CAS Number Validator
Verify a CAS Registry Number's check digit
Check-digit calculation
8×1 + 0×2 + 8×3 + 5×4 = 52
52 mod 10 = 2 → check digit matches 2 ✓
ƒ check digit = ( Σ digitᵢ × i ) mod 10
A CAS Registry Number uniquely identifies a chemical substance, and its final digit is a check digit calculated from the others. This validator applies the official checksum to confirm a CAS number is well-formed and flags transcription errors — and it shows exactly how the check digit is derived.
How the checksum is calculated
Drop the check digit, then read the remaining digits from right to left, multiplying each by its position (1, 2, 3, …). Add the products and take the remainder when divided by 10. For 58-08-2: (8×1)+(0×2)+(8×3)+(5×4) = 52; 52 mod 10 = 2 ✓.
Why validate a CAS number
CAS numbers are entered by hand on quotes, labels and safety documents, where a single transposed digit can point to the wrong substance. A checksum catches most typos instantly — useful when raising an RFQ or checking a Certificate of Analysis.
Frequently asked questions
What is a CAS Registry Number?
A CAS Registry Number (CAS RN) is a unique identifier assigned by the Chemical Abstracts Service to every chemical substance. It has the format up-to-7-digits–2-digits–1-digit, e.g. 58-08-2 for caffeine.
How does the check digit work?
Read the digits (excluding the check digit) right to left and multiply each by its position (1, 2, 3…). Sum the products and take mod 10 — that result is the check digit. The validator shows this calculation.
Does this look up the chemical's name?
No — it validates the number's format and check digit to catch typos. It does not identify the substance. For named products and specifications, browse the Tech Serve Solutions catalog.
Is 58-08-2 a valid CAS number?
Yes — 58-08-2 (caffeine) is valid: (8×1)+(0×2)+(8×3)+(5×4) = 52, and 52 mod 10 = 2, which matches the check digit.
Need the actual compound — not just the maths?
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