Calldata compression is the practice of encoding transaction data more compactly before posting it as calldata to a base-layer blockchain, reducing the number of bytes that must be paid for and stored. Rollups such as Arbitrum and Base apply techniques including signature aggregation, zero-byte-heavy encoding and custom serialisation formats to shrink batch size. Because calldata gas cost dominates rollup transaction fees, compression directly reduces the cost of settling Layer 2 state on the base chain.