CAS Registry Number (CAS RN)

A CAS number is a numeric identifier of up to 10 digits, split by hyphens into three parts. The registry holds more than 290 million unique substances (CAS REGISTRY). The first part has 2 to 7 digits and the second has 2. The third is a single check digit that validates the whole number (CAS check digit documentation).

The number has no inherent chemical significance (CAS FAQ). A CAS algorithm assigns it when a novel substance is added to the registry (CAS), so you cannot read a structure or property from it.

A CAS number does not tell you whether a substance is approved, safe, legal or controlled. It only identifies the substance. Regulators use several identifiers. The United States Drug Enforcement Administration (DEA) lists Schedule III anabolic steroids by chemical name, "including its salts, esters and ethers" (21 CFR 1308.13), so a CAS number alone does not show whether a substance is controlled. Other rules, such as United States Environmental Protection Agency (EPA) release reporting, also use CAS numbers.

Other identifiers are separate things. The US Food and Drug Administration (FDA) assigns its own Unique Ingredient Identifiers (FDA). The European Community (EC) number has a different format; testosterone enanthate, for example, also has the EC number 206-253-5 (PubChem). A customs tariff code classifies goods for shipping rather than naming a substance.

Each CAS number designates only one substance, so every distinct form gets its own number. A chemical name may have more than one CAS Registry Number because of the chemical's various forms (EPA).

Different crystal structures of one substance also get different numbers, while liquid water and ice share one (Wikipedia). So when two suppliers quote the same drug name with different numbers, the first question is which form each means. The next section shows how to tell.

Run three tests in order, and stop at the first fail. A passing check digit proves only that the number is well-formed, so the database and form checks do the real work.

1. Check digit. Multiply the last digit before the check digit by 1, the one before it by 2, and so on, add the results and take the remainder after dividing by 10. For 7732-18-5: 8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6 = 105, remainder 5, which matches. Pass: remainder equals the check digit. Fail: any other result, such as 7732-18-6.
2. Database match. Look the number up in CAS Common Chemistry, which covers nearly 500,000 substances and searches by name or number, or in PubChem. Pass: the name returned is the name you intended. Fail: a different substance. A miss is not proof of a fake, since Common Chemistry holds only a small part of the full registry.
3. Form match. Pass: the quote, COA and safety data sheet name the same form (base, salt, ester, hydrate) and the same number. Fail: any mismatch. Resolve any mismatch in writing before payment.

How much does the first test miss? In October 2026 the formula was run on the 22 numbers below. Every other digit was substituted at every position (1,332 variants) and every pair of different adjacent digits was swapped (112 variants), then the check was applied. It caught 1,210 single-digit changes (90.8%) and 110 swaps (98.2%). The rates depend on the length and digit positions of these particular numbers, so they are not a general rate for CAS numbers.

The 22 numbers: water 7732-18-5, sodium chloride 7647-14-5, ethanol 64-17-5, caffeine 58-08-2, acetaminophen 103-90-2, boric acid 10043-35-3 and 11113-50-1, epinephrine 51-43-4, 150-05-0 and 329-65-7, metformin 657-24-9, metformin hydrochloride 1115-70-4, testosterone 58-22-0, testosterone enanthate 315-37-7, testosterone cypionate 58-20-8, testosterone propionate 57-85-2, nandrolone decanoate 360-70-3, semaglutide 910463-68-2, liraglutide 204656-20-2, tirzepatide 2023788-19-2, BPC-157 137525-51-0 and GHK-Cu 89030-95-5.

The misses follow from the formula. The digit with weight 5 (the sixth from the right, counting the check digit) can change by 2, 4, 6 or 8 without changing the remainder. A digit with weight 2, 4, 6 or 8 can change by exactly 5 with the same result. A first digit with weight 9 is always caught.

We quote active pharmaceutical ingredients (APIs) from the API name and CAS number, so a number you have verified shortens the exchange. Our peptide raw material catalog and our steroid, hormone and general pharmaceutical API ranges serve business buyers. Our steroid API RFQ checklist lists what else a request should carry.

A request should state:

• the API name and form
• the CAS number
• quantity
• destination
• grade or purity
• documents needed (COA, specification, safety data sheet where applicable)
• payment terms
• Incoterms (International Commercial Terms, the shipping-responsibility codes)

Prices depend on the molecule and market conditions, so we quote product by product. We support samples and evaluation quantities for qualified business buyers. Permitted end use and import authorization depend on the molecule and destination country, and buyers confirm local registration before supply.

To send a request, email sunil@aarisepharma.com, message WhatsApp +91 9899783330, or use our contact form. For the wider term, see what an API is in pharma and drug substance vs drug product.