How a transaction enters the mempool
A transaction can be tracked with a block explorer as soon as it is broadcast to the network. Nodes check its signatures, the bitcoins being spent and the consensus rules, among other things. If it is valid but has not yet been included in a block, it is stored in the node’s local mempool and relayed to its peers.
There is no single centralized mempool. Each node applies its own capacity, relay policy and minimum threshold. As a result, two explorers may show slightly different numbers of pending transactions. A transaction rejected by one node may still be known to others.
When a miner builds a block, it selects transactions that comply with the rules and the block’s weight limit. Once the block is accepted by the network, the transaction receives its first confirmation.
Why fees are expressed in sat/vB
Absolute fees correspond to the difference between the bitcoins spent in the inputs and those created in the outputs. To compare transactions, the network focuses primarily on the fee rate, usually expressed in satoshis per virtual byte, or sat/vB.
A 300 vB transaction at 20 sat/vB pays approximately 6,000 satoshis. A 150 vB transaction at 30 sat/vB pays 4,500 satoshis, but it may be selected first because its fee rate is higher.
The amount sent is not the main factor determining a transaction’s size. A transaction consolidating many small inputs can be large even if it transfers little value. Consolidating UTXOs during a quiet period can reduce the cost of future spending, but it also has privacy implications.
The terms UTXO, satoshi and block are also defined in our crypto ecosystem glossary, which is useful for connecting these technical concepts.
Estimating confirmation times
An estimate is never a guarantee. You need to compare the proposed fee rate with the transaction bands already in the mempool and the speed at which blocks free up space. mempool.space documents its estimates and how it works, while also displaying projected blocks based on the transactions being observed.
If the selected fee rate is above most of the transactions projected for the next block, fast confirmation is likely. If it falls several bands lower, the wait may last hours or longer. A sudden influx of new high-fee transactions can push back previous estimates.
The average block interval is around ten minutes, but two blocks may be mined in quick succession or separated by a long period. Saying that a transaction will be confirmed “exactly in ten minutes” is therefore incorrect.
What one, three or six confirmations mean
One confirmation means that the transaction is included in a block. Each block built on top of it increases the cost of a reorganization that would reverse it. The required number depends on the amount, the risk of fraud and the recipient’s policy.
For a small payment, some services accept an unconfirmed transaction or one confirmation. For a large amount, waiting for several blocks reduces the risk. Exchanges set their own thresholds and may change them during periods of disruption.
A transaction visible in the mempool is not final. It may be replaced if it signals Replace-by-Fee, conflict with another transaction or disappear from some nodes after a long wait.
Accelerating a transaction with RBF
Replace-by-Fee allows an unconfirmed transaction to be replaced with a version that pays a higher fee rate. The wallet must support the feature, and the replacement transaction must comply with relay rules.
Bitcoin Optech’s documentation on Replace-by-Fee explains the different policies used across the network. For users, the key point is not to manually create a second spend without understanding the outputs and the change.
If the wallet displays an “increase fee” option, it generally reconstructs the transaction while keeping the same payment to the recipient. The transaction ID may change, which should be communicated to the recipient.
CPFP and child transactions
Child Pays For Parent involves spending an unconfirmed output in a new transaction with fees high enough to make the package attractive. A miner must include the parent before the child, so it evaluates the package’s average fee rate.
This method can be used by the sender if they control a change output, or by the recipient if they can spend the output received. It requires a compatible wallet and an accurate estimate of the total weight.
RBF and CPFP are not immediate guarantees. If the increase remains insufficient or the relay policy rejects the package, the transaction may continue waiting.
Can a transaction be lost?
An unconfirmed transaction may be removed from a node’s mempool when the node runs out of memory, when its fee rate is too low or after it expires. The bitcoins do not disappear: as long as no conflicting transaction has been confirmed, the original outputs remain available according to the state of the blockchain.
However, the wallet may continue to display the transaction as pending. Resynchronization or rebroadcasting may be necessary. Avoid “acceleration” tools that ask for a seed phrase; this information is never needed to broadcast a transaction.
Our guide to securing cryptocurrencies reviews the checks to perform before handling any keys.
Key takeaways
- The mempool is local to each node, not a single central queue.
- The sat/vB fee rate matters more than the amount sent.
- RBF and CPFP can accelerate a transaction, but they should be used with a reliable wallet.
Understanding the mempool makes it easier to choose fees that match the urgency of a transaction. A non-urgent transaction can wait for a quieter period; a critical transaction should use a fee rate compatible with current demand and be monitored until confirmation.
