Reading the Open Interest Heatmap

What the Colour Actually Encodes

Each cell on the board is one strike at one point in time. The number printed in the cell is the open interest standing at that strike. The coloured fill behind the number is that open interest divided by the largest open interest on the board, drawn as a width and an opacity. Call cells fill from the right in a red tone, put cells fill from the left in a green tone, so the two sides of the chain read as one continuous shape rather than two lists.

Both cells above sit on a board whose heaviest strike holds 20,00,000 contracts. That single strike is what sets the scale. The call cell holds 17,00,000, so it fills to 85 percent. The put cell holds 13,00,000, so it fills to 65 percent. Neither fill is a measurement of anything except its share of the largest number in front of you.

This is why the number stays printed in every cell. Use the colour when you want the shape of the board in one glance. Use the number the moment you want to know how big something actually is.

Every Shade Is Relative

This is the part that trips people up, so it is worth stating flatly: a dark cell does not mean a large position. It means a large position for this board, right now. Two consequences follow, and both of them cost people money.

You cannot compare shades across boards. A solid red cell on a BANKNIFTY heatmap and a solid red cell on the heatmap of a mid-cap F&O stock can differ by two orders of magnitude in contracts. Same colour, completely different amounts of committed capital. The same applies across expiries, because each series is normalised against its own maximum.

The scale moves under you during the session. When one strike takes on a large fresh position, the board maximum rises and every other cell fades in step, even though none of them lost a single contract. The table below is the same four strikes three hours apart. Three of them did not change at all.

None of this makes the shading useless. It makes it a tool for one specific job: seeing the shape of the distribution instantly. Rank, spacing and clustering are exactly what a relative scale is good at showing. Magnitude is what the printed number is for.

Call Walls Above, Put Walls Below

The heaviest call strike sitting above spot and the heaviest put strike sitting below it are the two cells most readers go to first. Heavy call open interest above the market is commonly read as resistance, heavy put open interest below it as support. That reading is a crowding observation and not a rule — the levels break regularly, usually in the session where that open interest starts unwinding quickly.

The words above and below are doing real work in those definitions. A heavy call strike that is now underneath spot is not resistance, and a heavy put strike now sitting above spot is not support. Those cells are heavy because of where price used to be, and the open interest in them is in the money and being managed rather than defended. Sorting the board purely by shade, with no regard for which side of spot a strike is on, is the single most common way to come away with the wrong level.

The mirror case is worth watching too. When the heaviest put strike is sitting above spot after a fall, the board is telling you that support was breached rather than that support is overhead. Read the geography before you read the colour.

One Dominant Strike, or a Broad Plateau

Once you have the walls, the next question the heatmap answers is how much conviction sits behind them. That is a question about shape, and it has three common answers.

A single dominant strike is the cleanest picture you get. One price has attracted the crowd, and the strikes on either side of it are thin. That thinness cuts both ways. Approaching the level, you have a precise number to work against. Once price is through it, there is very little standing behind it, which is why a break of a lone peak so often runs further and faster than the size of the wall would suggest.

A broad plateau is the opposite situation and is frequently misread as strength, because a wide band of colour looks impressive. It is not one wall — it is the absence of one. When eight strikes carry roughly equal open interest, no individual price has a constituency defending it, and the underlying tends to travel through the band without much resistance at any single strike. The usable level in that case is the outside edge of the band, not the darkest cell inside it.

Twin peaks usually resolve rather than persist. One of the two is typically legacy positioning that has not been closed out and the other is being built now. Watching which cell keeps its shade over the following sessions, and which one drains, tells you which level the market is actually working with.

How the Picture Changes Into Expiry

A heatmap read on the first day of a series and one read on the expiry day set by the exchange are not the same kind of object, and treating them the same is a reliable way to get confused.

Two things happen as expiry approaches. First, open interest in the front series concentrates. Far strikes decay towards nothing because there is no longer time for the underlying to reach them, while strikes near spot thicken. The board narrows into a tight band and everything outside it goes pale. That is partly genuine concentration and partly the scale doing its job again — as the maximum rises, distant strikes that lost nothing still fade.

Second, positions roll. Open interest starts building in the next series while the front one drains. If your board is pinned to the front expiry, the collapse you see in the final sessions is largely migration rather than a change of view.

Those two cells are drawn here on one common scale so they can be compared directly. Your board will not do that — it normalises each expiry separately, which is precisely why you have to switch series and look, rather than infer it from how the front month is shaded.

The practical consequence is that a heatmap gives you its most stable read in the middle of a series, when open interest has had time to build but the roll has not started. Early in a series the board is thin and a single trade can dominate the scale. In the last sessions it is compressed and contaminated by the roll.

Read the Range, Not the Direction

A heatmap is a map of where positions are, not of what anyone intends to do next. It answers where, and it is very good at that. It does not answer which way, and asking it to is where most of the bad conclusions come from.

Two readers with opposite views can look at the same board honestly. What they should agree on is the band: the heaviest call strike above spot, the heaviest put strike below it, and how thick or thin the strikes in between are. That band is the deliverable. Where the underlying goes inside it, and whether it leaves, is a question for price.

Direction, when it comes from the option chain at all, comes from what open interest is doing rather than what it is. Fresh writing into a level while price pushes against it is a different message from open interest draining out of that level while price pushes against it, and the total-OI heatmap shows neither — both boards look similar. That read lives in the change-in-OI view and in the price-plus-OI build-up matrix, which is covered in the open interest guide.

The same distribution the heatmap shades is also the input to the max pain calculation, which reduces the whole board to a single strike by asking where the open contracts would settle most cheaply. If the heatmap band and that strike point at the same neighbourhood, positioning is at least internally consistent. The max pain guide works through that arithmetic step by step.

Three things to keep in the back of your mind whenever you use the board. Heavy open interest at a strike is a crowd, not a barrier, and crowds get run over. A level that is holding is only visible as a level while it holds. And the shading tells you nothing at all about size until you have read the number underneath it.