The mempool is not a single global thing, every node keeps its own local mempool of transactions it has seen but that are not yet confirmed in a block. Nodes relay valid transactions to their peers, so mempools across well-connected nodes tend to converge, but they are never guaranteed to be perfectly identical at every moment, and nodes can apply different local policies for what they relay or store.
Miners typically build blocks by picking mempool transactions in order of fee rate (satoshis per virtual byte), since block weight is capped and higher fee-rate transactions earn them more revenue per unit of scarce space. When the mempool grows larger than a node's configured memory limit, it evicts the lowest fee-rate transactions to make room for higher-paying ones.
Two mechanisms let users adjust after broadcasting: Replace-By-Fee (RBF) lets the original sender rebroadcast a conflicting version of the same transaction with a higher fee, replacing the first in mempools that honour it; Child-Pays-For-Parent (CPFP) lets anyone, including the receiver, spend an unconfirmed output with a high enough fee that miners are incentivised to confirm both transactions together.
Explain more simply
When you broadcast a transaction, it does not go straight into a block. It first sits in a waiting area called the mempool (memory pool) until a miner picks it up.
How quickly it gets picked up mostly depends on the fee you attached: miners generally prefer transactions that pay more per byte of space, since block space is limited.
Real-world analogy
The mempool is like the waiting list at a busy restaurant with limited tables: bigger tips (fees) generally get you seated sooner, but each host (node) keeps their own list and can run it slightly differently.
Key facts
- Every node keeps its own local mempool of unconfirmed, relayed transactions.
- Miners generally prioritise transactions by fee rate in sat/vB when filling limited block space.
- RBF lets a sender rebroadcast with a higher fee; CPFP lets a receiver bump priority by spending an unconfirmed output with a high fee.
Common misconception
“There is one single, official mempool that everyone sees the same way.”
The mempool is a local, per-node view built from relayed transactions and each node's own policy; it is generally similar across well-connected nodes but not perfectly identical or centrally maintained.[5]
Go deeper
Because mempool state is local and probabilistic rather than globally agreed, fee estimators (which suggest a sat/vB rate for a target confirmation time) are approximations based on recent block history and a node's own current view of pending demand, they can disagree between services and can be wrong during sudden demand spikes.
A transaction that never accumulates enough fee pressure can sit unconfirmed until it is evicted from mempools entirely (after which it must be rebroadcast to be considered again) or ages out under a node's own expiry policy; it is never automatically 'cancelled' network-wide, since no central authority tracks or removes it everywhere at once.
Quick check
Answer every question correctly (100%) to complete this lesson.
1.What generally determines how quickly a transaction gets confirmed?
2.What does Child-Pays-For-Parent (CPFP) allow?
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Extra exam questions
Every question here counts towards your accuracy, XP and rank. No guessing: every answer is explained.
Quick check
Answer every question correctly (100%) to complete this lesson.
1.What is the mempool?
2.Is the mempool the same for every node on the network?
3.What typically determines which transactions a miner picks from the mempool first?
4.Is mempool acceptance policy part of Bitcoin's consensus rules?
5.What happens to a transaction with a very low fee rate during a busy period?
6.What is Replace-By-Fee (RBF)?
7.Why do mempool sizes vary between nodes?
8.What happens to a transaction once it is confirmed in a block?
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Sources
- mempool.space REST API, mempool.space (open source)
Live block, mempool, fee, mining and Lightning figures shown on this site come from this API.
- Bitcoin developer documentation, bitcoin.org / Bitcoin Optech glossary
- Blockstream Esplora API, Blockstream (open source)
Fallback block and transaction data provider with a compatible interface.