# Fee Calculation

## Bonder Fee

A bonder fee is required when sending L2->L2 or L2->L1.

The bonder fee calculation is a little complex since it involves getting AMM estimated amount outs on both source and destination chains and getting the estimated destination bond transaction fee.

The simplest way to get the estimated total bonder fee to use when sending transfers is to use the `getTotalFee()` JS SDK method.

## Relayer Fee

A bonder fee can be applied when sending L1->L2.

The relayer fee represents the cost of relaying the transaction on each L2. Since this is often low, bonders can choose to ignore this. When L2 gas is high, bonders might enforce this fee.

This fee can be retrieved in the same way as the bonder fee by use the `getTotalFee()` JS SDK method.

### Bonder fee implementation pseudo code

Here's an pseudo code example for calculating bonder fee.

*Note: The the live fee calculation may change and this may not be up-to-date.*

```javascript
bonderFee = getBonderFee(amountIn, source, destination)

getBonderFee(amountIn, source, destination, isHTokenSend) {
  hTokenAmount = getToHTokenAmount(amountIn, source)
  bonderFeeRelative = getBonderFeeRelative(amountIn, source, destination)
  destinationTxFee = getDestinationTxFee(source, destination)
  adjustedBonderFee = 0
  adjustedDestinationTxFee = 0
  totalFee = 0

  if (source != L1) {
    if (isHTokenSend) {
      adjustedBonderFee = bonderFeeRelative
      adjustedDestinationTxFee = destinationTxFee
    } else {
      adjustedBonderFee = getFromHTokenAmount(bonderFeeRelative, destination)
      adjustedDestinationTxFee = getToHTokenAmount(destinationTxFee, destination)
    }

    bonderFeeAbsolute = getBonderFeeAbsolute()
    if (adjustedBonderFee < bonderFeeAbsolute) {
      adjustedBonderFee = bonderFeeAbsolute
    }

    totalFee = adjustedBonderFee + adjustedDestinationTxFee
  }

  return totalFee
}

getToHTokenAmount(amount, chain) {
  if (chain == L1) {
    return amount
  }

  canonicalTokenIndex = 0
  hTokenIndex = 1
  amountOut = getAmmContract(chain).calculateSwap(canonicalTokenIndex, hTokenIndex, amount)

  return amountOut
}

getFromHTokenAmount(amount, chain) {
  if (chain == L1) {
    return amount
  }

  canonicalTokenIndex = 0
  hTokenIndex = 1
  amountOut = getAmmContract(chain).calculateSwap(hTokenIndex, canonicalTokenIndex, amount)

  return amountOut
}

getBonderFeeRelative(amountIn, source, destination) {
  if (source == L1) {
    return 0
  }

  hTokenAmount = getToHTokenAmount(amountIn, source)
  feeBps = getFeeBps(token, dest)
  bonderFeeRelative = (hTokenAmount * feeBps) / 10000

  return bonderFeeRelative
}

getBonderFeeAbsolute() {
  price = getPrice(token)
  minBonderFeeUsd = 0.25
  bonderFeeAbsolute = (minBonderFeeUsd / price)

  return bonderFeeAbsolute
}

getDestinationTxFee(source, destination) {
  if (source == L1) {
    return 0
  }

  nativeTokenPrice = getPrice(getNativeToken(destination))
  tokenPrice = getPrice(token)
  gasPrice = getGasPrice(destination)
  bondTransferGasLimit = getEstimatedBondWithdrawalGasLimit(destination)
  rate = nativeTokenPrice / tokenPrice
  settlementGasLimit = getSettlementGasLimit(destination)
  totalGasLimit = bondTransferGasLimit + settlementGasLimit
  txFeeEth = gasPrice * totalGasLimit
  fee = txFeeEth * rate

  return fee
}
```

The JS implementation is found in the [SDK here](https://github.com/hop-protocol/hop/blob/develop/packages/sdk/src/HopBridge.ts):

## Questions

If you have any questions on the bonder fee calculation, please reach out on [discord](https://docs.hop.exchange/faq#how-can-i-contact-the-hop-team) #dev channel!


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