Mempool

The waiting room of unconfirmed transactions — where every Bitcoin transaction sits before being included in a block.

When you broadcast a Bitcoin transaction, it doesn't land on the blockchain immediately. It goes into the mempool — the "memory pool" of unconfirmed transactions that every Bitcoin node keeps. Miners pick transactions from their mempool to include in the next block, prioritising those with the highest fees.

During quiet periods, the mempool is small and almost any transaction confirms in the next block. During busy periods (price spikes, ordinals frenzies), it bloats with thousands of transactions competing for the limited block space. Fees rise as users bid for inclusion.

You can monitor the mempool in real time on sites like mempool.space — it shows current fee rates, expected confirmation time at different fee levels, and the size of the backlog. Useful for deciding when to send a transaction if you're cost-sensitive.

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Course topic

Mining & nodes

How Bitcoin actually works. Miners, nodes, blocks, and what running your own changes.

Related terms

Glossary

Block

A bundle of Bitcoin transactions added to the blockchain roughly every ten minutes.

Glossary

Mining

The process of competing to add new blocks to the Bitcoin blockchain. Miners earn block rewards plus fees.

Glossary

RBF

Replace-By-Fee — a way to bump up the fee on an unconfirmed transaction so it confirms faster.

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