In Futures Contracts, How Is the Effective Spread Calculated When Trading a Large Block?

When trading a large block of futures, the effective spread is calculated similarly to other derivatives. It is twice the absolute difference between the execution price and the mid-price of the market's best bid and offer at the time the order was submitted.

However, for block trades, the execution price is often negotiated privately and reported after the fact, potentially allowing for a price that is better than the visible order book, thus narrowing the effective spread.

Why Is the Bid-Ask Spread on a Futures Contract Often Tighter than on the Spot Market?
What Is the Effective Spread and How Does It Differ from the Quoted Spread in a Thin Market?
What Is the Practical Implication of a “Wide Mid-Price” in an Illiquid Options Market?
Explain How the Effective Spread Is Used as a Metric for Broker Execution Quality
How Do Exchanges Ensure the Transparency of Block Trade Execution Prices?
What Is the “Mid-Point Peg” Order Type Commonly Used in Dark Pools and How Does It Function?
Can a Trader Switch between Isolated and Cross Margin Mid-Trade?
In an Option Spread Strategy (E.g. a Bull Call Spread), How Many Times Does the Bid-Offer Spread Cost Factor In?

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