What is block size?
Block size refers to the maximum amount of data that can be included in a single Bitcoin block. Originally set at 1 MB by Satoshi Nakamoto, this limit determines how many transactions Bitcoin can process per block. With the activation of SegWit in 2017, Bitcoin moved to a "block weight" system where the effective maximum is about 4 MB, though typical blocks average around 1.5-2 MB.
Why It Matters
Block size directly affects Bitcoin's throughput, fees, and decentralization. Larger blocks can include more transactions, reducing fees and wait times during busy periods. But larger blocks also require more bandwidth, storage, and processing power to validate, which raises the cost of running a full node. This trade-off was at the center of the "block size wars" from 2015-2017, one of the most significant debates in Bitcoin's history. The community ultimately chose to keep blocks small and scale through layers like the Lightning Network, preserving decentralization.
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 can hold up to 4 million weight units (4 MWU). Witness data (signatures) is discounted, counting as only 1 weight unit per byte instead of 4. This is why SegWit effectively increased capacity without changing the base block size. When blocks are full, miners prioritize transactions offering higher fees per weight unit.