Mempool
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The mempool (memory pool) is the set of unconfirmed transactions that have been broadcast to the network but not yet included in a block. Every full node maintains its own version of the mempool.
How it works
- A user broadcasts a transaction to the network
- Nodes validate the transaction and add it to their mempool
- Miners select transactions from the mempool — typically prioritizing higher fees
- Once a miner includes the transaction in a block and that block is confirmed, the transaction leaves the mempool
Mempool congestion
When the network is busy, transactions accumulate faster than miners can process them. This creates a backlog — the mempool grows, and users who want fast confirmation need to pay higher fees to jump the queue.
- Low fee → transaction may wait hours or days (or get dropped)
- High fee → miners prioritize the transaction, confirmed in the next block
What happens to stuck transactions?
Unconfirmed transactions can stay in the mempool for days. Nodes eventually drop them (typically after 2 weeks), returning the funds to the sender. Some wallets allow "fee bumping" (RBF — Replace By Fee) to resend the transaction with a higher fee.
Relevance to miners
Miners choose which mempool transactions to include in each block. A full mempool means higher fee revenue on top of the block reward — which is increasingly important as the reward halves over time.
