Blockchain Confirmations

When a transaction is created, it’s initially in an unconfirmed or pending transaction state and waiting to be included in a block. Block confirmation is the process of validating and adding transactions to a block in a blockchain network. Blockchain confirmations help secure transactions against malicious actors and ensure the finality of transaction settlement.

By the end of this guide, you will understand:

• The importance of confirmations
• How confirmations are shown in Circle’s API platform
• What to consider when balancing transaction settlement speed with the risk of a reorganization

Importance of confirmations

Each time a new block is validated in a blockchain network, it indicates a confirmation. Without confirmations, transactions are vulnerable to alteration or reversal through reorgs. A reorg is a rewriting of transaction history which invalidates recently generated blocks. With each additional block confirmation, however, it becomes more difficult for an attacker to invalidate previous blocks.

Confirmation number

The confirmation number is the number of times a block must be confirmed before all transactions included in that block are considered final. This allows the block to reach finality and ensures that all transactions included in that block cannot be altered or reversed.

The confirmation number varies across different blockchain networks. On Circle’s API platform, a transaction state is COMPLETED when the confirmation number has been reached for the specific blockchain network. In your application, you can choose to use a CONFIRMED state to consider a transaction as valid, though this means taking on some risk.

Transaction state

On Circle’s API platform, when a transaction is added to a block, it starts in a CONFIRMED state and the wallet balance is updated. The meaning of this varies for Solana and EVM L2s such as Arbitrum:

• For Solana, CONFIRMED means that the transaction is included in a block that has been voted on by a supermajority (> 2/3) of the network’s stake. This is known as optimistic confirmation. To this date, a block that has been optimistically confirmed has never been rolled back nor failed to finalize.
• For EVM L2s, CONFIRMED means that the transaction has been processed by the sequencer. This is known as soft finality. Your acceptance of finality completely depends on trusting the sequencer of the L2 chain.

To receive a notification of when a transaction is added to a block, subscribe to webhook notifications. For more, see Notifications Quickstart. You can also view transaction details by sending a POST request to the /transactions/{id} endpoint.

Once the confirmation number is reached, the transaction state transitions to COMPLETED. If subscribed, you will receive a webhook notification of the new state.

Confirmation numbers for supported chains

Taking into consideration historical performance, the potential for reorgs, and overall network architecture, Circle’s API platform uses specific confirmation numbers for each of its supported chains. This means that all transactions start in a CONFIRMED state and when the confirmation number is reached for a specific chain, transitions to a COMPLETED state.

The following table showcases the confirmation numbers used by Circle’s API platform for each supported chain.

These confirmation numbers are based on Circle’s considerations and do not necessarily reflect the inherent requirements or limits of the blockchain networks.

Chain	Confirmation number	Approximate time
Arbitrum (USDC)	12 ETH blocks	~3 minutes
Avalanche (USDC, EURC)	1	~2 seconds
Base (USDC, EURC)	12 ETH blocks	~3 minutes
Ethereum (USDC, EURC)	12	~3 minutes
Optimism (USDC)	12 ETH blocks	~4 minutes
Polygon PoS (USDC)	2	~5 seconds
Solana (USDC, EURC)	33	~13 seconds
Unichain (USDC)	300	~3 minutes
Aptos (USDC)	1	~500 ms

For transactions on Polygon PoS, Circle’s API platform requires 50 confirmations before transitioning their state to COMPLETED. This process generally takes around 2 minutes. The chosen confirmation number represents three times the number of confirmations needed for sufficient transaction finality, based on the largest reorg observed thus far.

Balancing risk and transaction processing speed

Waiting for multiple confirmed blocks mitigates the risk of a reorg but increases the transaction settlement time. As a developer, you decide how to balance the two in your application.

When a transaction state is CONFIRMED on Circle’s API platform, the wallet balance is updated. You can display this to your user as a pending transaction, similar to how banks display inbound deposits. Here are a few ways you could choose to handle transactions in different states:

• Wait for the transaction state to transition to COMPLETED, which ensures the confirmation number is reached, before allowing your user to withdraw from their wallet balance. There is no risk if you choose to implement this option.
• Allow your user to withdraw funds when the transaction state is CONFIRMED. The trust associated with this option varies by chain:
  • Solana: CONFIRMED means that you are trusting the blockchain’s optimistic confirmation prior to COMPLETED.
  • EVM L2s: CONFIRMED means that you are trusting the L2’s sequencer prior to achieving finality on Ethereum, which is indicated by COMPLETED.
  • Avalanche: Transactions immediately transition from CONFIRMED to COMPLETED as Avalanche supports 1 block finality.
  • Other EVM L1s: Risks may vary. Ensure you research the blockchain.

If a transaction is of significant value, it is best practice to wait for its state to transition to COMPLETED. By understanding blockchain confirmations and the risk of reorgs, you can enhance the security and reliability of your applications.