diff --git a/contracts/interfaces/IRip7560Paymaster.sol b/contracts/interfaces/IRip7560Paymaster.sol
index 7bc86da..10e3c63 100644
--- a/contracts/interfaces/IRip7560Paymaster.sol
+++ b/contracts/interfaces/IRip7560Paymaster.sol
@@ -1,6 +1,41 @@
 // SPDX-License-Identifier: GPL-3.0
 pragma solidity ^0.8.23;
 
+/**
+ * @title UsedGasBreakdown
+ * @dev The detailed breakdown of where the gas in the transaction was consumed.
+ * @dev This data is provided to the Paymaster contract in the 'postPaymasterTransaction' frame.
+ * @notice Gas used during 'postPaymasterTransaction' cannot be known ahead of time and may need to be guesstimated.
+ */
+struct UsedGasBreakdown {
+	uint256 preTransactionGasUsed;
+	uint256 accountDeploymentGasUsed;
+	uint256 accountValidationGasUsed;
+	uint256 paymasterValidationGasUsed;
+	uint256 executionGasUsed;
+	uint256 executionUnusedPenaltyGas;
+}
+
+/**
+ * @dev Calculate ETH gas cost of a transaction using the provided gas usage info plus an estimate for 'postOpGasUsed'.
+ * @param usedGasBreakdown - the actual gas cost of each completed frame of the transaction.
+ * @param postOpGasUsed - the best-effort estimation of the amount of gas consumed by the 'postPaymasterTransaction'.
+ * @notice 'postPaymasterTransaction' may be penalized for unused gas and this should be included in 'postOpGasUsed'.
+ */
+function calculateActualGasCost(
+	UsedGasBreakdown calldata usedGasBreakdown,
+	uint256 postOpGasUsed
+) returns (uint256){
+	uint256 actualGas = postOpGasUsed +
+					usedGasBreakdown.preTransactionGasUsed +
+					usedGasBreakdown.accountDeploymentGasUsed +
+					usedGasBreakdown.accountValidationGasUsed +
+					usedGasBreakdown.paymasterValidationGasUsed +
+					usedGasBreakdown.executionGasUsed +
+					usedGasBreakdown.executionUnusedPenaltyGas;
+	return actualGas * tx.gasprice;
+}
+
 /**
  * @title IRip7560Paymaster
  * @dev Interface for the paymaster contract.
@@ -23,18 +58,18 @@ interface IRip7560Paymaster {
 
 
 	/**
-	 * paymaster post transaction function.
-	 * This method is called after the transaction has been executed - if the validation function returned a context
-	 * @dev revert in this method will cause the account execution function to revert too
-	 *	(but the reverted transaction will still get on-chain and be paid for)
-	 * @param success - true if the transaction was successful, false otherwise
-	 * @param actualGasCost - the actual gas cost of the transaction
+	 * @dev The paymaster's post-transaction callback function.
+	 * @dev This method is called after the transaction has been executed if the validation function returned a context.
+	 * @dev Revert in this method will cause the account execution function to revert too.
+	 * @dev Notice that unlike an invalid transaction, the reverted transaction will still get on-chain and be paid for.
+	 * @param success - true if the transaction execution was successful, false otherwise
 	 * @param context - context data returned by validatePaymasterTransaction
+	 * @param usedGasBreakdown - the actual gas cost of each completed frame of the transaction
 	 */
 	function postPaymasterTransaction(
 		bool success,
-		uint256 actualGasCost,
-		bytes calldata context
+		bytes calldata context,
+		UsedGasBreakdown calldata usedGasBreakdown
 	) external;
 
 }