How is post-trade price different from effective price?

Post-trade or marginal price is the spot price of the pool immediately after your swap, computed from the new reserves. Effective price is the average price your whole trade got. Post-trade price is always worse than effective price, which is worse than the original spot.

What makes price impact bigger?

Price impact grows with the ratio of your trade size to the pool reserves. A trade that is one percent of the pool barely moves it; a trade that is 30 percent of the input reserve moves the price sharply because the constant-product curve steepens as reserves drain.

Does the fee change the price impact?

The fee reduces the input that actually reaches the curve, so it slightly lowers the realised output and the post-trade price shift. The tool deducts the fee from the input before applying the constant-product formula, matching on-chain behaviour.

Why do MEV researchers care about this?

Price impact is the opportunity that sandwich attacks exploit. A bot front-runs a high-impact swap to push the price, lets the victim trade at the worse price, then sells back. Quantifying impact shows how much value a large naive swap leaves on the table.

Is this the same as Uniswap V3?

No. This models the V2 style constant-product curve with full-range liquidity. V3 concentrates liquidity in price ranges, so impact depends on where the active tick sits. Use this for V2 pools and as an upper-bound intuition for concentrated pools.

AMM Price Impact Calculator

On a constant-product AMM your trade literally bends the price curve. This calculator takes pool reserves and a trade size and reports the spot price, the post-trade marginal price, the percentage impact, and the output you receive after the protocol fee.

How it works

The pool obeys x × y = k. Swapping dx of the input token (after fee f):

dx_eff   = dx × (1 − f)
new x     = x + dx_eff
new y     = k / new x          where k = x × y
dy        = y − new y          ← output received

spot price (out per in)       = y / x
post-trade price (out per in) = new y / new x
price impact                  = 1 − post-trade / spot

Because k is fixed, draining the output reserve raises the price of each remaining unit — the curve is convex, so impact accelerates with trade size.

Example and tips

Trading 300 input tokens into a pool of 1,000 in and 1,000 out (0.3% fee) drains the output reserve hard: post-trade price drops well below the 1.0 spot and price impact runs into double digits. The same 300-token trade against a 100,000-token pool is nearly impact-free. The lesson for large orders is to split across pools or use a TWAP — and for researchers, high single-swap impact is precisely the signal that a sandwich is profitable.