What Is Calldata?
Calldata is the payload of an Ethereum transaction: the bytes that say which function to call and with what arguments. A simple ETH send has empty calldata. A Uniswap swap has a lot of it. You do not write it by hand. The dapp and wallet encode it. You are still responsible for what it means.
How It Actually Works
- The first four bytes are usually the function selector. The rest are the arguments. Explorers decode this when the contract is verified. That is why Etherscan can say "swapExactTokensForTokens" instead of a hex blob.
- Calldata is stored on-chain forever and is priced as part of gas. That permanence is why early rollups posted batches here, and why blobs were invented as a cheaper, temporary lane for the same job.
- A malicious site can put a setApprovalForAll in calldata behind a button that says "claim." Simulation exists to catch that. Hex-blind signing is how people get drained.
Risks and Common Mistakes
- Signing what you cannot read. If the wallet shows raw data and a friendly icon, trust the data.
- Confusing "L2 posted data to Ethereum" with "Ethereum executed my swap." Calldata (or a blob) can be a bulletin board, not a settlement of your trade.
When It Matters
Reading a transaction you did not understand, and any scaling conversation that mixes blobs, calldata, and execution. Pair with data availability.
Related Terms
Glossary · Learn · Resource Library · Return to Official Home Page
Copyright © 2026 Crypto Guidance Inc.