Open Interest and Volume

Introduction

Look at any options quote and, beside the bid, the ask and the last price, you will find two more numbers: volume and open interest. They are easy to skip past, yet they answer a question that matters enormously in practice — how easily can I get in and out of this contract, and at a fair price? An option that looks cheap can cost you dearly if almost no one trades it, because the price you actually pay to enter and the price you receive to exit can be far apart.

Volume and open interest are the two liquidity gauges of the options market. They are often confused, because both count contracts, but they measure very different things. This lesson pulls them apart, shows what each tells you, and explains why neither one — despite a persistent myth — predicts which way the market will move.

Quick Definition

Volume is the number of option contracts traded during the current session; it resets to zero each day. Open interest is the running total of contracts that remain open — created but not yet closed, exercised or expired.

Put simply: volume is today's activity; open interest is the total pool of live positions.

Volume: The Day's Activity

Volume counts every contract that changes hands during the session. Buy ten contracts and sell them an hour later and you have added twenty to the day's volume (ten on the way in, ten on the way out). It is a measure of how busy a particular strike and expiration were today, and it resets to zero at the next open.

High volume tells you a contract is actively being traded right now — useful around news, earnings or a sharp move, when attention concentrates on particular strikes. Low volume warns that few people are trading it today, which often means you will pay more to get filled.

Open Interest: The Live Pool

Open interest is subtler. It is the total number of contracts that have been opened and not yet closed. It rises when new positions are created and falls when positions are retired — it does not simply tick up every time a contract trades.

The bookkeeping depends on whether each side of a trade is opening or closing:

• Both sides opening a new position → open interest rises by one.
• Both sides closing existing positions → open interest falls by one.
• One opening, one closing → open interest is unchanged (the position simply transfers hands).

So a busy day (high volume) can leave open interest higher, lower or flat, depending on whether traders were net opening or net closing. Open interest is best read as the depth of standing participation in a contract — how many live bets are riding on that strike.

Why They Matter: Liquidity And The Spread

The practical reason to care about both numbers is liquidity — the ease of trading a contract without moving its price against yourself. Liquidity shows up most directly in the bid-ask spread, the gap between the best price to buy and the best price to sell.

• Liquid options (healthy volume and open interest) attract many competing market makers and traders. Their quotes bunch together, so the spread is narrow — you buy near fair value and sell near fair value, losing little to the round trip.
• Illiquid options (thin volume and open interest) have few competing quotes. The spread is wide, so you overpay to enter and get shortchanged to exit — and in a fast market you may not be able to close near fair value at all.

That spread is a real, recurring cost that never appears on a commission statement. On a heavily traded near-the-money option it might be a penny or two; on an obscure, far out-of-the-money strike it can be a large fraction of the option's whole price. Checking volume and open interest before you trade is how you avoid that trap.

Reading Them Together

Volume and open interest are most informative side by side:

• High open interest, healthy volume: a liquid, well-established contract — usually the easiest and cheapest to trade.
• High open interest, low volume today: lots of standing positions but a quiet session — still likely tradeable, just not busy right now.
• Low open interest, high volume: fresh attention, perhaps around news — participation may be building, but depth is still thin.
• Low open interest, low volume: an illiquid contract to approach with caution; expect a wide spread and difficulty exiting.

As a rule of thumb, the most liquid options are near-the-money, near-dated strikes on large, popular underlyings; far out-of-the-money and long-dated strikes are frequently thin.

Risks & Considerations

• Neither number tells you direction. Every open contract has a buyer and a seller; open interest measures participation, not sentiment.
• Thin contracts have wide spreads — a hidden cost that can dwarf commissions and quietly erode returns.
• Liquidity can vanish in a fast-moving market exactly when you most want to exit, widening spreads further.
• Volume can be misleading in isolation — a single large trade can spike it without meaning the contract is generally liquid.
• Always sanity-check the spread itself, not just the headline liquidity numbers; the spread is what you actually pay.

Common Misconceptions

• "Rising open interest means the price is about to rise." No — it means more contracts are open. It is a liquidity and participation gauge, not a directional forecast.
• "Volume and open interest are the same thing." Volume is today's trades and resets daily; open interest is the standing total of live positions.
• "A high last price means the option is liquid." Price says nothing about liquidity. Check volume, open interest and the spread.
• "If it's listed, I can always get out easily." Illiquid strikes can be very hard to exit near fair value; being listed is not the same as being tradeable.

Real-World Application

An investor spots two calls on the same stock with the same expiry. The near-the-money strike shows thousands of contracts of volume, tens of thousands of open interest, and a bid-ask spread of just £0.03. A far out-of-the-money "lottery" strike shows a volume of 4, open interest of 11, and a spread of £0.35 on an option worth £0.60 — meaning more than half its value could be lost simply crossing the spread to get in and out. The headline premium made the long-shot look cheap; its liquidity revealed the truth. The investor sizes down and sticks to the liquid strike, because an option you cannot exit at a fair price is not the bargain it appears to be. Reading volume and open interest turned a tempting number into an informed decision.

Key Takeaways

• Volume is the contracts traded during the session (it resets daily); open interest is the running total of contracts still open.
• Open interest rises when both sides open, falls when both sides close, and is unchanged when a position merely transfers hands.
• Together they gauge liquidity, which shows up in the bid-ask spread — the real, hidden cost of entering and exiting.
• The most liquid options are usually near-the-money, near-dated strikes on large, popular underlyings.
• Neither number predicts direction — every open contract has a buyer and a seller. They tell you how tradeable an option is, not which way it will go.