Standards

Reading the IATA check digit: how the MOD-7 rule works

Every 11-digit air waybill number carries its own proof. The last digit is computed from the rest by a single arithmetic rule — so the number can validate itself, no database required.

Aug 27, 2026· 2 min read· Validation

turns air waybills into clean, validated shipment records — no keying, no OCR clean-up.

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An air waybill number is 11 digits in a 3–7–1 layout: a 3-digit airline prefix, a 7-digit serial, and a final check digit that must equal the serial modulo 7. Because the last digit is derived from the rest by arithmetic, the number validates itself — recompute the serial mod 7, compare it to the printed digit, and a mismatch means a digit is wrong. No carrier account, no lookup. One division.

One example, two outcomes

mod-7 check · 999-12345675

# 999 is a stand-in prefix, not a real carrier
prefix = 999   serial = 1234567   check = 5
1234567 mod 7 = 5      ✓ equals the printed check digit → PASS

# transpose the last two serial digits (…67 → …76):
1234576 mod 7 = 0      ✗ printed check digit is still 5 → FAIL

The transposition is the point. Both versions are crisp, legible digits — an OCR confidence score sees nothing wrong with either. The arithmetic does: swapping two digits almost always changes the remainder, so the printed check digit no longer agrees and the number is held before it mis-routes a shipment or breaks an EDI message downstream.

With , you can catch a bad AWB check digit automatically on every document.

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What it doesn't do

MOD-7 proves the digits are consistent with each other — not that this is the right AWB for the shipment, or that it was ever issued. A few errors leave the remainder unchanged and slip through. So it's never the only check: AWBGuru pairs it with an airline-prefix check and a route cross-check, and the result feeds the document's confidence score. A failing check digit flags the field and routes the document to review instead of delivering it downstream.

Frequently asked

What is the AWB check digit?

The last of the 11 digits. It equals the 7-digit serial number modulo 7, so any AWB number can be checked by recomputing the remainder and comparing it to the printed digit.

Can MOD-7 catch every error?

No. It catches most single-digit misreads and transpositions, because those change the remainder. An error that leaves the remainder unchanged passes, and the check can't tell whether the AWB belongs to the shipment — which is why it's layered with a prefix check and a route cross-check.

Does AWBGuru need airline credentials for this?

No. It's arithmetic on the number itself — no lookup, no carrier account. AWBGuru runs it on every document.

Run the check digit on your own waybills.

Start with 30 free scans — no card. Watch a mis-keyed AWB number route to review while the clean ones clear.

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