What is the EIP-55 checksum?

EIP-55 adds a typo-detecting checksum to Ethereum addresses without changing their length. The lowercase address is hashed with keccak-256, and each letter is uppercased when the matching nibble of the hash is 8 or greater. Wallets that show mixed-case addresses are encoding this checksum, and a single wrong-case letter reveals a transcription error.

Why does an all-lowercase address pass without a checksum verdict?

EIP-55 is only meaningful when an address contains letters in mixed case. An all-lowercase or all-uppercase address carries no checksum information, so the tool reports it as a valid 20-byte address and shows you the correctly checksummed version to adopt.

Does this use SHA3-256 or keccak-256?

Ethereum uses original keccak-256, which differs from the finalised NIST SHA3-256 in its padding byte. This tool implements keccak-256 exactly, so the checksum it computes matches what MetaMask, ethers.js, and web3.js produce.

Can it resolve an ENS name to an address?

No. Resolving name.eth to its address requires an on-chain call to the ENS registry via an RPC endpoint, which a static page cannot do without a key. The tool validates only that the ENS label follows the allowed character and length rules.

Can it tell a contract from a wallet?

Not from the string alone. Externally-owned accounts and contract accounts share the same 20-byte address format. Only an on-chain code lookup distinguishes them, so always confirm an unfamiliar address on a block explorer before sending.

Ethereum Address Format Validator

Ethereum addresses are 20 raw bytes with no built-in checksum — which is why EIP-55 retrofitted one into the letter casing. This validator recomputes that checksum with a local keccak-256 implementation, so you can confirm an address was copied correctly and get the canonical checksummed form back, all without sending the address to any server.

How it works

A valid address is exactly 40 hexadecimal characters, optionally prefixed with 0x. To check the EIP-55 checksum, the tool lowercases the 40 hex characters, hashes that ASCII string with keccak-256, and walks the result nibble by nibble. For each hex letter (a–f) in the address, it uppercases the letter when the corresponding hash nibble is 8 or higher and leaves it lowercase otherwise; digits are untouched. The recomputed casing is then compared to what you entered.

If your input is all lowercase or all uppercase, there is no casing signal to verify, so the tool simply returns the correctly checksummed address. For ENS names ending in .eth, it validates the label against the allowed pattern — three or more characters of a–z, 0–9, or hyphen, not starting or ending with a hyphen — but does not resolve the name on-chain.

Example and notes

Paste 0x5aAeb6053F3E94C9b9A09f33669435E7Ef1BeAed and the checksum matches. Flip the case of a single letter and the verdict turns to “checksum FAILS” — exactly the typo EIP-55 was designed to catch. Because contract and wallet addresses look identical, treat a pass as “well-formed,” not “safe recipient”; verify unfamiliar addresses on a block explorer first.