Three Ways to Confirm a CAS Number Is the Right One

A wrong CAS number is one of the quieter ways a cosmetic filing goes sideways. Nobody catches it in a visual scan of the spreadsheet because it still looks like a normal number, formatted correctly, sitting in the right column. It's only wrong. Maybe a supplier's spec sheet had a typo, maybe someone copied the wrong row from an old formula, maybe two digits got transposed somewhere along the way. Here are three checks worth running before you trust the number you've got.

1. Run the check-digit math

CAS numbers are not arbitrary strings, they follow a specific format: up to seven digits, then a hyphen, then two digits, then a hyphen, then a single check digit. That last digit is not decorative, it is calculated from everything before it, and this makes it possible to catch a lot of transposition errors with simple arithmetic.

The check digit is calculated by taking all the digits before the final hyphen, multiplying the last digit by 1, the second-to-last by 2, the next by 3, and so on, summing the results, and taking that sum modulo 10. Whatever comes out should equal the check digit.

Take a hypothetical CAS number like 64-17-5, which is ethanol. Reading right to left before the final hyphen: 7, 1, 6. Multiply by position: 7x1 + 1x2 + 6x3 = 7 + 2 + 18 = 27. Take 27 mod 10 and you get 7... which does not match the check digit of 5, so if you actually saw "64-17-5" claimed for ethanol you'd want to go verify it, because either the digit sequence or the check digit is off somewhere in this example. That's the point of the exercise: the math either holds or it doesn't, and when it doesn't, you know before you file rather than after.

This check is fast to do by hand for a handful of ingredients, and it is exactly the kind of mechanical verification that is easy to automate if you're doing it across a whole formula sheet.

2. Cross-reference the CAS number against the INCI name independently

The check digit catches transposition and typo errors, but it will not catch a CAS number that is internally valid and simply belongs to the wrong substance. For that, you need a second, independent source. Look the INCI name up separately and see what CAS number comes back, then compare it to what your supplier or your formula sheet gave you.

A few things worth knowing going in:

• Some ingredients legitimately have more than one valid CAS number, often because they were registered separately as different grades, isomers, or historical entries. Seeing a mismatch does not automatically mean an error, it might mean you're looking at two valid numbers for related material.
• Some botanical extracts have no CAS number at all, because they are variable natural mixtures rather than single defined substances. If a supplier hands you a CAS number for something like "Rosa Damascena Flower Extract," that is worth a second look rather than automatic trust.
• Trade names never carry their own CAS number for filing purposes. If a spec sheet lists a trade name alongside a CAS number, confirm that number actually maps to the correct INCI name for the labeled component, not just to something plausible-sounding.

3. Check it against the source document, not the summary

The safest single habit is going back to the original documentation, a supplier's certificate of analysis or a safety data sheet, rather than trusting a number that has been copied from spreadsheet to spreadsheet across several formula revisions. Section 3 of an SDS lists composition, and that is generally a more reliable source than a marketing-facing product spec sheet, which sometimes simplifies or omits details that matter for filing purposes.

If a number has been sitting in your ingredient database for a couple of years and gotten copied into three or four formulas since, it is worth tracing it back to its original source at least once rather than assuming it was always right.

Check	Catches
Check-digit math	Transposed digits, typos in the number itself
Independent INCI-to-CAS lookup	Wrong substance entirely, mismatched entries
Original SDS or CoA review	Copy-paste drift across spreadsheets and formula versions

Running all three on every ingredient in a formula is tedious, and it is exactly the kind of repetitive verification that benefits from being automated rather than done by hand each time. Cosmetic Comply maps every ingredient in your list to its INCI name and CAS number, expanding supplier blends into their real components, and flags anything that does not check out cleanly before a human reviewer signs off. A wrong CAS number rarely causes a dramatic problem on its own, but it undermines the accuracy of everything downstream of it, so it is worth catching early rather than late.