Counting the open contracts

Next to a future's price and its volume sits a third number that beginners often ignore and experienced traders watch closely: open interest. On its own it is just a count. Read alongside the price, it starts to whisper about whether a move has real conviction behind it or is just the market tidying up old positions. This chapter teaches that reading, and, just as importantly, its limits.

What open interest is

Open interest is the number of futures contracts that are currently open, that is, positions that have been opened and not yet closed or expired. Every futures contract has two sides, a long and a short, and open interest counts the contract as one: one open long matched with one open short is one unit of open interest.

Open interest goes up when new positions are created and down when existing ones are closed. If a fresh buyer and a fresh seller both open a new position, open interest rises by one. If an existing long and an existing short both close, it falls by one. And if a new trader simply takes over from one who is leaving, one opening while the other closes, open interest does not change at all, because the number of open positions is the same.

Open interest is not volume

It is easy to confuse open interest with volume, but they measure different things. Volume is the number of contracts traded during the day, counting all the buying and selling activity, including a trader who opens and closes within the same day. Open interest is the number of positions still standing at a moment in time, regardless of how much trading it took to get there.

Think of a hall. Volume is how many people walked through the door today, including those who came in and left again. Open interest is how many people are actually in the room right now. A day can have huge volume, lots of coming and going, and end with open interest barely changed, if most of the activity was traders opening and closing rather than building new positions.

The four combinations

The reading that traders care about comes from putting the day's price change beside the day's open-interest change. There are four combinations, and each tells a small story about who was driving the move.

The useful intuition is this. When price and open interest rise together, new longs are backing the move with fresh commitment, which suggests conviction. When price rises but open interest falls, the rise is being driven by shorts closing out rather than by new buyers, which can be a weaker, less durable move once the covering is done. The same logic mirrors on the downside: a fall on rising open interest is fresh shorts pressing, while a fall on falling open interest is just longs giving up. A move backed by new positions generally carries more weight than one driven by old positions closing.

A quirk to know: the F&O ban

There is one India-specific consequence of open interest worth knowing, because it can catch a stock-futures trader by surprise. To limit concentration, the exchange sets a market-wide limit on how much open interest can build up in a single stock's derivatives. When a stock's open interest crosses that limit, it enters a ban period, in which no new positions may be opened, and traders may only reduce or close existing ones, until the open interest falls back under the threshold. A stock in this ban still trades, but you cannot start a fresh position in it. It is a reminder that open interest is not just a sentiment gauge; at its extreme it becomes a hard rule..

The honest limit

Read open interest as a clue, never as an instruction. The four combinations above describe what happened, not what will happen next, and the same open-interest change can have more than one story behind it. A rise in open interest with price could be genuine new conviction, or it could be one large trader building a position that reverses tomorrow. Traders use open interest alongside price, volume, and the basis to build a picture, but no single reading of it is a signal to trade on by itself.

What to carry forward

Open interest counts the futures positions currently open, rising when new positions form and falling when they close, and it is different from volume, which counts the day's activity. Beside the price it gives four readings, from long build-up to long unwinding, that hint at whether a move is backed by fresh money or by old positions closing, and at its extreme it becomes the F&O ban that stops new positions in a crowded stock. But it describes the past, not the future, and belongs in a wider picture rather than a lone signal. The next chapter turns to the most respectable thing you can do with a future, and the one where your risk goes down rather than up: hedging.