What is a CAS number and why does it matter?
A CAS number (CAS Registry Number) is the unique registry number assigned to a chemical substance by Chemical Abstracts Service (CAS), a division of the American Chemical Society. A substance may have dozens of names, but its CAS number is unique, making it the most common way to identify a substance precisely in supply.
How to read a CAS number
A CAS number has three parts separated by hyphens: the first has 2–7 digits, the second 2 digits, and the last is a single check digit. For example, water is 7732-18-5 and glycerin is 56-81-5.
How the check digit catches errors
The last digit is calculated from the others: excluding the check digit, the digits are multiplied by 1, 2, 3… from right to left, added up, and the remainder of the sum divided by 10 is taken. For 7732-18-5: 8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6 = 105 → check digit 5. A typing error in a CAS number is usually caught by this calculation; the CAS numbers on ISF CHEM product pages have been checked this way.
What a CAS number does not tell you
- Purity and grade: Food, pharmacopoeia and cosmetic grades of the same substance share the same CAS number. Grade is defined by the specification and certificate of analysis.
- Composition of mixtures: For natural mixtures such as vegetable oils and extracts, one CAS number may cover different product forms.
- Regulatory status: Whether a substance may be used in cosmetics, food or medicines is assessed separately under the relevant legislation.
CAS vs EC number
In the European Union, substances are also identified by an EC number (from the EINECS, ELINCS or NLP lists), used in REACH and CLP records. Giving the CAS number with the product name — and the INCI name for cosmetic ingredients — in quote and document requests reduces the risk of receiving the wrong product.
Sources: CAS — CAS Registry, ECHA — Information on Chemicals, PubChem — Water (7732-18-5).