Check the final digit
Compare the final digit with the CMS calculation. A checksum match does not verify a provider.
Entered identifier
- Supplied final digit
- Expected final digit
Inspect the nine-digit calculation
Add the nine contributions and the CMS prefix contribution of 24: total . The check digit brings this total to a multiple of 10. If the total already ends in zero, the check digit is 0.
The checker runs on this page. It does not query NPPES, save the input or send it in a URL. It does not generate or repair NPIs.
What to check after the arithmetic
A matching check digit answers one narrow question: does the final digit agree with the other nine under the CMS formula? It cannot establish whether the identifier was issued, remains active, identifies the intended person or organization, or is appropriate for a prescription or claim. Compare the full identifier and provider details through your authorized verification process. The official NPI Registry is a separate destination; this page does not transfer your input to it.
For a mismatch, return to the original source and verify all 10 digits. A different final digit is not permission to change a claim. A transcription error can occur earlier in the identifier, and some errors can still produce a checksum match.
How the NPI check digit works
CMS describes the NPI as a 10-digit, intelligence-free numeric identifier: the digits do not encode a provider’s state or specialty. The CMS check-digit specification, January 23, 2004, pages 1–2, defines nine identifier digits followed by one check digit. The arithmetic below follows that specification; it is not a registry lookup.
- Take the first nine digits. Starting with the rightmost of those nine, double alternating digits.
- Add the individual digits of each doubled product and the undoubled digits.
- Add 24, the contribution of the 80840 prefix used in the CMS calculation. Do not type that prefix into this checker.
- Find the digit needed to reach a multiple of 10. That is the expected final digit; use 0 if the total already ends in zero.
CMS uses 1234567893 as its worked arithmetic example. For the first nine digits, the contributions are:
2 + 2 + 6 + 4 + 1 + 6 + 5 + 8 + 9 = 43
43 + 24 = 67
70 − 67 = 3
The supplied final digit 3 matches. This is the CMS document’s calculation example, not a verified provider record or a number to use in a claim.
Practice: distinguish a checksum from verification
These original questions use the CMS arithmetic example. They do not identify a provider or establish professional competency.
A different final digit
Enter
1234567894. Does the check digit match, and should you submit a number ending in 3 instead?Show the answer
The expected final digit is 3, so 4 does not match. Do not substitute 3 automatically. Recheck the entire identifier against its source; no issued or correct provider identifier has been established.
The omitted prefix contribution
The nine contributions add to 43. What goes wrong if you use that total alone?
Show the answer
It would suggest a final digit of 7. The CMS calculation includes 24: 43 + 24 = 67, which needs 3 to reach 70. The prefix contributes to the arithmetic even though it is not entered as part of the 10-digit NPI.
A matching result
The checker reports a match for the CMS example. Can you infer the provider’s specialty, active status or eligibility for a claim?
Show the answer
No. A checksum is not a registry or credential check, and CMS describes the identifier as intelligence-free. No provider record or payer response was consulted.
Sources3 linked sources
- CMS, Requirements for NPI and NPI Check Digit, January 23, 2004, both pages checked October 16, 2026. Supports the digit count, Luhn procedure, prefix contribution and worked example. Its historical initial-issuance discussion is not applied as a current issuance eligibility rule here.
- CMS, National Provider Identifier Standard, page last modified August 14, 2026; accessed October 16, 2026. Supports the 10-digit, intelligence-free identifier description.
- Official NPI Registry, linked as a separate destination. No provider record was queried or verified by this tool.
The explanations, interface and practice questions are original PillPilot educational material. Primary documents remain with CMS; no copy of the PDF or registry data is redistributed. No new reuse license for this material or qualified pharmacist, CMS or payer approval is claimed.