Estimating Transaction Costs on OP Mainnet

Make a Project Folder

mkdir op-sample-project
cd op-sample-project

Initialize the Project

pnpm init

Install the Optimism SDK

pnpm add @eth-optimism/sdk

Install ethers.js

pnpm add ethers@^5

Import the Optimism SDK

const optimism = require("@eth-optimism/sdk")

Import ethers.js

const ethers = require("ethers")

Load your private key

const privateKey = process.env.TUTORIAL_PRIVATE_KEY

Create the RPC provider

Here you're creating a standard Ethers RPC provider and wrapping it as an L2Provider, a class provided by the Optimism SDK. This will add a few extra functions to the provider object that you'll use later in this tutorial.

const provider = optimism.asL2Provider(new ethers.providers.StaticJsonRpcProvider("https://sepolia.optimism.io"))

Create the wallet instance

const wallet = new ethers.Wallet(privateKey, provider)

Create the unsigned transaction

Ethers 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. You can also create unsigned transactions that interact with contracts using Contract.populateTransaction (opens in a new tab).

const tx = await wallet.populateTransaction({
  to: '0x1000000000000000000000000000000000000000',
  value: ethers.utils.parseEther('0.00069420'),
  gasPrice: await provider.getGasPrice(),
})

Estimate the execution gas fee

You can estimate the execution gas fee the same way you'd estimate the gas fee for any transaction on Ethereum. Simply multiply the gas limit by the effective gas price.

const gasLimit = tx.gasLimit
const gasPrice = tx.maxFeePerGas
const l2CostEstimate = gasLimit.mul(gasPrice)
console.log(ethers.utils.formatEther(l2CostEstimate))

Estimate the L1 data fee

You can estimate the L1 data fee with the estimateL1GasCost (opens in a new tab) function. Under the hood, this function is estimating the amount of Ethereum gas required to publish this transaction on Ethereum and multiplying it by the current Ethereum gas price (as tracked by the L2). This function returns the current cost estimate in wei.

const l1CostEstimate = await provider.estimateL1GasCost(tx)
console.log(ethers.utils.formatEther(l1CostEstimate))

Estimate the total cost

Once you've individually estimated the execution gas fee and the L1 data fee, you can sum these two values together to get the total cost of the transaction.

const totalSum = l2CostEstimate.add(l1CostEstimate)
console.log(ethers.utils.formatEther(totalSum))

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 res = await wallet.sendTransaction(tx)
const receipt = await res.wait()
console.log(receipt.transactionHash)

Check the actual execution gas fee

Once you get back the transaction receipt, check the actual execution gas fee.

const l2CostActual = receipt.gasUsed.mul(receipt.effectiveGasPrice)
console.log(ethers.utils.formatEther(l2CostActual))

Check the actual L1 data fee

You can also check the actual L1 data fee.

const l1CostActual = receipt.l1Fee
console.log(ethers.utils.formatEther(l1CostActual))

Check the actual total cost

Sum these two together to get the actual total cost of the transaction.

const totalActual = l2CostActual.add(l1CostActual)
console.log(ethers.utils.formatEther(totalActual))

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 = totalActual.sub(totalSum).abs()
console.log(ethers.utils.formatEther(difference))