Measuring Market Impact Without Trading

The most direct way to learn what a large order costs is to execute it and measure the result. That is also the most expensive way. Before trading, desks need an estimate, and order-book data provides a transparent starting point.

Walking the book

Given a snapshot of the book, the cost of an immediate market order of a given size can be computed by consuming resting quantity level by level until the order is filled. The difference between the resulting average price and the mid is the static impact for that size at that moment.

Why static impact underestimates

• Displayed quotes may be withdrawn as soon as aggressive flow appears.
• Other participants may trade ahead of a detected large order.
• Splitting an order over time reduces immediate impact but introduces exposure to price drift.
• Hidden liquidity can partially offset these effects, but cannot be observed in advance.

Empirical work across electronic markets has repeatedly found that impact grows with order size less than linearly — often approximated by a square-root relationship to size relative to traded volume. Book-based estimates are therefore best used as a floor, calibrated upward with observed fills.

This publication is provided for informational purposes only and does not constitute investment, legal, or tax advice, or an offer or solicitation to buy or sell any asset. Live figures are computed from third-party public market data and may be delayed, incomplete, or inaccurate.