Price impact and slippage: a numerical comparison

Measure price impact against the price before your trade changes the pool; measure execution slippage against the quote you accepted. A single swap can have both. Using the same word for both obscures where the difference arose.

Follow one timeline

Assume a hypothetical fee-free constant-product pool starts with 10,000 units of each token. Its local rate is one output token per input token. You request a swap of 100 input tokens.

The constant-product pool calculation produces 10,000 × 100 / 10,100, approximately 99.009901 output tokens. Relative to the 100-token output implied by the initial local price, the quoted shortfall is about 0.990099%. That is the average execution effect of the requested size under this model.

Now suppose the accepted quote is 99.009901, but intervening state changes mean the completed swap delivers 98.80. Quote-to-fill deterioration is approximately (99.009901 − 98.80) / 99.009901 = 0.212%. The entire gap from the initial 100-token reference is 1.20%.

Why the percentages do not simply add

The impact figure uses 100 as its output benchmark. The slippage figure uses approximately 99.009901. Their bases differ, so adding 0.990099% and 0.212% is only an approximation to the combined shortfall.

Uniswap's terminology distinguishes your trade's price effect from the difference between expected and actual execution. The numerical example deliberately excludes fees to keep those effects visible.

Apply the distinction to a real quote

Record the reference price, the size-specific quote and the final received amount separately. Identify whether an interface includes fees in its displayed impact statistic. Do not add a quoted impact percentage to an output amount that already contains that effect.

Separate gas remains a further economic cost. It is neither pool-price impact nor quote-to-fill slippage, even when you convert it into output-token units for a complete cost ledger.