Explainer, in Market Microstructure
Order Book Imbalance
OBI = (bid_volume − ask_volume) / (bid_volume + ask_volume), aggregated over some depth (the touch only, top-N levels, or volume-weighted depth). Values near +1 indicate dominant buying interest; near −1, selling. Depth-weighting and the choice of which level to include dominates how the metric behaves.
1 question. Published May 10, 2026. By AI Fin Hub Research.
Definition
Order book imbalance
OBI = (bid_volume − ask_volume) / (bid_volume + ask_volume), aggregated over some depth (the touch only, top-N levels, or volume-weighted depth). Values near +1 indicate dominant buying interest; near −1, selling. Depth-weighting and the choice of which level to include dominates how the metric behaves.
Why it matters
Short-term price changes are driven by who exhausts the queue first. OBI captures the imbalance that predicts which side the next trade is most likely to consume. It's the strongest single short-horizon predictor on most equity venues — and it's been heavily harvested by HFT, so the alpha decay is fast.
How it works
Snapshot the order book at decision time. Choose depth (top-1, top-5, full book). Compute weighted volume on each side. Aggregate to OBI. Microprice = (bid × ask_volume + ask × bid_volume) / (bid_volume + ask_volume) is a related quantity that already incorporates OBI into a price estimate.
Example
5-level deep book on a liquid name
Top-of-book bid size
1,200 sh
Top-of-book ask size
300 sh
OBI (top-of-book)
+0.60
OBI (top-5 levels)
+0.18
Top-of-book signals strong buying pressure; depth-weighted signals only mild. Choosing the depth changes the strategy entirely — top-of-book OBI is a millisecond signal, deep OBI is a minute signal.
Key takeaways
OBI signal strength decays with depth; top-of-book is fastest, deepest is slowest.
Microprice is the natural mid-quote when OBI is non-zero.
OBI alpha has been heavily harvested by HFT — pure OBI strategies rarely survive in retail.
Related terms
Run the numbers next
- Playgrounds Order Book Replay Visualizer
Drop a Level-2 snapshot CSV and replay the book snapshot by snapshot. Animated depth bars, best bid/ask, spread and imbalance. Understand microstructure before.
- Playgrounds Execution Simulator
Estimate execution cost in closed form — square-root permanent impact, linear temporary impact, half-spread, and a latency-drift band. See the slippage a naive backtest hides.
Questions people ask next
Does OBI work outside of equities?
Yes — the structural imbalance signal is generic to any limit-order-book market. It's been documented in futures, spot FX, and crypto. Strength varies by venue depth and HFT participation.
Sources
- Order Book Dynamics in Liquid Markets , Cont, Stoikov, Talreja (2010), arXiv:1101.5392
Keep the topic connected
- Learn Bid-Ask Spread
Bid-ask spread defined: quoted vs effective vs realized spread, why the touch isn't the cost you actually pay, and how to measure each.
- Learn Latency Arbitrage
Latency arbitrage: cross-venue price discrepancies exploited by being faster than the slowest replicator. Why the game is mostly won at the cable layer.