ARN Decoder

What an ARN is for

The Acquirer Reference Number is the identifier that survives. A STAN is unique only within a day and only per acquirer. An authorisation code is six characters with no structure. The ARN is 23 digits assigned at clearing, and it is what schemes, acquirers and issuers all use to point at the same transaction weeks later — which is why it turns up on every chargeback notice.

If a cardholder disputes a payment and you need to find it in your own records, the ARN is the key you want. If you only have an auth code, you are going to have a bad afternoon.

The layout

Digit 1 is a prefix indicating the processor or reference type. Digits 2-7 are the acquiring institution BIN — the same value that appears in ISO 8583 DE 32, and the fastest way to work out which acquirer handled a transaction you do not recognise. Digits 8-11 are the processing date as a Julian YDDD. Digits 12-22 are an acquirer-assigned sequence. Digit 23 is a mod-10 check over the preceding 22.

Be aware that only the BIN, the date and the check digit are dependable. The prefix and the sequence layout are conventions rather than requirements, and some acquirers put a terminal or batch reference inside the sequence portion. This tool flags anything that looks unusual rather than pretending certainty.

Why the date is ambiguous

A Julian YDDD carries only the last digit of the year. 5222 is day 222 of a year ending in 5 — which could be 2025, 2015 or 2005. In practice the transaction context resolves it, but a decoder cannot, so this one shows you the alternatives instead of guessing silently. Day 366 is a useful sanity check: it only exists in a leap year.

Related identifiers

Do not confuse the ARN with the RRN (DE 37), a 12-character retrieval reference set by the acquirer at authorisation time, which often embeds its own Julian date. Both are "reference numbers", neither is globally unique, and they are not interchangeable. The ARN is created at clearing; the RRN exists from the authorisation onwards.