Full Bitcoin blocks turn each byte into a fee auction

When a Bitcoin block reaches the consensus limit of 4 million weight units, the available space in the next block is allocated to the transactions offering the highest fee per unit of block space, measured in satoshis per virtual byte (sat/vB). Miners construct block templates to maximize fees, so transactions with higher sat/vB are more likely to be included regardless of when they were first broadcast.

Each node maintains its own mempool of validated, unconfirmed transactions and applies local policies to admission and eviction. At 8 a.m. EDT on Monday, about 90,617 transactions were unconfirmed across node mempools. Bitcoin Core nodes typically allow roughly 300 MB of memory for a mempool and may retain transactions for up to 336 hours. When a node reaches its memory limit it trims the economically weakest connected groups of transactions and raises a local fee floor. An evicted transaction remains valid on the blockchain but is no longer carried by that node.

Bitcoin Core 31.0 introduces a cluster mempool that groups related unconfirmed transactions and ranks them by the fee rate at which the group is expected to be mined. The default cluster limits are 64 transactions and 101 kB of virtual size. The change affects how nodes construct blocks, evict transactions and relay packages, while the 4 million weight-unit consensus ceiling remains unchanged.

Users and businesses can experience longer waits and higher fees during congestion. Wallets may show payments as pending for hours while later, higher-fee transactions are confirmed. Traders, exchanges and treasury operations can see deposits or transfers delayed, which can affect liquidity or the timing of trades. Because fees are charged by transaction weight rather than by transferred value, payments of very different monetary amounts can face similar fee competition when their transaction structures are comparable.

There are three common ways to raise a stalled transaction’s effective fee. Replace-by-fee (RBF) allows a sender to broadcast a replacement transaction that pays a higher fee, subject to node policy and market conditions. Child-pays-for-parent (CPFP) uses a high-fee child transaction that spends an output of the delayed parent to raise the combined fee rate of the package. Some mining pools operate transaction accelerators that submit or prioritize a transaction directly to participating miners; such services have different eligibility rules, limits and fees and some offer a limited number of free pushes per hour.

Engineering work aims to reduce propagation delay and the risk of competing chain tips. Compact block relay lets a node reconstruct much of a new block from transactions it already holds, reducing the data and time needed to share full blocks. Faster effective propagation can lower short-term fee pressure but increases bandwidth and validation demands. New blocks are found at irregular intervals, averaging about one every 10 minutes.

A confirmation visible in a block explorer indicates that some nodes know about a transaction but does not prove every miner has seen it. An unconfirmed transaction can be replaced or evicted from individual nodes while remaining a valid spend on the network. Organizations that accept zero-confirmation receipts assume the operational risk that comes with potential eviction or replacement during periods of congestion.