Market Impact: How Larger Orders Worsen Average Execution Prices

A market buy may seem likely to fill at the displayed price, but the actual fill is higher. If quantity exceeds the first ask, the remainder fills at the next level, and then the next. The amount your own order pushes price upward is market impact. You can calculate it beforehand from order-book quantities rather than guessing.

A Fill Is a Weighted Average Across Levels

A market buy consumes asks from the lowest price upward. When one level has insufficient quantity, it moves to the next. The final execution price is the quantity-weighted average of the consumed levels.

At this size the cost is effectively negligible. Problems begin when the order cannot finish within the first two levels. See reading the order book for structure and depth, and limit versus market orders for order differences.

Six Times the Quantity Can Mean 21 Times the Cost

Increasing quantity in the same book shows the worsening unit price.

Costs do not scale linearly because two factors multiply: larger quantity reaches worse prices, and the worse unit price applies to more quantity. Doubling size can therefore bring costs nearer four times than twice. See why slippage increases and how to reduce it for situational causes.

The Same $20,000 Has Different Effects Across Coins

What matters is your order's share of that order book, not its absolute amount. $20,000 cannot consume even the first BTC ask above, but it crosses five levels in a thin altcoin.

The same amount and market-order button cost 0% in one market and 0.87% in the other. The reason is explained by liquidity.

Leverage can obscure this gap. With $1,000 margin at 20×, the order hitting the book is $20,000, not $1,000. Small capital can still create a large order. See leverage for the relationship between the multiple and actual exposure.

You Pay on Entry and Again on Exit

Market impact does not end at entry. A market exit consumes bids downward.

Targeting 2% but retaining only 0.18% is a cost-structure problem, not a direction-prediction problem. See round-trip trading costs to combine fees, spread, slippage, and funding, and spread costs to isolate the bid-ask spread.

Urgent stops can be even more expensive. During fast movements, depth thins and an order normally slipping 0.2% can slip 1%. The moment you most want to leave can be the most expensive moment to do so.

How Much Can Splitting Save?

Instead of crossing five levels at once, four slices can avoid the higher levels as quotes replenish between orders.

This depends on replenishment. In a persistent directional move, price can rise while you split, leaving the remaining purchase at worse prices than the impact saved. The exit-side process is covered in scaling out.

A limit order reduces market impact to zero but accepts nonexecution risk. See limit-order queue priority for why other orders at the same price fill first, and unfilled limit orders for reasons an order may not execute at all.

One Number to Check Before Ordering

You do not need to multiply every level each time. Comparing your order with cumulative depth across the top five levels usually suffices.

The 20% and 30% levels are starting points, not absolute rules. Measure several orders in your asset and trading hours, then adjust. Build a habit of checking the book before ordering. If your share is large, reduce size or use a limit. See position sizing for working backward from risk to order size.

Someone also earns these costs. Market makers place limits on both sides and absorb others' market orders. Costs paid by people entering urgently become their source of revenue.

Recap

Without knowing your order's share of the book, you cannot know the entry price. For short-term trades targeting 1–2%, this calculation can determine the result before the win or loss itself.