The mempool is where all the valid transactions wait to be confirmed by the Bitcoin network. A high mempool size indicates more network traffic which will result in longer average confirmation time and higher priority fees. The mempool size is a good metric to estimate how long the congestion will last whereas the {mempoolcount} chart tells us how many transactions are causing the congestion.
In order to be confirmed, a transaction from the mempool needs to be included in a block. The size of a block cannot exceed 4 million weight units (1 million vbytes) , and each transaction has its own weight depending on the type of transaction, the UTXOs it spends (inputs) and the addresses it sends to (outputs).
Each Bitcoin node builds its own version of the mempool by connecting to the Bitcoin network. The mempool content is aggregated from a few instances of up to date Bitcoin nodes maintained by the Blockchain.com engineering team; this way, we gather as much information as possible to provide accurate mempool metrics.
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