Guides

Plain-language explainers for the codes behind products, books, bank accounts and industries — written against the official standards, with worked examples you can verify in our tools.

Understand the codes

How check digits work	Mod-10, Mod-11 and Mod-97 explained step by step — the math that catches typos in barcodes, ISBNs and IBANs.
SIC vs NAICS	Two industry classification systems, one frozen in 1987 — which one you need and how they map.
HS vs HTS vs Schedule B	Customs codes untangled: the 6-digit international core and the 10-digit import/export extensions.
Find your NAICS code for SAM.gov	Picking the right industry code for US government registration, without guesswork.

Avoid the classic mistakes

Excel eats your barcodes	Leading zeros vanish and long numbers turn into scientific notation — how to stop both.
Fix "invalid GTIN" on Amazon	What Amazon actually checks and how to get a listing through.

For developers

Working code to validate these identifiers yourself — every sample is tested, with the check-digit math explained and a worked example you can paste and run.

GTIN check digit in Python	Compute and validate GTIN-8/12/13/14 — with the leading-zero trap handled.
GTIN check digit in Excel & Sheets	One formula to calculate or verify a barcode check digit down a whole column.
GTIN check digit in SQL	Audit a product table in place — validate check digits inside a query.
Validate an IBAN in Python	Full MOD-97 with country lengths and letter transliteration.
Validate an IBAN in JavaScript	MOD-97 done right — the chunked-modulo trick that avoids the big-number bug.
Convert ISBN-10 ↔ ISBN-13 in Python	Recompute the check digit both ways, including the 979 caveat.
Luhn algorithm in JavaScript	Validate cards and IMEIs — and why barcodes are not Luhn.
Validate EU VAT format in Python	Per-country structure for all 27 states — plus what only VIES can confirm.
VIN check digit in Python	The 49 CFR 565.15 algorithm — and why EU VINs legitimately fail it.
Container (ISO 6346) check digit in Python	For logistics scripts and EDI pipelines.
ABA routing checksum in Excel	Audit a vendor payment file with the 3-7-1 formula.
Validate a whole CSV via the API	A ready-to-run script that checks a column of codes and writes the results back.

Fix a specific error

Hit an actual error message — Amazon 5665 or 8572, Merchant Center "Invalid GTIN", a rejected ISBN on KDP, "IBAN invalid" at the bank? There's a dedicated step-by-step library for those: the error troubleshooting library →