The biggest rate factors
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Market price movement while you review the route.
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The network you choose for the payout asset.
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Route availability at the moment you continue.
What the quote does and does not cover
Stable assets can still show a different final amount when the sending asset is moving quickly or the route changes to keep the swap available.
It is also important to separate the swap quote from wallet and network costs. Those outside costs depend on the wallets and chains involved.