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CAS number validator

Check whether a CAS registry number is well formed before it goes on a quote, a label or a customs document. See the check digit worked out step by step, validate a whole list from a spreadsheet at once and jump straight to the chemical on CompoundList.

Check

Hyphens are optional. 7732185 is read as 7732-18-5.

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Valid check digit

7732-18-5 is well formed. This proves the number has no typing error that the check digit can catch, not that it belongs to the chemical you expect.

Look up 7732-18-5

The check digit calculation

Digit (right to left)WeightProduct
8× 18
1× 22
2× 36
3× 412
7× 535
7× 642
Sum105
Last digit of the sum5 = check digit

Take every digit except the last, multiply from the right by 1, 2, 3 and so on, add them up and keep the last digit of the total. It must equal the check digit.

How the CAS check digit works

A CAS registry number has three parts: two to seven digits, two digits and a single check digit, for example 7732-18-5 for water. The numbers are assigned in sequence by CAS, a division of the American Chemical Society, and carry no chemical meaning. The last digit is calculated from the others so that typing mistakes can be caught.

check digit = ( 1×d1 + 2×d2 + 3×d3 + … + n×dn ) mod 10
d1 is the digit just before the check digit, d2 the one before that, and so on

Ignore the hyphens, take every digit except the last, and multiply them from right to left by 1, 2, 3 and upwards. Add the products. The last digit of the total must be the check digit.

The check catches most typing mistakes, but not all of them. Swapping two neighbouring digits in front of the check digit always breaks it, because the swap changes the total by the difference between the two digits. A single wrong digit is always caught at weights 1, 3, 7 and 9, but at the other weights some changes slip through. For example, a digit that is off by 5 at weight 2 adds or removes exactly 10, so the last digit of the total stays the same.

A number that passes is only well formed. It may still belong to a different substance than you think, or to a record CAS has since deleted. Always confirm the name, the formula or the structure as well.

Worked example

The same steps the calculator follows, done by hand

Check caffeine, 58-08-2. The digits before the check digit are 5, 8, 0 and 8.

Digit from the rightWeightProduct
818
020
8324
5420
  1. 1
    Add the products: 8 + 0 + 24 + 20 = 52.
  2. 2
    The last digit of 52 is 2, which matches the check digit, so 58-08-2 is valid.
  3. 3
    If someone types 58-08-3 the sum is still 52, the check digit no longer matches and the number is rejected.

Frequently asked questions

If a CAS number passes the check, is it correct?
It is well formed, but that is all. A completely different valid number also passes. Confirm that the number matches the chemical name and structure, for example by looking it up on CompoundList or on the supplier's safety data sheet.
Why does the same chemical have several CAS numbers?
CAS gives separate numbers to distinct substances, so hydrates, salts, stereoisomers and mixtures each have their own. Copper sulfate anhydrous is 7758-98-7 and the pentahydrate is 7758-99-8. Old numbers can also be replaced when CAS merges records.
Can I enter a CAS number without hyphens?
Yes. The validator reads the last digit as the check digit and the two before it as the middle part, so 7732185 is read as 7732-18-5.
Where do I find the CAS number of a product?
On the label, the safety data sheet (section 1 or 3), the certificate of analysis and most supplier catalogues. On CompoundList every chemical page shows its CAS number together with other identifiers such as the EC number and InChIKey.

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