EIP-55: the checksum in letter case

How it works

An Ethereum address is 20 bytes written as 40 hex characters. EIP-55 takes the Keccak-256 hash of the lower-case address and, for every letter a–f, writes it in upper case when the matching hex digit of the hash is 8 or higher. Digits stay digits.

So 0x5aAeb6053F3E94C9b9A09f33669435E7Ef1BeAed (a test vector from EIP-55) carries a checksum, and changing any character almost always breaks the pattern of upper and lower case.

When there is no checksum

An address written all in lower case (or all in upper case) carries no checksum at all. It is still valid, but a typo in it gives another valid-looking address. That is why the checker shows “no checksum in this form” for it and offers the EIP-55 form.

The same on every EVM chain

BNB Smart Chain, Polygon, Arbitrum, Base and other EVM chains use the same 20-byte address and the same EIP-55 rule. (EIP-1191 defines a chain-specific variant that only a few chains use; the checker follows EIP-55.)