Delta is the difference between volume that traded at the ask and volume that traded at the bid. It is one of the most useful numbers on a footprint chart and one of the most misread. The misreading is usually the same: positive delta means buyers are winning, so price should go up. Sometimes. The number says who was impatient, not who was right.
What the sign actually records
A trade at the ask is a buyer who crossed the spread to get filled now. A trade at the bid is a seller who did the same. Delta therefore counts aggression. It says nothing about the passive side of each trade, and the passive side is where the size usually lives. When a bar prints heavy positive delta and price does not rise, the aggressive buyers were met by a seller large enough to fill every one of them at the same price. That is absorption, and it is bearish, and the delta was positive the whole time.
So the honest read of delta is always a comparison: what did this much aggression buy? Heavy positive delta with a big up move is buyers in control. Heavy positive delta with no move is buyers being sold to. Light positive delta with a big up move is a thin book, a vacuum, and vacuums fill in both directions.
Cumulative delta and its blind spot
Cumulative delta runs the sum across a session and is read for divergence: price makes a new high and cumulative delta does not, so the move is "weak". The logic is sound and the number is fragile. Cumulative delta depends entirely on where you start the sum and on the feed's ability to classify every trade. Start it at a different time and the divergence appears or disappears. Feed a classification rule that guesses on trades inside the spread, and the line drifts in a direction that has nothing to do with the market.
Imbalances are a ratio, not a signal
A footprint imbalance compares the ask volume at one price to the bid volume one tick below it. A ratio of three to one or more is flagged. Stacked imbalances in the same direction show a sequence of prices where one side kept crossing and the other side did not keep up. That is genuinely informative about who was in a hurry. It is not informative about whether they were filled by someone bigger and calmer, which is the thing you need. An imbalance that resolves with price moving away from it was initiative. An imbalance that resolves with price sitting on it was absorbed. Same print, opposite meaning, and the ratio cannot tell the two apart.
A practical order of reading
- Price first: did it go anywhere? How far, how fast, on what volume?
- Delta second: how much aggression did that take? Little aggression for a large move means a thin book. A lot for no move means someone absorbed it.
- Imbalances last, as a location for where the aggression concentrated, so you know which prices to watch for the reaction.
Read in that order, delta becomes what it is: the cost of the move. Read as a direction, it is a coin that lands heads about as often as the market goes up anyway.