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The mempool (memory pool) is a pool of transactions that are not included in the timechain yet. It is a waiting room for transactions, if you will. Every full node has its own mempool—there is no single mempool that is shared by nodes, there is an individual mempool for every single node. Mempools might differ depending on node configuration. Nodes can decide which transactions to keep, which transactions to pass on to other nodes, and which transactions to drop.

Miners will consult their mempools to construct candidate blocks. Once a transaction is included in a valid block—and your node confirms the blocks validity—it is considered confirmed and removed from the node's mempool.

Learn me a bitcoin: Memory Pool

Miners are incentivised to include transactions that reward them the most, i.e. transactions that pay the highest mining fees. Because mining fees are set by the users, you can view mining fees as a kind of bribe that is paid by the users to the miners. If the bribe is high, the chance of your transaction being included is high. If it is low, the chance of inclusion is lower.

Paying Mining Fees

Mempool and Transaction Fees

Bitcoin block space is scarce. If onchain activity is high, the mempool will be crowded. The more crowded the mempool, the higher the "bribe" you will have to pay to miners.

Fee estimation is an attempt to guess how high the fee needs to be for your transaction to be included in the future. It is more an art than a science, since the future is always uncertain and mempool conditions might change quickly.

Most mempool explorers have some sort of fee estimation. You can also use the estimatesmartfee command to do a fee estimation based on your mempool:

bitcoin-cli estimatesmartfee 21 economical

Mempool Instances

We recommend that you use your own node to inspect the mempool. For example, you can use the getrawmempool command to inspect your mempool:

bitcoin-cli getrawmempool true

Of course, plenty of graphical user interfaces exist too. The software that is powering is free software and can be installed quite easily on your node.

Mempool One-Click Installation

Mempool Visualisations

You can also refer to public mempool instaces in case you don't have access to the mempool of your own node. Note, however, that using public mempool and block explorers has certain privacy implications, as whoever is running these websites will be able to correlate your IP with the blocks and transactions you look up. Use Tor or a VPN to mitigate this.

There are also great visualisations that help you understand what's going on, Bitfeed and Mempool Observer being two of them.

Visualisation: Mempool Observer

Visualisation: Bitfeed