The process of a transaction being included in a block and added to the blockchain. Each subsequent block adds another confirmation, increasing the transaction's security. Six confirmations is widely considered irreversible.
The process of a transaction being included in a block and added to the blockchain. Each subsequent block adds another confirmation, increasing the transaction's security. Six confirmations is widely considered irreversible.
When you send Bitcoin, the transaction is first broadcast to the network and enters the mempool (the waiting area for unconfirmed transactions). A miner then includes your transaction in a block they are mining. When that block is successfully added to the blockchain, your transaction has one confirmation. Each subsequent block mined on top of that block adds another confirmation.
Confirmations matter because they represent the cumulative proof of work securing your transaction. To reverse a confirmed transaction, an attacker would need to re-mine the block containing that transaction and all blocks built on top of it — which requires enormous computational power. With each additional confirmation (roughly every 10 minutes), the energy cost of a reversal increases exponentially.
The standard threshold for considering a transaction irreversible is six confirmations (approximately one hour). This threshold was established because the probability of a successful attack drops to near zero after six blocks. For small amounts, one or two confirmations may be sufficient. For very large transactions, some recipients wait for more than six confirmations. The Lightning Network enables instant transactions by settling off-chain and only recording final balances on the main chain.
A single Bitcoin confirmation takes approximately 10 minutes on average, as that is the target time between blocks. However, actual block times vary — sometimes blocks are found in 1 minute, other times in 20+ minutes. If the mempool is congested and your transaction fee is low, it may take longer to be included in a block. The standard six confirmations take roughly one hour.
Six confirmations require approximately one hour of proof-of-work. To reverse a transaction with six confirmations, an attacker would need to control more than 50% of the network's hash rate and sustain that advantage long enough to re-mine six blocks — a so-called 51% attack. Given Bitcoin's enormous hash rate, this is prohibitively expensive and has never been successfully executed on the Bitcoin network.