Checksum EIP-55 Ethereum

EIP-55: how Ethereum turns letter casing into a checksum

EIP-55 is the clever scheme that gives a raw 0x hex Ethereum address built-in typo protection without adding any characters. It does this by deciding, for each letter a–f, whether to print it upper- or lower-case based on a hash of the address itself. Tools that don't understand EIP-55 simply ignore case and still work; tools that do can catch mistyped addresses. This page detects and explains the checksum on a given address.

The algorithm, step by step

• 1. Take the 40-hex address body, lowercased, and compute its Keccak-256 hash (not SHA3-256 — Ethereum uses original Keccak).
• 2. Walk the 40 characters. For each letter (a–f), look at the matching hex nibble of the hash.
• 3. If that nibble is ≥ 8, print the letter uppercase; otherwise lowercase. Digits 0–9 are unaffected.
• 4. The resulting mixed-case string is the checksummed address; re-running the process must reproduce the exact casing.

Reading the result

• All lowercase / all uppercase: valid format but no checksum applied — nothing to verify.
• Mixed case: an EIP-55 checksum is present; full verification means recomputing the Keccak hash and matching every letter's case.
• Mismatch: if even one letter's case is wrong, the address is a typo and a strict wallet rejects it.

Gotchas

• Keccak-256 ≠ SHA-3: Ethereum froze the pre-standard Keccak padding; using NIST SHA3-256 yields the wrong casing.
• This tool flags presence, not full validity: a complete check requires a Keccak-256 implementation in the browser.
• Don't normalise away the casing: lower-casing destroys the checksum a downstream tool might rely on.
• EIP-1191: some chains mix the chain ID into the hash; an address checksummed that way won't match plain EIP-55.

FAQ

Why use casing instead of extra digits? It keeps the address the same length and stays backward-compatible: case-insensitive tools still treat it as the same address.

Does every address have a checksum? No — only mixed-case addresses. All-lowercase ones are valid but unprotected.

Is EIP-55 the same on Polygon or BNB Chain? The base algorithm is identical; some networks adopt EIP-1191, which folds in the chain ID and produces a different casing.