What is block size?
Block size is the maximum amount of data a single Bitcoin block can hold. Originally 1 MB. Set by Satoshi. This limit determines how many transactions Bitcoin can process per block. With SegWit's 2017 activation, Bitcoin moved to a "block weight" system where the effective maximum is about 4 MB, though typical blocks average 1.5-2 MB.
Why It Matters
Block size directly affects Bitcoin's throughput, fees, and decentralization. Larger blocks fit more transactions, reducing fees and wait times during busy periods. But larger blocks require more bandwidth, storage, and processing power to validate. That raises the cost of running a full node. The tradeoff matters.
This was the center of the "block size wars" from 2015-2017, one of the most consequential debates in Bitcoin's history. The community ultimately chose to keep blocks small and scale through layers like the Lightning Network, preserving decentralization at the base layer.
How It Works
Every 10 minutes on average, a miner assembles a block of pending transactions from the mempool. Transactions are measured in "weight units" since SegWit. Each block holds up to 4 million weight units. Witness data (signatures) is discounted at 1 unit per byte instead of 4. That's how SegWit effectively expanded capacity without changing the base block size limit that had been the subject of years of contentious debate. When blocks are full, miners prioritize transactions offering higher fees per weight unit.