International · Validation
New Zealand NHI number validator.
Check a National Health Index (NHI) number against the validation routine Health NZ publishes. It handles the original format (three letters, three digits, numeric check digit) and the newer one (three letters, two digits, two letters, letter check character).
How it works: letters count A=1 up to Z=24 with I and O left out. Multiply the first three letters by 7, 6, 5, the two digits by 4 and 3, and the sixth character by 2, then add. Original format: 11 minus (total mod 11) must equal the last digit, where 10 counts as 0 and 11 can never be valid. Expanded format: 23 minus (total mod 23), turned back into a letter, must equal the last letter.
How the NHI check character works
Many New Zealand national and local health systems use the NHI number as a reference key for patient data. It has seven characters, and the last one is a check character so that a mistyped number can be caught before it is used. Health NZ publishes the routine for it, and this tool follows that document step by step.
| Step | What happens |
|---|---|
| 1 | Characters 1 to 3 must be letters other than I or O; characters 4 and 5 must be digits |
| 2 | Characters 6 and 7 must be both digits (original format) or both letters (expanded format) |
| 3 | Convert each letter with the table A=1, B=2, C=3, D=4, E=5, F=6, G=7, H=8, J=9, K=10, L=11, M=12, N=13, P=14, Q=15, R=16, S=17, T=18, U=19, V=20, W=21, X=22, Y=23, Z=24 |
| 4 | Multiply characters 1 to 6 by 7, 6, 5, 4, 3, 2 and add the six products |
| 5 | Original format: divide by 11 and subtract the remainder from 11. A result of 11 means the number can never be valid; a result of 10 becomes 0 |
| 6 | Expanded format: divide by 23, subtract the remainder from 23, and convert the result back to a letter with the same table |
| 7 | The seventh character must equal that check digit or letter |
Health NZ's own examples all check out: ZZZ0016 (total 434, remainder 5, check digit 6), ZZZ0024 (total 436, remainder 7, check digit 4), ZZZ00AC (total 434, mod 23 remainder 20, 23 − 20 = 3 = C) and ZVU27KE (total 432, mod 23 remainder 18, 23 − 18 = 5 = E). Its fifth example, ZZZ0044, fails: the total of 440 leaves a remainder of 0, the calculation gives 11, and no single digit can complete ZZZ004.
Original and expanded formats
The original format is three letters, three digits and a numeric check digit. Health NZ's routine also defines an expanded format of three letters, two digits, two letters, ending in a letter check character. Health NZ says the expanded format's check algorithm uses modulus 23 instead of modulus 24 (testing found a weakness in modulus 24), that providers must validate the letter check character with a modulus 23 operator, and that new numbers in the expanded format start to be issued from 1 July 2027.
A format and check-character test, not a lookup
Health NZ says the routine is there to catch mistyped numbers. It does not confirm that an NHI number was assigned to a particular person, or that it is registered: pre-allocated numbers are not active until they are sent to the NHI in a registration transaction. Only an authorised NHI query can tell you that.
Sources
- Health NZ: NHI Validation Routine (April 2023, v6)
- Health NZ: Upcoming changes to the NHI (expanded format, modulus 23, 1 July 2027)
Retrieved October 2026. Agencies can change these rules; check the issuing authority if a result matters.
FAQ
What does an NHI number look like?
Seven characters. The original format is three letters, three digits and a numeric check digit (for example ZZZ0016). The expanded format is three letters, two digits and two letters, ending in a letter check character (for example ZVU27KE). The letters I and O are never used.
How is the NHI check digit calculated?
Letters are converted with a table that runs A=1 to Z=24 without I and O. The first six characters are multiplied by 7, 6, 5, 4, 3 and 2 and added. For the original format the remainder after dividing by 11 is subtracted from 11 (10 becomes 0, 11 is never valid). For the expanded format the remainder after dividing by 23 is subtracted from 23 and turned back into a letter.
Why does ZZZ0044 fail whatever last digit I try?
Health NZ uses it as its own example of an impossible number: the total of the first six characters is 440, the remainder after dividing by 11 is 0, and 11 minus 0 is 11, which cannot be written as a single digit.
Does a valid result mean the NHI is registered?
No. Health NZ says the routine does not confirm that a number has been assigned to a person or registered on the NHI. It is there to catch typing errors.
Is the expanded NHI format in use yet?
Health NZ's guidance, retrieved October 2026, says new numbers in the expanded format start to be issued from 1 July 2027 and that providers must validate its letter check character with modulus 23. This tool already checks both formats.
Is the number I type sent anywhere?
No. The check runs in your browser and nothing you type is transmitted, stored or logged.
Check character only, not an NHI lookup
This tool tests the structure and check character of the number in your browser. It does not query the National Health Index, and a passing result does not show the number was issued to anyone. Nothing you type is transmitted, stored or logged.