What Is a Block Trade?
A block trade is a privately negotiated purchase or sale of a large quantity of securities executed outside the open order book. The traditional threshold is 10,000 shares of stock or $200,000 in value, but real institutional blocks frequently run into the millions of shares and hundreds of millions of dollars. Mutual funds, pension funds, and hedge funds use blocks when their order size would overwhelm normal market liquidity.
The problem a block solves is price impact. Dumping a huge sell order into the public market would signal the seller's intentions and push the price down before the order finished executing. By negotiating the trade privately, often through a bank's block desk or in a dark pool, the institution transfers the position at a single agreed price.
How Block Trades Work
Most blocks go through an investment bank's equity trading desk, historically called the upstairs market. In a risk trade, the bank buys the entire block from the client at a discount to the market price, then works out of the position over time, earning money if it can resell above its purchase price and losing if the stock drops first. Alternatively, the desk can act as agent, quietly finding institutional buyers to cross the block without taking the shares onto its own balance sheet.
A prominent variant is the overnight block offering, where a large shareholder such as a private equity sponsor sells a stake in a public company. Banks bid after the close for the right to buy the block, typically at a 2% to 8% discount to the last trade, then place the shares with institutions before the next open. Executed blocks must still be reported to the tape, so the market learns of the trade after the fact.
Why Block Trades Matter
Block trading is where institutional size meets limited liquidity, and handling it well is a defining skill of equity sales and trading. Pricing a block requires judging how much stock the market can absorb, who the natural buyers are, and how much discount compensates for the risk. Misjudging a block can cost a desk millions in minutes, as the wave of Archegos-related blocks in March 2021 demonstrated when banks raced to sell more than $20 billion of stock.
Blocks also raise sensitive information issues. Advance knowledge that a large block is coming is market-moving, and in 2022 US regulators investigated whether banks had tipped favored clients ahead of block sales; Morgan Stanley paid $249 million in 2024 to settle related charges. Understanding block mechanics, including discounts and price impact, is useful in both markets and banking interviews.
