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SSCC-18 check digit

Enter the seventeen digits — extension, company prefix and serial — and get the eighteenth. No signup, nothing stored.

How the digit is computed

Take the seventeen digits, ignoring the check digit. Working from the right, multiply each digit alternately by 3 and by 1, starting with 3 on the rightmost digit. Sum the results, then subtract that sum from the next multiple of ten. The remainder is the check digit.

17 digits in, one digit out. The check digit is computed, never chosen.
A barcode can scan perfectly and still carry an invalid check digit. The scanner reads what is printed; it does not know what you meant.

A worked example

Base 00340123450000012. From the right, the digits are weighted 3, 1, 3, 1 and so on: 2×3 + 1×1 + 5×1 + 4×3 + 3×1 + 2×3 + 1×1 + 4×1 + 3×3 = 47. The next multiple of ten is 50, so the check digit is 3 and the SSCC is 003401234500000123.

Where it goes on the label

The eighteen digits are encoded as a GS1-128 barcode under application identifier (00). The human-readable digits sit underneath, because a person has to key them when a scanner will not read the bars.

Pallet: at least two adjacent sides, so a forklift driver can scan without turning the load.
Carton: one label per carton when the retailer requires carton-level labelling.
The same numbers must appear in the 856, in the same order, as the physical shipment.

Why labels still fail

Four causes, and only one of them is the printer.

A serial reused — reprinted from last week, so the DC has already seen that number.
A check digit appended as a zero — generated as text rather than computed.
Print quality — too small, wrong stock, or angled so a handheld rejects it.
ASN mismatch — valid numbers on the pallet, different numbers in the 856.