Definition
Block size is a metric that specifies the maximum amount of data that a single block on a blockchain is allowed to hold. This data mainly consists of transactions, along with supporting information stored in the block header and other block fields. Block size is usually measured in units like bytes, kilobytes, or megabytes, depending on the design of the blockchain. By setting an upper limit on how large a block can be, the protocol defines how much information can be recorded in each block.
Because each block has a fixed block size limit, only a certain number of transactions can be included in a block at any given time. This directly influences the network’s throughput, or how many transactions can be processed over a period, and is closely related to overall scalability. A larger block size allows more transaction data per block, while a smaller block size restricts it, shaping how the blockchain grows and how quickly new data is added.
Context and Usage
In practice, block size defines the upper bound on the combined size of all transactions and metadata inside a block. The block header itself occupies only a small portion of this limit, leaving most of the space for transaction data. When a block is created, its total size must not exceed the protocol’s block size constraint, or it will be considered invalid by network participants.
Block size also affects how blocks move through the network during block propagation. Larger blocks can take longer to transmit and verify across nodes, while smaller blocks tend to spread more quickly. As a result, block size is a central parameter in discussions about scalability, since it balances the desire for higher throughput with the need to keep blocks easy to share and validate across the entire network.