How to: Display Transaction Fees

import { createPublicClient, http, formatUnits } from "viem";
import { arc } from "viem/chains";

const client = createPublicClient({
  chain: arc,
  transport: http("https://rpc.arc.circle.com"),
});

// Simple: current gas price in USDC wei
const gasPrice = await client.getGasPrice();
console.log("Gas price (wei):", gasPrice);

// Detailed: recent base fee history
const feeHistory = await client.getFeeHistory({
  blockCount: 4,
  rewardPercentiles: [25, 50, 75],
});
console.log("Latest base fee (wei):", feeHistory.baseFeePerGas.at(-1));

// Native USDC transfer: ~21,000 gas
const simpleTransferGas = 21_000n;

// ERC-20 token transfer or contract call: use estimateGas
const contractGas = await client.estimateGas({
  account: "0xYourAddress",
  to: "0xContractAddress",
  data: encodedCalldata,
});

const latestBlock = await client.getBlock();
const baseFee = latestBlock.baseFeePerGas!;

// 2x base fee is generous—Arc's EWMA smoothing keeps fees stable
const maxFeePerGas = baseFee * 2n;

// Priority fee of 0 is acceptable on Arc (validators don't require tips)
const maxPriorityFeePerGas = 0n;

function calculateMaxFeeUsd(gasLimit: bigint, maxFeePerGas: bigint): string {
  const maxCostWei = gasLimit * maxFeePerGas;
  // USDC has 18 decimals as the native gas token on Arc
  // Since 1 USDC = $1, the numeric value IS the USD cost
  const usdCost = formatUnits(maxCostWei, 18);
  return usdCost;
}

// Example: simple transfer
const maxFee = calculateMaxFeeUsd(21_000n, maxFeePerGas);
console.log(`Max fee: $${Number(maxFee).toFixed(6)}`);

function formatFeeDisplay(gasLimit: bigint, maxFeePerGas: bigint): string {
  const maxCostWei = gasLimit * maxFeePerGas;
  const usdValue = Number(formatUnits(maxCostWei, 18));

  if (usdValue < 0.01) {
    return "< $0.01";
  }
  return `~$${usdValue.toFixed(2)}`;
}

// Given: base fee = 20 Gwei (minimum), gas limit = 21,000
const baseFee = 20_000_000_000n; // 20 Gwei in wei
const gasLimit = 21_000n;
const maxFeePerGas = baseFee * 2n; // 40 Gwei

const maxCostWei = gasLimit * maxFeePerGas;
// 21,000 * 40,000,000,000 = 840,000,000,000,000 wei
// = 0.00000084 USDC = $0.00000084

const display = formatFeeDisplay(gasLimit, maxFeePerGas);
// Output: "< $0.01"

// Given: base fee = 500,000 Gwei (0.0005 USDC), gas limit = 65,000
const baseFee = 500_000_000_000_000n; // 500,000 Gwei in wei
const gasLimit = 65_000n;
const maxFeePerGas = baseFee * 2n; // 1,000,000 Gwei

const maxCostWei = gasLimit * maxFeePerGas;
// 65,000 * 1,000,000,000,000,000 = 65,000,000,000,000,000,000 wei
// = 0.065 USDC = $0.065

const display = formatFeeDisplay(gasLimit, maxFeePerGas);
// Output: "~$0.07"