The format provides for 13,824,000 unique NHI numbers, although 1,256,727 of these combinations cannot generate a check digit.
Any NHI Number that does not fit the correct format or that has an incorrect check digit is referred to as invalid.
Open-source packages are available to check the validity of NHI numbers: The existing range is expected to be exhausted after 2025.
In 2019, a revised standard introduced a new format of LLLNNLX, where X is a letter check digit (e.g. ABC12DV).
[4] The new algorithm implementation however creates a situation where collisions in the check-digit are much more likely to happen for simple single character transcription errors.
Each character’s equivalent numeric value is then multiplied by its reverse ordinal position within the NHI number.
The algorithm for generating the digit is described below: Each alpha character is given a numeric representation equivalent to its ordinal position within the alphabet, starting at A through to Z.
Each alpha character's numeric representation is multiplied by the inverse of its ordinal position within the NHI Number.
The sum of these multiplications modulus 11 subtracted from 11 is taken as the check digit (a result of 10 is translated to 0).