Order Book ImbalanceOBI
Order book imbalance compares the volume of resting buy orders with resting sell orders near the current price, as a hint of short-term pressure.
Order book imbalance compares resting buy size with resting sell size near the price. A heavy bid side hints at upward pressure and a heavy ask side at downward pressure, but only over very short horizons.
- Category
- Market Structure
- Reading time
- 3 min
- Also called
- Order flow imbalance, Book imbalance
- Updated
Order book imbalance in plain words
A study of 50 US stocks found that short-term price moves are explained mainly by order-flow imbalance — the mismatch between buying and selling interest at the best bid and ask.1
The relationship is roughly linear, and its strength depends on depth: the thinner the book, the bigger the move.1
A simple ratio is (bid size − ask size) ÷ (bid size + ask size), from −1 to +1.
How to measure order book imbalance
Pick levels
Use the best bid and ask, or a few levels.
Sum sizes
Total resting bid and ask size.
Compute the ratio
(Bid − Ask) ÷ (Bid + Ask).
Track changes
Track how the balance of buying and selling at the best prices is changing.1
Scale by depth
Adjust for depth — the same imbalance moves price more in a thin book.1
Prefer it to raw volume
Prefer imbalance over raw volume; the link between volume and price is much noisier.1
Worked examples
Example 1 — Bid-heavy
+0.50Hypothetical: bid size 6,000, ask size 2,000.
| Ratio | (6,000 − 2,000) ÷ 8,000 = +0.50 |
More demand than supply at the best prices points to short-term upward pressure.1
Example 2 — Ask-heavy
−0.60Hypothetical: bid 1,000, ask 4,000.
| Ratio | −0.60 |
A negative reading suggests sellers dominate; the effect is larger when depth is thin because the slope is inversely proportional to the market depth.1
Example 3 — Auction imbalance
Buy-sideHypothetical close: 1,000 buy shares unmatched.
| Side | Buy |
Exchanges also publish a related figure in auctions: the shares that would go unmatched at the current reference price.2
Book imbalance vs volume delta
| Aspect | Order book imbalance | Volume delta |
|---|---|---|
| Uses | Resting orders | Executed trades |
| Timing | Before trades | After trades |
| Can be faked | Yes, by cancels | Harder |
| Horizon | Seconds | Seconds to minutes |
Where imbalance is used
Short-term trading
Timing entries in Scalping.
Order flow
Part of Order Flow analysis.
Depth
Reading the Order Book.
Execution
Choosing when to place a Limit Order.
Advantages and disadvantages
Advantages
- Simple to compute.
- Backed by market microstructure research.
- Reacts before trades print.
Disadvantages
- Very short-lived signal.
- Spoofed orders can distort it.
- Needs fast, deep data.
- Hidden orders are not counted.
Going deeper
Read as far as you need. Each level builds on the one before it.
Core finding
Beginner
In the short run, the push and pull at the best bid and ask explains most price changes.1
Depth matters
Intermediate
Price responds roughly in proportion to imbalance, more strongly when depth is low.1
Volume vs imbalance
Advanced
Volume alone predicts price changes far less reliably than imbalance does.1
Common mistakes
Holding too long
The edge fades in seconds.
Trusting all size
Orders can be cancelled.
Ignoring hidden orders
Icebergs hide size.
Ignoring fees
Small edges vanish after costs.
Check your understanding
Test yourself before moving on. Each answer comes with an explanation.
Key points to remember
- Imbalance compares bid and ask size.
- Ratio runs from −1 to +1.
- It predicts very short-term moves.
- Thin books magnify it.
- Spoofing and hidden orders distort it.
Frequently asked questions
Is imbalance a reliable signal?
Which levels to use?
Does it work in crypto?
Further learning
- Order Book
Where imbalance lives.
- Order Flow
Wider method.
- Scalping
Short-term use.
- Liquidity
Depth and impact.
Sources
This article is written in our own words. The sources below support the specific facts, rules and figures marked with a number in the text. Links open the original publisher’s page.
- arXiv (Cont, Kukanov, Stoikov). The Price Impact of Order Book Events (opens in a new tab) Checked .
- Nasdaq. Opening and Closing Cross FAQs (opens in a new tab) Checked .
Educational content, not financial advice. Examples use hypothetical prices and simplified costs; actual fees, margin rules and regulations vary by broker, market and country. We are not affiliated with any publisher linked above. Last reviewed . Report an error or request a source credit.