What is a function selector?

The first four bytes of calldata are the function selector, computed as the first four bytes of the keccak256 hash of the function signature, such as transfer(address,uint256). The EVM uses it to route the call to the right function in the contract.

Why are some selectors shown as unknown?

The tool ships a built-in table of common ERC-20, ERC-721, and DEX selectors rather than hashing in the browser. If a selector is not in that table the function name cannot be resolved, but the raw 32-byte argument words are still displayed for manual inspection.

Can it decode strings, bytes, and arrays?

Static types like address, uint256, and bool are decoded directly from the head words. Dynamic types such as string, bytes, and arrays are encoded as tail-pointer offsets; fully resolving them requires the contract ABI, so the tool shows the offset rather than guessing.

How is an address extracted from a 32-byte word?

An Ethereum address is 20 bytes, left-padded to fill a 32-byte word. The tool takes the last 40 hex characters of the word and prefixes 0x. Integers use the full word parsed as a BigInt, and booleans treat any non-zero word as true.

Is my calldata sent anywhere?

No. Decoding happens entirely in your browser using JavaScript. No node, RPC endpoint, or external API is contacted, so you can safely inspect sensitive or unbroadcast transactions offline.

EVM Transaction Calldata Decoder

Every Ethereum contract interaction is encoded as a hex calldata string: a four-byte function selector followed by the arguments, each padded to a 32-byte word. This decoder pulls that apart in the browser so you can see which function a transaction calls and with what values, without spinning up a node or trusting a third-party API.

How it works

The decoder splits the calldata and decodes each argument by its ABI type:

selector  = first 4 bytes (8 hex chars) of the calldata
argData   = everything after, in 32-byte (64 hex) words
address   = last 20 bytes of a word, prefixed with 0x
uint256   = the whole word parsed as a BigInt
bool      = non-zero word is true

The selector is matched against a built-in table of common ERC-20, ERC-721, and Uniswap-style functions to recover the human-readable signature. Static types are decoded inline; dynamic types such as string, bytes, and arrays appear as tail-pointer offsets because resolving them needs the full contract ABI.

Example and notes

The calldata 0xa9059cbb followed by two padded words is an ERC-20 transfer(address,uint256): the first word decodes to the recipient address and the second to the token amount in base units. Remember that a uint256 amount is in the token’s smallest unit, so an 18-decimal token shows 1000000000000000000 for one whole token. When a selector is unknown, look it up in a public 4-byte signature database, then read the raw words here against the recovered argument types.