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Calldata Decoder Guide

Last updated: April 2026

What is Ethereum calldata and how is it encoded?

Calldata is the raw input data sent with an Ethereum transaction when a user or contract calls a function on another smart contract. Every smart contract call on the EVM (Ethereum Virtual Machine) encodes the function to call and its arguments into a compact binary format called ABI encoding. This encoded data is what you see in a transaction's input field on a block explorer — a long hexadecimal string that looks completely unreadable without the right tools.

For example, a simple ERC-20 token transfer might appear as:

0xa9059cbb000000000000000000000000d8da6bf26964af9d7eed9e03e53415d37aa9604500000000000000000000000000000000000000000000003635c9adc5dea00000

Without decoding, this tells you almost nothing. With the Calldata Decoder, it becomes immediately clear that this is a transfer(address,uint256) call sending tokens to a specific address.

How does Ethereum ABI encoding work?

The Ethereum ABI (Application Binary Interface) specification defines how function calls and their parameters are encoded. Every ABI-encoded calldata follows the same structure:

  • Function Selector (4 bytes) — The first 4 bytes of the calldata are the function selector. This is computed as the first 4 bytes of the Keccak-256 hash of the function signature. For example, transfer(address,uint256) hashes to 0xa9059cbb.
  • Encoded Parameters — The remaining bytes encode the function arguments using ABI encoding rules. Fixed-size types like uint256 and address are padded to 32 bytes. Dynamic types like string and bytes use a two-part encoding with an offset pointer followed by the actual data.

The Calldata Decoder uses the 4byte.directory and on-chain ABI data to look up the function selector and then decodes the remaining bytes according to the matched function signature.

How do I decode raw calldata hex?

Follow these steps to decode any Ethereum calldata:

  • Step 1: Navigate to the Calldata Decoder page.
  • Step 2: Paste the raw calldata hex string into the input field. Make sure to include the 0x prefix.
  • Step 3: Click the decode button. The tool will extract the 4-byte function selector, look it up against known function signatures, and decode the parameters.
  • Step 4: Review the decoded output, which shows the function name, parameter types, and their values in a human-readable format.

Supported Input Formats

The Calldata Decoder accepts the following input formats:

  • Full calldata with 0x prefix — The standard format copied from block explorers or wallet interfaces.
  • Full calldata without 0x prefix — The tool handles both formats automatically.
  • Minimum 4 bytes — At least 4 bytes (8 hex characters) are required for the function selector. If no parameters are present, the tool will still identify the function name.

Example: Decoding a USDT Transfer Call

Let's walk through a real example. The following calldata represents a USDT (Tether USD) transfer on Ethereum mainnet:

0xa9059cbb000000000000000000000000ab5801a7d398351b8be11c439e05c5b3259aec9b0000000000000000000000000000000000000000000000000000000077359400

Breaking this down:

  • a9059cbb — Function selector for transfer(address,uint256)
  • 000...ab5801a7d398351b8be11c439e05c5b3259aec9b — The recipient address, padded to 32 bytes
  • 000...77359400 — The amount in the token's smallest unit (hex 77359400 = decimal 2000000000, which is 2,000 USDT with 6 decimals)

After pasting this into the Calldata Decoder, you will immediately see:transfer(address to, uint256 amount) with the recipient address and amount clearly displayed.

When should I use a calldata decoder?

The Calldata Decoder is most useful when you need to:

  • Debug a failed transaction — When a transaction fails, understanding exactly what function was called with what parameters helps identify why it reverted.
  • Verify a pending transaction — Before approving a transaction in your wallet, decode the calldata to confirm what the contract will actually execute.
  • Audit a smart contract interaction — When reviewing on-chain activity for a protocol or a suspicious transaction, decoding calldata reveals the exact function call.
  • Learn ABI encoding — The decoded output with parameter types is a practical way to understand how the Ethereum ABI works in practice.
  • Build developer tooling — Quickly prototype calldata parsing logic by verifying your expected decoding against the tool's output.