Mempool

‘Mempool’ is short for memory pool: the working memory in which a nodeGlossaryNodeA computer that runs Bitcoin software and exchanges transactions and blocks with other nodes. Together, the nodes form the Bitcoin network; full nodes check every rule themselves.On the learning path: Stage 1 · Step 3 – How does Bitcoin work? →In the glossary →× collects checked transactions that aren’t in a block yet. There is no single ‘mempool’ – every node keeps its own, and the contents can differ slightly. Miners usually pick the transactions with the highest fee rate in sat/vBGlossarysat/vB (satoshis per virtual byte)Unit for the fee rate of a Bitcoin transaction: how many satoshis you pay per virtual byte of its size. The higher the rate, the sooner miners will include the transaction in a block.In the glossary →×.^[1]

When it gets full

By default, Bitcoin Core reserves 300 MB of memory for the mempool. When it’s full, the transactions with the lowest fee rate are dropped first, and new ones are only accepted above a higher minimum fee. Bitcoin Core also removes unconfirmed transactions after 336 hours – two weeks – by default.^[1],[2]

If your payment drops out, the money isn’t lost – it never left your wallet. However, other nodes may still know the old version. So don’t send a new payment using different coins, or you may end up paying twice. Instead, spend the same coins again with a higher fee (replace-by-feeGlossaryReplace-by-fee (RBF)A method of replacing a still-unconfirmed transaction with a new version that pays a higher fee. This lets you speed up a stuck payment, as long as it isn’t in a block yet.On the learning path: Stage 5 · Step 4 – First withdrawal →In the glossary →×). While the payment is still waiting, CPFPGlossaryCPFP (Child Pays for Parent)A way to speed up a stuck transaction: a follow-up transaction (the ‘child’) spends one of its outputs and pays a high fee. It then pays off for miners to include both transactions together.On the learning path: Stage 5 · Step 4 – First withdrawal →In the glossary →× can help too.

What the mempool tells you

Block explorers show how full the mempool is and which fee rate is currently needed. Current estimates are around 2 sat/vB for the next block and 1 sat/vB for payments that aren’t urgent. When it’s busy, they rise considerably – so check the situation right before sending. More under Transactions & fees.