Updated: 2026-02-20
In trading, Market impact is the price movement your own order causes when you trade enough size to move the book. This glossary entry explains why market impact matters, how traders use it, and how to track it with evidence instead of vibes.
Quick definition
Market impact: the price movement your own order causes when you trade enough size to move the book.
Market impact is the price movement your own order causes when you trade enough size to move the book. The practical version is: can you define it as a field you can log and audit later?
Most trading terms become confusing when they are used as vibes instead of variables. Your goal is a definition that helps you decide size, stop, entry timing, or whether to skip the trade.
Traders sometimes confuse Market impact with slippage. Treat them as separate variables in your journal so your reviews stay honest.
Market impact is the execution tax that appears when your size is large relative to available liquidity. It can quietly erase edge, especially in small caps or during thin windows.
If Market impact never changes your decision, it is just jargon. The term earns its place when it improves your process consistency under real market pressure.
A useful mental model: plan first (risk and invalidation), execute second (order type and fills), review last (tags and metrics).
Use it to make one decision pre-trade. Example decisions: where the stop goes, whether to take partials, how to scale size, or whether conditions are too thin to trade.
Write the rule in one sentence, then run it consistently for a week. Consistency matters because it creates comparable data for review.
If the rule fails, adjust slowly. Do not rewrite the whole system after one bad session.
Track average slippage versus order size and measure how impact changes with volatility. If slippage increases non-linearly with size, you are hitting impact.
Use tags so you can slice results by regime and behavior state. The same term behaves differently when volatility changes or when you are fatigued.
Your review question should be binary: did this variable improve outcomes or reduce rule breaks? If not, simplify.
A $5,000 market order moves price 2 bps, but a $50,000 order moves price 25 bps. The curve is not linear, and your strategy does not scale the way you think.
The point of an example is not to predict price. It is to show what you would log before the trade and what you would audit after the trade.
Assuming you can scale size linearly without measuring how execution costs change.
The fastest way to improve market impact is to remove one failure mode at a time. If you try to fix everything, you will fix nothing.
Market impact matters most when volatility is high and the book is thin. That's where small execution errors compound into expectancy drag.
Before you trade, decide what matters more: price control (limits) or fill certainty (markets/stops). Then trade the choice consistently for one week so your data is comparable.
If you change order types every time you feel stressed, your metrics will lie to you.
Market impact is the price movement your own order causes when you trade enough size to move the book. In practice, it matters when it changes a concrete decision like size, stop placement, or whether you skip a trade.
They are related but not identical. In your journal, track Market impact as its own variable and treat slippage as a separate context factor so you can audit each cleanly.
Track average slippage versus order size and measure how impact changes with volatility. If slippage increases non-linearly with size, you are hitting impact.
Assuming you can scale size linearly without measuring how execution costs change.
See plans and run one weekly review loop with Tiltless: edges, leaks, and enforceable next actions.
