Order book imbalance

What it is

Order book imbalance is a measurable skew between the resting bid quantity and the resting ask quantity in the depth ladder. The most common definition takes the top N price levels on each side, sums their sizes, and expresses the result as either a ratio (bid size divided by ask size) or a normalized score between -1 and +1. When bids vastly outweigh asks, the book is bid-heavy; when asks dominate, it is offer-heavy.

The metric exists because the visible order book is the only forward-looking liquidity signal a trader has. Trade prints tell you what already happened; the resting book tells you what is prepared to happen. Imbalance compresses that prepared liquidity into a single comparable number that can be tracked across time.

Why it matters

Persistent order book imbalance is interpreted by some intraday traders as short-term directional pressure: if buyers consistently stack more size below the inside bid than sellers stack above the inside ask, that asymmetry can precede upward moves. Combined with delta and tape behavior, imbalance becomes a confirmation input — does the resting book back up what aggressive flow is doing, or contradict it?

Practical uses include:

• Confirming a breakout where aggressive flow is matched by thinning opposing book.
• Fading moves where the book against the trade direction stays thick.
• Spotting regime changes — sudden flips in imbalance often coincide with absorption or order pulls.

How it appears on Sierra Chart

Sierra Chart records full historical market depth in its .depth files when the data feed supports it, and exposes the live order book through the DOM and Market Depth Historical Graph. Imbalance itself is not a single built-in indicator number, but custom studies built on ACSIL's depth APIs can compute and plot any imbalance metric — top-N ratio, weighted imbalance, time-series of imbalance — over time.

The Market Depth Manager from SCS works with this depth data directly, focused on practical visualization and depth-history management rather than imbalance scoring.

Common patterns / pitfalls

• Order book imbalance is contaminated by spoofing — large displayed orders that get pulled before they fill. Treat lone giant bids or offers with skepticism unless they actually transact.
• The signal degrades quickly in thin overnight conditions or around news events when participants pull liquidity entirely.
• Top-of-book imbalance and N-levels-deep imbalance can disagree. Pick a definition and stick with it across the playbook.
• Imbalance lags structurally on slow-moving instruments and leads on fast-moving ones — calibrate to the specific market.

Related SCS studies

The Market Depth Manager surfaces the depth data that any order book imbalance computation requires, with a focus on managing historical depth recording and visual inspection of the book. Custom imbalance studies built on top of c_ACSILDepthBars or live sc.GetBidMarketDepthEntryAtLevel calls can layer onto the same data.