How the NPI Check Digit Works

The tenth digit of an NPI is a check digit. It's computed from the first nine with the Luhn formula, so most single-digit typos are caught.

The method

NPIs use the Luhn algorithm (the same family of check as credit card numbers), with the constant 80840 placed in front of the nine-digit base. In practice that means:

Take the first 9 digits. Starting from the right, double every other digit (the 9th, 7th, 5th, 3rd and 1st). If doubling gives a number over 9, subtract 9.
Add up all nine results.
Add 24 (that's what the 80840 prefix contributes).
The check digit is the amount that brings the total up to the next multiple of 10. If the total already ends in 0, the check digit is 0.

Worked example: 1234567893

First nine digits: 1 2 3 4 5 6 7 8 9. Doubling the 9th, 7th, 5th, 3rd and 1st digits gives 18→9, 14→5, 10→1, 6 and 2. The undoubled digits are 8, 6, 4 and 2. Sum: 9+8+5+6+1+4+6+2+2 = 43. Add 24 = 67. To reach 70 you need 3, so the check digit is 3. It matches the last digit, so 1234567893 passes.

What a pass proves

Only that the number is formatted like an NPI. Many valid-looking numbers were never issued. To confirm one exists, look it up in the registry.

Try the checker: it shows the digit that would be correct when a number fails.

Sources

Reviewed October 2026. Federal rules and CMS pages change; check them for the latest.

Look one up

Need a number now? Search the registry by NPI, name or organization.