What was Bitcoin’s original maximum block size limit?

Bitcoin’s original maximum block size limit was 1 megabyte.

The limit was introduced in Bitcoin’s early software to restrict how much transaction data could fit into a single block. Because blocks are added to the blockchain roughly every 10 minutes, the restriction also limited Bitcoin’s transaction capacity and helped keep the amount of data that network participants needed to store manageable.

As Bitcoin usage grew, the 1-megabyte limit became the focus of the block-size debate. Some developers and users supported larger blocks, while others preferred changes that preserved decentralisation and encouraged second-layer systems. This disagreement contributed to the 2017 split that created Bitcoin Cash, whose initial block-size limit was larger than Bitcoin’s.

Bitcoin later activated Segregated Witness in 2017. SegWit changed how transaction data was counted and allowed the effective capacity of blocks to exceed the old 1-megabyte data limit, although the traditional block-size wording is often still used when discussing Bitcoin’s original design. This fact is distinct from Bitcoin’s mining interval: block size measures data capacity, while the interval concerns how frequently blocks are produced.