EIP-712 Typed Data Hash Calculator

When a dapp asks your wallet to “sign typed data”, the wallet does not sign the JSON you see. It hashes it first, in a fixed order set by EIP-712, and signs a single 32-byte digest. If a contract computes that digest even slightly differently, every signature fails with nothing more helpful than “invalid signature”. This calculator shows each step of the hash.

Paste the object a dapp passes to eth_signTypedData_v4, with its types, primaryType, domain and message. You get the digest a wallet signs, the domain separator, hashStruct of the message, the typeHash and the encodeType string behind it, and the 32-byte word every field turns into. Nothing is signed and no wallet is connected, so it is also a safe way to see what a signature request actually commits you to.

Worked Example: The EIP's Own Mail Message

The tool opens with the example from the EIP-712 specification: a Mail from Cow to Bob reading "Hello, Bob!", on the “Ether Mail” domain at chain 1. The encodeType reads Mail(Person from,Person to,string contents)Person(string name,address wallet), the domain separator is 0xf2cee375…a912090f, and the digest is 0xbe609aee343fb3c4b28e1df9e632fca64fcfaede20f02e86244efddf30957bd2, the same value the specification publishes. Press Load Sample to switch to an ERC-2612 Permit on the USDC contract. That is the gasless-approval message most wallets show, and its digest is 0x106980d0…3ede1bd9d7.

Two outputs are easy to miss. Solidity constant gives a ready line such as bytes32 constant MAIL_TYPEHASH = keccak256("…"); for the verifying contract. The field tables let you copy any single encoded word, which is the fastest way to find the one field where your contract and your frontend disagree.

Encoding Method, Field by Field

• Numbers, bool, address: one 32-byte word, left-padded; negative intN values use two's complement.
• bytes1 to bytes32: right-padded to 32 bytes, and the value must be exactly N bytes long.
• string and bytes: replaced by their keccak256, so a 5,000-character string still takes one word.
• Structs: replaced by their own hashStruct, which is keccak256(typeHash ‖ encoded fields).
• Arrays: keccak256 of every item's encoding laid end to end.

The digest is keccak256 of 0x19 0x01, the domain separator and hashStruct(message). Results were checked against the ethers.js 6.13.4 TypedDataEncoder on 2,000 randomly generated schemas and messages, and a signature made by ethers recovered to the same key from this digest.

Edge Cases That Change the Digest

• Large numbers: a JSON number above 9,007,199,254,740,991 loses its last digits before any code sees it. Instead of hashing a different value, the tool stops with, for example, “"value" is a number above 9,007,199,254,740,991, which JSON numbers cannot hold exactly. Put it in quotes as a string.”
• No EIP712Domain in types: the domain type is built from the keys present, in the order name, version, chainId, verifyingContract, salt, as ethers does. If EIP712Domain is given but the domain has a key it does not list, an amber note warns that the key is not hashed.
• No primaryType: the one struct that no other type refers to is used, and the result says so. If two could qualify, you are asked to name it.
• Extra message keys are ignored by the standard and listed in a note. A missing field is an error, not a zero.
• Mixed-case addresses must pass the EIP-55 checksum, and plain uint is rejected because EIP-712 needs the full name, uint256.