Notes
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).
Methodology
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.