A floor, not the expected amount
A swap shows two numbers that matter here. The expected output is what you would receive if the price did not move between quoting and settlement. The minimum received is lower, and the gap between them is your slippage tolerance. If the price drifts within that band, the swap fills. If it would land below the minimum, the swap reverts instead.
So the minimum received is not a prediction of what you will get. It is the worst outcome you have agreed to accept, and in calm conditions you usually receive something close to the expected figure.
How slippage sets the floor
The minimum is the quote reduced by your slippage tolerance. A 0.5% tolerance sets the floor roughly 0.5% below the expected output. Widen the tolerance and the floor drops, which makes a revert less likely but allows a poorer fill. Tighten it and the floor rises, which protects the price but reverts more often when the market is moving.
- Tighter tolerance: higher floor, more protection, more reverts in volatile conditions.
- Wider tolerance: lower floor, fewer reverts, more room for a poor fill.
- Net-of-fee quotes fold routing and network costs into the numbers you compare.
- The minimum is enforced by the transaction itself on-chain, not by a promise from the venue.
Price impact and volatility set the gap
Two forces decide how far the minimum sits below the expected output. Price impact is the move your own order causes as it consumes liquidity, and it grows with order size relative to the pool. Volatility is the market moving on its own between the moment you see the quote and the moment the block settles.
A large order on a thin pair has high price impact, so it may need a wider tolerance to fill at all, which lowers the floor. A fast-moving pair has high volatility, so even a small order can drift past a tight floor and revert. Reading the minimum tells you how much of either the current settings allow.
Reading it before you confirm
Before approving, glance at both numbers. If the minimum received sits far below the expected output, either the pair is volatile or the tolerance is wide. That is a signal to reconsider the size, the timing, or the route, not a reason to raise tolerance blindly.
This is also the mechanism behind a swap that 'failed' on slippage. The execution price would have breached the minimum, so the network reverted to protect you rather than filling at a loss. A fresh quote, a smaller size, or a slightly adjusted tolerance usually clears it.