# Fees Calculation & Gas Estimation Logic

Fee calculation and gas estimation help you understand the costs involved in cross-chain transactions and deployments. These processes account for things like gas limits, payload size, and currency prices on the source and destination chains to ensure your operations run smoothly. Entangle provides [APIs](https://docs.entangle.fi/universal-token-standard/developer-guides/estimations) to estimate fees for bridging and deploying tokens, making it easier to plan and execute your transactions.

This page explains the logic for calculating transaction fees and estimating gas costs for cross-chain operations, including base fees, payload costs, and optimization strategies handled off-chain by the backend.

## **Base Transaction Fee**

* **dstGasLimit**: User-defined gas limit to be used on the destination network during the redeem transaction.
* **dstGasPriceInWei**: Stored in the contract and updated by the backend; represents the gas price on the destination network based on `chainId`.
* **dstNativePriceInUSDe18**: Stored in the contract and updated by the backend; indicates the USD price of the native currency on the destination network, depending on `chainId`.
* **curNativePriceInUSDe18**: Stored in the contract and updated by the backend; reflects the USD price of the native currency on the source network.

## **Base Fee Calculation**

To calculate the base fee in terms of the current native currency:

```
baseFeeAtCurrentNative = dstGasLimit * dstGasPriceInWei * dstNativePriceInUSDe18 / curNativePriceInUSDe18
```

This formula determines the amount of native currency required on the source network to execute the transaction (excluding payload), which is then compared to `msg.value`.

## **Payload Cost Calculation**

To calculate the cost related to the payload, use these parameters:

* **payloadLength**: User-defined length of the payload in bytes.
* **dstPricePerByteInWei**: Cost per byte of payload in the native currency on the destination network.
* **dstNativePriceInUSDe18**: Represents the USD price of the native currency on the destination network.
* **curNativePriceInUSDe18**: Represents the USD price of the native currency on the source network.

The payload cost calculation in the current native currency is:

```
payloadPriceAtCurrentNative = payloadLength * dstPricePerByteInWei * dstNativePriceInUSDe18 / curNativePriceInUSDe18
```

This formula calculates how much native currency is needed on the source network to cover the payload cost, which is also compared to `msg.value`.

## **Final Calculation**

To determine the total required amount, sum `baseFeeAtCurrentNative` and `payloadPriceAtCurrentNative`, then compare to `msg.value`. Note that users may incur small losses if `dstGasLimit` exceeds the actual gas consumed.

## **Optimization of Calculations**

To optimize the process, the backend handles calculations off-chain to minimize contract updates across different destination networks:

* **dstGasPriceInWei**
* **dstPricePerByteInWei**
* **dstNativePriceInUSDe18**
* **curNativePriceInUSDe18**

Using these parameters, the backend performs these calculations:

* **Calculate `dstGasPriceAtCurrentNative`**:

  ```
  dstGasPriceAtCurrentNative = dstGasPriceInWei * dstNativePriceInUSDe18 / curNativePriceInUSDe18
  ```
* **Calculate `dstPayloadPriceAtCurrentNative`**:

  ```
  dstPayloadPriceAtCurrentNative = dstPricePerByteInWei * dstNativePriceInUSDe18 / curNativePriceInUSDe18
  ```

The backend then sends only the `dstGasPriceAtCurrentNative` value to the contract. Optionally, it can also send `dstPayloadPriceAtCurrentNative` if a dynamic price per byte is preferred; otherwise, a static price is more efficient.