Block capacity refers to the maximum amount of data that a single block in a blockchain can hold. This limit is typically determined by the size of the block, which is measured in bytes. For example, Bitcoin blocks are limited to 1 megabyte, meaning they can only contain a specific number of transactions at a time.

If the total size of transactions waiting to be added exceeds this limit, some transactions will remain unconfirmed until space becomes available in future blocks. This capacity impacts transaction speed and fees. During periods of high activity, users may need to pay higher fees for their transactions to be prioritized.

Consequently, block capacity plays a crucial role in determining a blockchain’s scalability and efficiency. Solutions like layer 2 protocols, or increasing the block size, are often explored to improve capacity and user experience.