Crypto glossary

Mempool

The mempool, short for memory pool, is a waiting area for valid transactions that have been broadcast but not yet included in a block. Miners or validators pick transactions from it to build the next block.

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How the mempool works

When you send a transaction, your wallet broadcasts it to the network. Each node checks it, for example that the signature is valid and the coins are not already spent, and then keeps it in its own mempool while passing it on to peers. There is no single global mempool; every node has its own, and they are usually similar but not identical.

Block space is limited. Miners on Bitcoin, or validators on Ethereum, generally choose the transactions that pay the most per unit of block space. When many people want to transact at once, the mempool fills up and fees rise as users compete for the next blocks.

An example

Say you send bitcoin with a low fee during a busy period. Your transaction sits in the mempool while higher-paying ones get confirmed first. It may wait hours or longer. If demand drops, it eventually gets picked up; if it stays unconfirmed long enough, many nodes drop it from their mempool by default, after which the coins are effectively back in your wallet.

Getting unstuck

On Bitcoin, replace-by-fee (RBF) lets you rebroadcast the same transaction with a higher fee, and child-pays-for-parent (CPFP) lets the recipient speed it up by spending the unconfirmed output with a high fee. On Ethereum, you can send a new transaction with the same nonce and a higher fee to replace a pending one.

Risks to know

Pending transactions are public. On Ethereum and similar chains, bots watch the mempool and can place their own trades before and after yours to profit from the price movement you cause, a practice known as MEV, with sandwich attacks as a common form. An unconfirmed transaction is also not final, so merchants should not treat it as a completed payment for significant amounts.

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Frequently asked questions

Why is my transaction stuck in the mempool?

Usually because the fee was too low for current demand. You can wait, or speed it up with replace-by-fee or a similar method if your wallet supports it.

Can a transaction in the mempool be cancelled?

Not directly, but it can often be replaced with a higher-fee transaction that sends the funds back to yourself, which makes the original invalid.

Does a full mempool mean the network is broken?

No. It means demand for block space is high. Transactions still confirm, prioritized by fee.

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All glossary terms · Educational reference only — not investment, legal, tax or financial advice.