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A weight unit is the unit used to measure the size of transactions and blocks as of the SegWit upgrade. Weight units are abbreviated as ‘wu’. Prior to SegWit, transactions and blocks were measured in bytes, and blocks were limited to 1MB or 1 million bytes. After the SegWit upgrade, transactions and blocks are measured by weight units, and blocks are limited to 4 million weight units.
In a legacy transaction, a single byte is equivalent to four weight units. However, for SegWit transactions, any bytes in the Witness, which usually includes the signatures, are counted as one weight unit each. This discount allows SegWit transactions to pay lower fees relative to non-SegWit transactions. Thus, if a block solely consists of legacy transactions, the block size limit of 4 million weight units is still equivalent to 1MB, but if SegWit transactions are included in the block, the block size limit allows for up to 4MB of data.