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In this tutorial, you’ll learn how to use viem to estimate the cost of a transaction on OP Mainnet. You’ll learn how to estimate the execution gas fee and the L1 data fee independently. You’ll also learn how to estimate the total cost of the transaction all at once.
Check out the full explainer on OP Stack transaction fees for more information on how OP Mainnet charges fees under the hood.

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Supported networks

Viem supports any of the Superchain networks. The OP Stack networks are included in Viem by default. If you want to use a network that isn’t included by default, you can add it to Viem’s chain configurations.

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Dependencies

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Create a demo project

You’re going to use Viem for this tutorial. Since Viem is a Node.js library, you’ll need to create a Node.js project to use it.
1

Make a project folder

mkdir op-est-cost-tutorial
cd op-est-cost-tutorial
2

Initialize the project

pnpm init
3

Install viem

pnpm add viem
Want to create a new wallet for this tutorial? If you have cast installed you can run cast wallet new in your terminal to create a new wallet and get the private key.

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Get ETH on Sepolia

This tutorial explains how to bridge ETH from Sepolia to OP Sepolia. You will need to get some ETH on Sepolia to follow along.
You can use this faucet to get ETH on Sepolia.

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Get ETH on OP Sepolia

This tutorial explains how estimate transaction costs on OP Sepolia. You will need to get some ETH on OP Sepolia in order to run the code in this tutorial.
You can use the Superchain faucet to get ETH on OP Sepolia.

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Add a private key to your environment

You need a private key in order to sign transactions. Set your private key as an environment variable with the export command. Make sure this private key corresponds to an address that has ETH on Sepolia. 
export TUTORIAL_PRIVATE_KEY=0x...

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Start the Node REPL

You’re going to use the Node REPL to interact with Viem. To start the Node REPL run the following command in your terminal: 
node
This will bring up a Node REPL prompt that allows you to run JavaScript code.

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Set session variables

You’ll need a few variables throughout this tutorial. Let’s set those up now.
1

Import viem and other necessary modules

const { createPublicClient, createWalletClient, http, parseEther, parseGwei, formatEther } = require('viem');
const { privateKeyToAccount } = require('viem/accounts');
const { optimismSepolia } = require('viem/chains');
const { publicActionsL2, walletActionsL2 } = require('viem/op-stack');
2

Set up the account

const privateKey = process.env.TUTORIAL_PRIVATE_KEY
const account = privateKeyToAccount(privateKey)
3

Create the public client

const publicClient = createPublicClient({
  chain: optimismSepolia,
  transport: http("https://sepolia.optimism.io"),
}).extend(publicActionsL2())
4

Create the wallet client

const walletClientL2 = createWalletClient({
  chain: optimismSepolia,
  transport: http("https://sepolia.optimism.io"),
}).extend(walletActionsL2())

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Estimate transaction costs

You’re now going to use the Viem to estimate the cost of a transaction on OP Mainnet. Here you’ll estimate the cost of a simple transaction that sends a small amount of ETH from your address to the address 0x1000000000000000000000000000000000000000.
1

Create the unsigned transaction

Viem makes it easy to create unsigned transactions so you can estimate the cost of a transaction before you a user to sign it. Here you’ll create an unsigned transaction that sends a small amount of ETH from your address to the address 0x1000000000000000000000000000000000000000.
  const transaction = {
  account,
  to: '0x1000000000000000000000000000000000000000',
  value: parseEther('0.00069420'),
  gasPrice: await publicClient.getGasPrice() 
  }
2

Estimate the total cost

With Viem you can estimate the total cost of a transaction using the estimateTotalFee method.
  const totalEstimate = await publicClient.estimateTotalFee(transaction)
  console.log(`Estimated Total Cost: ${formatEther(totalEstimate)} ETH`)
3

Send the transaction

Now that you’ve estimated the total cost of the transaction, go ahead and send it to the network. This will make it possible to see the actual cost of the transaction to compare to your estimate.
  const txHash = await walletClientL2.sendTransaction(transaction)
  console.log(`Transaction Hash: ${txHash}`)

  const receipt = await publicClient.waitForTransactionReceipt({ hash: txHash })
  console.log('receipt', receipt);
4

Check the actual execution gas fee

Once you get back the transaction receipt, check the actual execution gas fee. You can do so by accessing the gasUsed and effectiveGasPrice from the transaction receipt. You can then multiply these values to get the actual L2 cost of the transaction
  const l2CostActual = receipt.gasUsed * receipt.effectiveGasPrice
  console.log(`Actual Execution Gas Fee: ${formatEther(l2CostActual)} ETH`)
5

Check the actual L1 data fee

You can also check the actual L1 data fee.
  const l1CostActual = receipt.l1Fee
  console.log(`Actual L1 Data Fee: ${formatEther(l1CostActual)} ETH`)
6

Check the actual total cost

Sum these two together to get the actual total cost of the transaction.
  const totalActual = l2CostActual + l1CostActual
  console.log(`Actual Total Cost: ${formatEther(totalActual)} ETH`)
7

Check the difference

Finally, check the difference between the estimated total cost and the actual total cost. This will give you a sense of how accurate your estimate was. Estimates will never be entirely accurate, but they should be close!
  const difference = totalEstimate >= totalActual ? totalEstimate - totalActual : totalActual - totalEstimate
  console.log(`Estimation Difference: ${formatEther(difference)} ETH`)
Estimates will never be entirely accurate due to network conditions and gas price fluctuation, but they should be close to the actual costs.

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Next steps

  • Always estimate before sending: Estimating costs before sending a transaction helps prevent unexpected fees and failed transactions.
  • Account for gas price volatility: Gas prices can change rapidly. Consider adding a buffer to your estimates or implementing a gas price oracle for more accurate pricing.
  • Optimize transaction data: Minimize the amount of data in your transactions to reduce L1 data fees.
  • Monitor network conditions: Keep an eye on network congestion and adjust your estimates accordingly.
  • Use appropriate gas limits: Setting too low a gas limit can cause transactions to fail, while setting it too high can result in unnecessary costs.
  • Implement retry mechanisms: If a transaction fails due to underestimated gas, implement a retry mechanism with adjusted gas parameters.
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