FBA box ID format and label builder.

How a box ID is built

Amazon’s FBA guide on shipping label specifications describes the rule. For small parcel shipments each package’s label should carry a unique barcode. To build the values, take the shipment ID and append U and 000001 for the first package; for each next package, increase the number by one and append U and that number. Its worked example uses shipment FBA1MMD8D0 with three packages, giving FBA1MMD8D0U000001, FBA1MMD8D0U000002 and FBA1MMD8D0U000003.

The same guide adds a note that a box label with its own unique number must follow the six-digit format after the U (U000001, U000002 and so on) and be affixed to each carton sent to a fulfillment center. It also lists the barcode format as Code 128A and recommends a label of 4 inches by 6 inches, up to 6 by 8, on permanent adhesive.

Two cautions apply. The guide comes from Amazon MWS, which Amazon retired on 31 March 2024, so we cite an archived copy and Seller Central’s current shipping label requirements govern. And the list this page builds is only the construction rule: for a real shipment, prefer the labels your Seller Central shipment workflow generates, and use a hand-built list only if you produce your own labels following the guide’s specifications.

How the check works, variable by variable

The checker removes spaces, capitalises letters and splits the value at the last U that is followed by exactly six digits. The part before it must look like an FBA shipment ID (FBA plus 6 to 10 letters or digits, an observed pattern). The six digits become the box number, and U000000 is rejected because Amazon’s example starts at U000001.

If you paste only a shipment ID, it tells you to add U and six digits and shows an example. If you paste something starting FBA with a U but the wrong digits afterwards, it says the part after the final U is not exactly six digits.

The list builder

Enter a shipment ID and a count from 1 to 500 and the tool prints the box IDs in order, one per line, ready to copy into a label tool or spreadsheet. It follows the guide’s rule exactly: the shipment ID, U, and a zero-padded six-digit counter starting at 1. It does not make labels or barcodes. To render the barcodes, paste the lines into a Code 128 generator.

Worked example

A seller has shipment FBA1MMD8D0 (Amazon’s example ID) and 12 cartons. The builder returns FBA1MMD8D0U000001 through FBA1MMD8D0U000012. Checking FBA1MMD8D0U000012 returns shipment part FBA1MMD8D0 and box part U000012, box number 12. Checking FBA1MMD8D0U00012 fails because only five digits follow the U, and the message says so.

The numbers are Amazon’s documentation example, not a real shipment of yours.

Box content information and why labels matter

Amazon’s v0 inbound model records where box contents information came from: NONE, FEED, 2D_BARCODE or INTERACTIVE. With NONE, Amazon says it will process the box contents manually and that may incur a fee, and the model carries an estimated manual processing fee field for that case. So an unlabelled or unreported box has a cost, not just a delay. The model also describes the seller printing box labels for small parcel shipments or pallet labels for less-than-truckload shipments.

Check carton size and weight against Amazon’s guidance with the carton size checker before you label, because relabelling after a repack means new box numbers.

Label and barcode specifications from the archived guide

The same archived guide gives print specifications for a seller-generated shipping label. They are reproduced here as published, with the caveat above that MWS has been retired:

Common mistakes

Putting the plain shipment ID on every carton instead of a unique value per package, which the guide says to avoid for small parcel shipments. Counting from 000000, writing five digits, leaving out the U, or using a lower-case u. Carrying numbering over between shipments: each shipment has its own ID, so each starts again at U000001. And rebuilding the list after a repack without reprinting every label, which leaves two cartons with the same barcode.

Who this is for

FBA sellers and prep centers who produce their own carton labels, developers reading ID fields in the inbound API, and operations staff matching a scanned barcode back to its shipment.

What this does not cover

• It does not make or print labels and does not read Seller Central.
• It does not confirm that a shipment or box exists, or its status.
• It does not cover pallet labels for less-than-truckload shipments, whose format we did not find in a source we could read.
• It does not store, send or log what you type.