Quick definition Order book imbalance occurs when there is significantly more volume on the bid side (buy orders) than the ask side (sell orders), or vice versa. A large imbalance suggests directional pressure on price. What it is The order book has two sides: buy orders and sell orders. If 1 million shares are bid at multiple price levels and only 100,000 shares are offered, there is a large buy-side imbalance. This imbalance suggests that buyers are more aggressive than sellers and price may move higher to attract selling. Imbalances are temporary. As price moves, the imbalance usually corrects (buyers sell into higher prices, sellers buy into lower prices, new orders arrive). Why it matters Order book imbalance is a signal of short-term price direction. A large buy-side imbalance is often followed by a price increase. A large sell-side imbalance is often followed by a price decrease. High-frequency traders use order book imbalance as a trading signal. They have proprietary models that incorporate imbalance data and predict short-term price moves. Measuring imbalance Imbalance can be measured many ways. A simple measure is (bid volume - ask volume) / (bid volume + ask volume). This produces a number between -1 (all sell) and +1 (all buy), with 0 meaning balanced. Practical example You are watching the Apple order book. At 10:32:15 AM: - Bid: 150 million shares across all price levels - Ask: 40 million shares across all price levels - Imbalance: +78 percent This large buy-side imbalance suggests strong buying pressure. You expect the price to move higher as buyers exhaust the available ask liquidity. You buy shares anticipating the move. By 10:32:20 AM, the price has moved from 150.50 to 150.60 and new sellers have arrived. The imbalance is now balanced again. Mechanical imbalances Some imbalances are mechanical. If large buyers route orders to a venue and large sellers route orders elsewhere, the imbalance at this venue is not reflective of overall market direction. Imbalances that persist across all venues are more meaningful than imbalances at a single venue. See also - Order Book - Depth-of-Book - Buying Pressure - Price Discovery
