Block Weight

Definition

Block weight is Bitcoin's accounting system for limiting block size while distinguishing transaction data from segregated witness data. Under the SegWit rules, a block can contain at most four million weight units. Data outside the witness receives four weight units per byte, while witness data receives one. The equivalent formula is three times the stripped size plus the total serialized size. Transaction virtual size, commonly expressed in virtual bytes, is its weight divided by four and rounded upward. This lets wallets compare fee rates across transactions with different structures. Weight is not simply the number of transactions or the number of coins transferred. Two transactions sending the same value may consume different space because they use different inputs, outputs, or spending conditions. Understanding weight helps explain why batching payments and selecting suitable input types can change transaction costs.