What was the Bitcoin block size limit before the SegWit upgrade?

Before SegWit, Bitcoin’s block size limit was 1 megabyte. The limit capped the amount of transaction data that could be included in each block and became one of the central constraints in Bitcoin’s early scaling debate.

Bitcoin’s original design did not initially impose the modern limit in the same way. A one-megabyte cap was introduced to prevent attackers from creating extremely large blocks that would consume excessive bandwidth, storage, and processing time for other nodes. The trade-off was a smaller transaction capacity, especially as adoption increased.

SegWit, activated on 24 August 2017 at block 481,824, did not simply replace the old limit with a straightforward 4-megabyte block. It changed transaction accounting by separating witness data and assigning it lower weight, allowing a larger effective capacity while preserving a 1-million-byte base-block limit. The proposed 2 MB SegWit2x hard fork was a separate plan and was never activated.