What is price impact on a DEX, and does splitting a big trade help?

Swapping against a pool

Uniswap's documentation says its swaps work differently from an order book. They are not matched against separate orders. They trade against a shared pool of tokens, supplied by liquidity providers who earn fees.¹

During a swap, the pool's price shifts continuously. So the final price you get lands somewhere between the price at the start and the price at the end.¹ Uniswap calls this price impact, and says every swap on its protocol has some.¹

Big trade, small pool

Uniswap's documentation says the more liquidity there is at a price, the lower the price impact of a swap of a given size.¹ Its blog gives an example: trading $10,000 in a pool holding $200,000 moves the price more than the same trade in a pool holding $20 million.²

Its app estimates price impact before you confirm, and shows a warning when the impact is unusually high.¹ Uniswap Labs also warns that small pools are prime targets for MEV bots (Lesson 6), because even small orders can swing their prices.⁵

Price impact and slippage are separate things. Uniswap's documentation says a slippage tolerance sets the change you will accept beyond price impact.¹ In the made-up case below, the app's quote already includes Maya's price impact.

Splitting a trade

In share markets, big investors often split large orders. In 2010 the SEC described broker software that takes a large "parent" order, divides it into many smaller "child" orders, and sends them out over time.⁶ It also described "dark pools": trading venues that do not show their best orders publicly, used by big investors who want to trade in size without moving prices against themselves.⁶

Crypto tools do something similar. Uniswap Labs suggests splitting a large swap into smaller chunks.² CoW Protocol, one example, offers "TWAP" orders, short for time-weighted average price. They break a large order into a series of smaller limit orders, traded at set intervals.³ CoW says spreading the trade over time reduces price impact.³

Some tools split across places instead of over time. The International Organization of Securities Commissions (IOSCO) says DEX aggregators can divide one trade across several trading protocols to get the best overall rate.⁷

The catch

Splitting swaps one risk for another. While you wait, the price can move against you as well as for you. FINRA reminds investors that no type of order can fully remove market risk.⁴

Pieces with a price limit may also not trade. CoW's TWAP orders, for example, let you set a minimum price, and a piece will not trade until the price recovers above it.³ FINRA makes the same point about limit orders in general: they can sit unfilled.⁸