Market Impact
Understand the price movement caused by your own order, how it differs from ordinary market movement and slippage, and why impact usually grows non-linearl
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Core concept
Market impact is the change in market price or execution path caused by the trader’s own order. It can be temporary—liquidity replenishes after the order—or more persistent if the trade conveys information or shifts other participants' expectations.
Impact is related to slippage but narrower: slippage includes any difference between expected and realised execution, while market impact isolates the component attributable to the order itself.
How the mechanics fit together
Notional and urgency.
Share of available liquidity / market volume consumed.
Depth is consumed and other participants respond.
VWAP and post-trade price differ from the no-trade counterfactual.
| Execution choice | Impact trade-off |
|---|---|
| Execute immediately | Higher certainty, usually more impact and spread crossing. |
| Slice over time | Can reduce instantaneous impact but increases market-movement and information-leakage risk. |
| Passive limits | Potentially lower impact/fees, but uncertain fills and adverse-selection risk. |
| Route across venues | May access more liquidity but adds fragmentation, latency and operational complexity. |
Evidence to inspect
- Order size relative to near-touch depth and recent traded volume.
- Participation rate over the intended execution window.
- Historical implementation shortfall for similar size buckets.
- Post-trade price reversion: rapid reversion suggests temporary impact; persistence may indicate information or broader flow.
- Cross-venue reaction and whether liquidity providers pull quotes as the order executes.
- Volatility regime and scheduled event risk.
Practical workflow
- Choose an arrival/reference price and intended quantity.
- Estimate depth and expected fill cost if executed immediately.
- Calculate expected participation rate if sliced over time.
- Compare immediate, passive and staged execution scenarios.
- Set maximum acceptable implementation shortfall or price range.
- After execution, compare realised cost with the original benchmark and review how much price reverted.
Worked example / thought exercise
A venue shows £40,000 of cumulative asks within 10 bps and £100,000 within 50 bps. A trader needs to buy £150,000 immediately.
Even if the displayed spread is only 4 bps, the order exceeds near-touch depth and is likely to move through substantially worse prices. The trader’s own demand therefore becomes a material component of execution cost.
If the same £150,000 is split over 30 minutes, instantaneous impact may fall—but the trader accepts the risk that the market moves higher during the execution window.
Which choice is better depends on what: spread alone, or the trade-off between urgency, impact and market-movement risk?
Common mistakes and misunderstandings
Calling every adverse move market impact
Price can move for unrelated reasons while an order is executing. Impact is specifically the component caused by the trade.
Assuming impact scales linearly
Liquidity curves and books are not uniform. Larger orders can experience disproportionately higher cost.
Ignoring participation rate
A £1m order can be trivial in one market and enormous in another.
Slicing without considering information leakage
Repeated predictable child orders can allow other participants to anticipate the remaining flow.
Knowledge checkpoint
- What distinguishes market impact from general slippage?
- Why can doubling order size more than double execution cost?
- What new risk is introduced when you reduce impact by extending the execution horizon?
FAQs
❓ Is market impact always permanent?
No. Some impact reverses as liquidity replenishes; some persists if the trade conveys information or coincides with broader order flow.
❓ How is impact reduced?
Common methods include reducing order size, slicing execution, using passive orders, routing across venues or using specialised execution algorithms.
❓ Is low spread evidence of low market impact?
Not by itself. A book can have a tight spread but very little depth behind the best quote.
❓ Why does participation rate matter?
The larger your order relative to available trading activity, the more likely it is to consume liquidity and alter the price path.
📋 Summary
Market impact is the price effect caused by your own order. It depends on size, urgency, depth, participation rate and market regime. Execution design balances impact against non-fill, timing and information-leakage risks.
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