# Swap fee

A swap fee is charged for each swap, as well as on the non-proportional amounts in add/remove liquidity operations. When a pool is registered, the initial swap fee is passed as a parameter and stored as part of the pool's configurationopen in new window. The swap fee is always charged on the amount in (i.e., on the given amount for EXACT_IN, and the calculated amount for EXACT_OUT).

# Setting a static swap fee

Users who have been granted authorization have the capability to set a fixed swap fee percentage for liquidity pools. This can be done by invoking the vault.setStaticSwapFeePercentage(address pool, uint256 swapFeePercentage) function.

If users prefer not to have the swap fee of a pool controlled by Balancer governance (through the Authorizer), they can opt out by providing a non-zero address for the swapManager. This address could be a multi-sig or custom contract.

Unlike in v2, v3 does not impose limits on the swap fee percentage at the Vault level. Rather, these limits are set at the pool level (0.001% - 10% for standard Balancer Weighted, and 0.0001% - 10% for Stable pools).

# Swap fees by pool type.

Different types of pools can have varying minimum and maximum swap fees. These variations are determined by the mathematical security properties and specific product requirements. Maximum and minimum swap fees are set on a per pool basis implemented via the ISwapFeePercentageBounds interface, which is inherited by IBasePool:

/// @return minimumSwapFeePercentage The minimum swap fee percentage for a pool
    function getMinimumSwapFeePercentage() external view returns (uint256);
/// @return maximumSwapFeePercentage The maximum swap fee percentage for a pool
    function getMaximumSwapFeePercentage() external view returns (uint256);

This means that all new pool types (assuming they implement IBasePool) will need to think about what the swap fee range should be, according to the pool type's math and other constraints, then override and set the values accordingly. (Note that invariant limits must also be defined for new pool types, by implementing IUnbalancedLiquidityInvariantRatioBounds.)

# Dynamic swap fee (using Hooks)

Using Hooks a pool can be set up to use dynamic swap fees. When registering a pool with a dynamic swap fee, shouldCallComputeDynamicSwapFee should be true in the HooksConfig.

Instead of getting the swap fee from the pool's configurationopen in new window, the Vault uses the onComputeDynamicSwapFeePercentage() hook to fetch the dynamic swap fee. This function returns the swap fee percentage to be used for the current swap. It's important to note that even when a pool is set to use dynamic swap fees, it still maintains a static swap fee, which is not directly used (though it is sent to the dynamic fee hook for reference).