Bitcoin White Paper: 7. Reclaiming Disk Space

Too Long; Didn't Read

Once the latest transaction in a coin is buried under enough blocks, the spent transactions before it can be discarded to save disk space. To facilitate this without breaking the block's hash,transactions are hashed in a Merkle Tree [7], with only the root included in the hash. Old blocks can then be compacted by stubbing off branches of the tree. A block header with no transactions would be about 80 bytes. Moore's Law predicts current growth of 1.2GB per year.