How EIP-55 works
An EVM address is 20 bytes, written as 40 hex characters. EIP-55 adds a checksum without changing the address: it takes the Keccak-256 hash of the lowercase address and upper-cases each letter whose matching hash nibble is 8 or higher. A wallet that sees mixed case can then catch almost all typos: the EIP estimates that only about 0.0247% of mistyped addresses would still pass the check.
- Valid checksum: the capitalisation matches, so the address was very likely copied correctly.
- Checksum mismatch: mixed case that doesn't match. Treat it as a typo and do not send funds.
- No checksum: all lowercase or all uppercase is a valid format but carries no typo protection.
The checksum does not tell you whether an address belongs to anyone, or which chain it's on. The same address exists on every EVM chain.
FAQ
Does this work for Base, Arbitrum and Polygon addresses?
Yes. They all use the same 20-byte address format and EIP-55 checksum. (EIP-1191 chain-specific checksums, used by RSK, are not covered.)
Is anything sent to a node or API?
No. Hashing uses an embedded Keccak-256 implementation (js-sha3, MIT) in your browser.