CAS number check digit validator
Paste a CAS Registry Number and see whether its check digit adds up.
What a CAS number is
A CAS Registry Number is an identifier assigned by the Chemical Abstracts Service to a specific substance. It carries no information about structure: it is a label, allocated in sequence, and its only job is to say "this substance and not another one" across every language and naming convention. Our note on CAS numbers covers why that matters when a compound has six names.
How the check digit works
The last digit is a checksum over the rest, not part of the identifier itself. Take all the digits before the final hyphen, number them from right to left starting at one, multiply each by its position, add the products, and the remainder when you divide by ten is the check digit.
For 7732-18-5, which is water: reading right to left from the digit before the check digit, 8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6 comes to 105, and 105 leaves a remainder of 5. The number ends in 5, so it is consistent.
The arithmetic is public and the test takes a second, which is the whole point. A single mistyped digit breaks it in almost every case.
What a pass does not prove
Three things, and they matter more than the pass itself.
It does not prove the number is assigned. The checksum is arithmetic over digits, so a number can be perfectly consistent and correspond to nothing. Only a lookup in a registry answers that.
It does not prove the number belongs to the compound on the label. Pairing a real CAS number with the wrong name passes this check every time, because the check never sees the name. The useful follow-up is to look the number up and read back which substance it returns.
It does not prove anything about the material in the vial. A CAS number identifies a substance in the abstract. What is in a particular batch is a question for a certificate of analysis, and specifically for the mass measurement on it. Our note on reading a certificate covers that.
What a failure does mean
That the number as written cannot be a valid CAS number at all. Usually a transposed pair of digits in a catalogue entry copied by hand. Occasionally a number invented to make a listing look documented, which the arithmetic catches because inventing a consistent one takes deliberate effort.
Frequently asked questions
How is a CAS check digit calculated?
Take the digits before the final hyphen, number them from right to left starting at one, multiply each digit by its position, sum the products and divide by ten. The remainder is the check digit.
Does a valid check digit mean the CAS number is real?
No. It means the number is arithmetically consistent, which rules out most typos and casually invented numbers. A number can pass and still be unassigned, because the checksum only sees digits.
Can a CAS number be valid but wrong for the compound?
Yes, and this is the common case. The check never sees the compound name, so a real CAS number paired with the wrong substance passes. Look the number up and read back which substance it returns.
Why do some research peptides have no CAS number?
A number is assigned when a substance is indexed in the chemical literature. A compound that is new or has had little published work on it may not have one yet, which is not in itself a problem.
What does a failed check usually mean?
Most often a digit transposed while copying a catalogue entry by hand. Sometimes a number invented to make a listing look documented, since producing an arithmetically consistent fake takes deliberate effort.
Does the number tell me anything about purity?
Nothing at all. A CAS number identifies a substance in the abstract. What is in a particular vial is a question for that batch certificate of analysis.