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Gas and Transaction Fees

Concept​

To conduct any transaction with the Aptos blockchain on the mainnet, you are required to pay a processing fee. This fee is derived from transactions with a client application, stake owner, node operator, or voter. The processing fee you are required to pay is based on the computing and storage resources you use on the blockchain to:

  1. Process your transactions on the blockchain.
  2. Propagate the validated record throughout the distributed network of the mainnet.
  3. Store the validated record in the distributed blockchain storage.
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Conceptually, this fee can be thought of as quite similar to how we pay for our home electric or water utilities.

Prioritizing your transaction​

You can even bump up your transaction to a higher priority level on the blockchain by paying a larger processing fee. In your transaction, you can commit to paying a gas price that is higher than the market gas price. This is one way you can move your transaction higher in the priority list and have it processed more quickly.

Unit of gas​

You can make a simple and inexpensive transaction or a complicated transaction that requires the blockchain to perform much computation and distributed storage. In either case, you will be required to spend a processing fee sufficient to complete the transaction. This is where the notion of gas becomes clear. Here is how it works:

In the Aptos blockchain a unit of gas represents a basic unit of resource consumption. A single unit of gas is a combined representation of:

  • Computation resource, and
  • Storage resource.

When your transaction is executed on the blockchain, instead of separately presenting you an account of each unit of the specific resource consumed, the blockchain simply presents an account of the number of units of gas being consumed by the transaction.

See How Base Gas Works for a detailed description of gas fee types and available optimizations.

Gas price​

In the Aptos network, the Aptos governance sets the minimum gas unit price. However, the market determines the actual minimum gas unit price. See Ethereum Gas Tracker, for example, which shows the market price movements of Ethereum gas price.

Unit of gas

👉 A unit of gas is a dimensionless number, expressed as an integer. The total gas units consumed by your transaction depends on the complexity of your transaction. The gas price, on the other hand, is expressed in terms of Aptos blockchain’s native coin (Octas). Also see Transactions and States for how a transaction submitted to the Aptos blockchain looks like.

Gas and transaction fee on the Aptos blockchain​

When a transaction is submitted to the Aptos blockchain, the transaction must contain the following mandatory gas fields:

  • max_gas_amount: The maximum number of gas units that the transaction sender is willing to spend to execute the transaction. This determines the maximum computational resources that can be consumed by the transaction.

  • gas_price: The gas price the transaction sender is willing to pay. It is expressed in Octa units, where:

    • 1 Octa = 10-8 APT and
    • APT is the Aptos coin.

    During the transaction execution, the total gas amount, expressed as:

    (total gas units consumed) * (gas_price)

    must not exceed max_gas_amount, or else the transaction will abort the execution.

The transaction fee charged to the client will be at the most gas_price * max_gas_amount.

Gas parameters set by governance​

The following gas parameters are set by Aptos governance.

On-chain gas schedule

These on-chain gas parameters are published on the Aptos blockchain at 0x1::gas_schedule::GasScheduleV2.

  • txn.maximum_number_of_gas_units: Maximum number of gas units that can be spent (this is the maximum allowed value for the max_gas_amount gas parameter in the transaction). This is to ensure that the dynamic pricing adjustments do not exceed how much you are willing to pay in total.
  • txn.min_transaction_gas_units: Minimum number of gas units that can be spent. The max_gas_amount value in the transaction must be set to greater than this parameter’s value.

Dynamic gas pricing for storage​

Aptos gas pricing uses dynamic prices for the storage operations. This means that the storage costs, hence the gas_used, can increase exponentially as the Aptos blockchain state database is filled up. The storage cost can become as high as 100x at 100% utilization. However, the expectation is that the validators will make use of larger and cheaper storage hardware to mitigate such exponential rise in the storage costs.

Dynamic pricing is used to protect the Aptos network in the worse-case scenarios. However, we expect upgrades to Aptos network to happen well before the network gets into the high-cost zones.

Examples​

Example 1: Account balance vs transaction fee​

The sender’s account must have sufficient funds to pay for the transaction fee.

If, let's say, you transfer all the money out of your account so that you have no remaining balance to pay for the transaction fee. In such case the Aptos blockchain would let you know that the transaction will fail, and your transfer wouldn't succeed either.

Example 2: Transaction amounts vs transaction fee​

Transaction fee is independent of transfer amounts in the transaction.

In a transaction, for example, transaction A, you are transferring 1000 coins from one account to another account. In a second transaction B, with the same gas field values of transaction A, you now transfer 100,000 coins from one account to another one account. Assuming that both the transactions A and B are sent roughly at the same time, then the gas costs for transactions A and B would be near-identical.

Estimating the gas units​

The gas used for a transaction can be estimated by simulating the transaction. When simulating the transaction, the simulation results represent the exact amount that is needed at the exact state of the blockchain at the time of simulation. These gas units used may change based on the state of the chain. For this reason, any amount coming out of the simulation is only an estimate, and when setting the max gas amount, it should be increased above by an amount you’re comfortable with.

Simulating the transaction to estimate the gas​

Transactions can be simulated with the SimulateTransaction API. This API will run the exact transaction that you plan to run.

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Note that the Signature provided on the transaction must be all zeros. This is to prevent someone from using the valid signature.

To simulate the transaction, there are two flags:

  1. estimate_gas_unit_price: This flag will estimate the gas unit price in the transaction using the same algorithm as the estimate_gas_price API.
  2. estimate_max_gas_amount: This flag will find the maximum possible gas you can use, and it will simulate the transaction to tell you the actual gas_used.

Simulation steps​

The simulation steps for finding the correct amount of gas for a transaction are as follows:

  1. Estimate the gas via simulation with both estimate_gas_unit_price and estimate_max_gas_amount set to true.
  2. Use the gas_unit_price in the returned transaction as your new transaction’s gas_unit_price.
  3. View the gas_used * gas_unit_price values in the returned transaction as the lower bound for the cost of the transaction.
  4. To calculate the upper bound of the cost, take the minimum of the max_gas_amount in the returned transaction, and the gas_used * safety factor. In the CLI a value of 1.5 is used for safety factor. Use this value as max_gas_amount for the transaction you want to submit. Note that the upper bound for the cost of the transaction is max_gas_amount * gas_unit_price, i.e., this is the most the sender of the transaction is charged.
  5. At this point you now have your gas_unit_price and max_gas_amount to submit your transaction as follows:
    1. gas_unit_price from the returned simulated transaction.
    2. max_gas_amount as the minimum of the gas_used * a safety factor or the max_gas_amount from the transaction.
  6. If you feel the need to prioritize or deprioritize your transaction, adjust the gas_unit_price of the transaction. Increase the value for higher priority, and decrease the value for lower priority.