Table of Contents


Following the Fjord upgrade, three additional values used for L1 fee computation are:

  • costIntercept
  • costFastlzCoef
  • minTransactionSize

These values are hard-coded constants in the GasPriceOracle contract. The calculation follows the same formula outlined in the Fjord L1-Cost fee changes (FastLZ estimator) section.

A new method is introduced: getL1FeeUpperBound(uint256). This method returns an upper bound for the L1 fee for a given transaction size. It is provided for callers who wish to estimate L1 transaction costs in the write path, and is much more gas efficient than getL1Fee.

The upper limit overhead is assumed to be original/255+16, borrowed from LZ4. According to historical data, this approach can encompass more than 99.99% of transactions.

This is implemented as follows:

function getL1FeeUpperBound(uint256 unsignedTxSize) external view returns (uint256) {
    // Add 68 to account for unsigned tx
    uint256 txSize = unsignedTxSize + 68;
    // txSize / 255 + 16 is the pratical fastlz upper-bound covers 99.99% txs.
    uint256 flzUpperBound = txSize + txSize / 255 + 16;

    int256 estimatedSize = costIntercept + costFastlzCoef * flzUpperBound;
    if (estimatedSize < minTransactionSize) {
        estimatedSize = minTransactionSize;
    uint256 l1FeeScaled = baseFeeScalar() * l1BaseFee() * 16 + blobBaseFeeScalar() * blobBaseFee();
    return uint256(estimatedSize) * feeScaled / (10 ** (DECIMALS * 2));

L1 Gas Usage Estimation

The getL1GasUsed method is updated to take into account the improved compression estimation accuracy as part of the Fjord upgrade.

function getL1GasUsed(bytes memory _data) public view returns (uint256) {
    if (isFjord) {
      // Add 68 to the size to account for the unsigned tx
      int256 flzSize = LibZip.flzCompress(_data).length + 68;

      int256 estimatedSize = costIntercept + costFastlzCoef * flzSize;
      if (estimatedSize < minTransactionSize) {
        estimatedSize = minTransactionSize;

      // Assume the compressed data is mostly non-zero, and would pay 16 gas per calldata byte
      return estimatedSize * 16;
    // ...

The getL1GasUsed method is deprecated as of Fjord because it does not capture that there are two kinds of gas being consumed due to the introduction of blobs. This function will revert when called in a future upgrade.

Users can continue to use the getL1Fee method to estimate the L1 fee for a given transaction, or the new getL1FeeUpperBound method introduced by Fjord as a lower gas alternative.