than a line, known as the volatility smile, or a skew when it is lopsided.

On the call side the driver is demand. Cheap, far out of the money calls are popular lottery tickets, costing little and paying handsomely if the share surges, and that steady speculative demand lifts their implied volatility. The result is genuinely counter-intuitive and worth sitting with: the 110p call is less likely to finish in the money than the 100p call, yet it carries the richer implied volatility, because people keep buying the long shot.

In real equity markets the curve is usually lopsided rather than symmetrical, and the heavier side is the put side. Investors holding shares buy out of the money puts as insurance against a crash, and that demand is more persistent than the demand for lottery ticket calls, because it comes from people protecting portfolios rather than speculating. So out of the money puts typically carry the highest implied volatility on the chain. If you ever wonder why downside protection feels expensive, this is the reason, and it is not a market inefficiency. It is what insurance costs when many people want it.

IV rank: are options cheap or expensive right now?

A single implied volatility figure tells you very little on its own. Is 27% high? For a utility it would be enormous. For a small biotechnology company it would be unusually calm. The number needs a reference point.

IV rank supplies one by placing the current implied volatility against that share’s own 52 week range, on a scale from 0 to 100.

The broad logic follows from that. A high IV rank favours strategies that sell premium, because you are being paid unusually well by this company’s own standards. A low IV rank favours strategies that buy it, for the mirror reason. You are judging expensive or cheap relative to normal, never in absolute terms, and normal is specific to the share in front of you.

One limitation to carry with you. IV rank compares against the past twelve months only, so a company whose circumstances have genuinely changed can show a low rank while its options are perfectly fairly priced for its new situation. Cheap by historical standards is not the same as cheap.

Choosing your expiry: days to expiry

Days to expiry is the calendar time remaining on the contract, and it governs how much time value you are paying for.

There is a genuine trade-off here rather than a right answer. Too little time and decay works against you brutally, as module 3 shows when it explains why theta accelerates towards the end. Too much time and you are paying for weeks you may not need, since a longer dated option costs more and every extra day of life is priced in. For most directional trades a 30 to 60 day window is the practical middle ground, long enough that decay is still gentle and short enough that you are not overpaying for time.

One rule overrides all of that. Always choose an expiry that falls after any event you are trading. It sounds too obvious to state, and it is a mistake people make regularly, usually because the nearer expiry was cheaper. An option expiring the week before earnings captures none of the move you are betting on. The catalyst arrives after your contract is already dead, and being right becomes irrelevant.

Picking a contract: a liquidity checklist

Put the columns together and choosing a contract stops being a guess and becomes a quick screen. Before committing to any option, run it past four checks.

• Open interest of 500 or more. Enough standing liquidity that you can get in and out at a fair price.
• Spread under 10% of the ask. So that friction is not quietly taxing the trade before it starts.
• 30 to 60 days to expiry, and always past your catalyst date.
• Use a limit order, never a market order. On a wide options spread a market order can fill a long way from the midpoint, and unlike shares there is often no crowd of competing quotes to protect you. A limit order at or near the midpoint controls your entry, and it is worth waiting a few minutes for a fill rather than paying up.

Applied to four candidate calls, the screen is decisive. Note that these candidates deliberately sit across different expiries, which is why the 110p line differs from the 45 day chain above.

The 100p at the money call wins on all three counts, with the deepest liquidity, the tightest spread and a days to expiry figure in the useful window. Look at the bottom row for the lesson, though. The 110p call at 7 days is the cheapest contract on the list in cash terms, and it fails every single test. Cheap in premium and cheap to trade are different things, and the second is the one that decides whether you keep your gains.

One final point about size, and it catches people out when they move between markets. UK listed equity options typically represent 1,000 shares per contract, where the US standard is 100. So one HLXB.L call at 4.4p is not 4.4p of exposure, it is 4.4p × 1,000 = £44 for a single contract, and it controls 1,000 shares worth £1,000. Always confirm the contract multiplier before you work out what a position actually costs, because a factor of ten in the wrong direction is not a rounding error.