NHS Number Check Digit Calculation

The NHS number comprises 10 digits. The first nine are the identifier and the tenth is a check digit used to confirm the number's validity. The check digit is calculated using the Modulus 11 algorithm. There are four steps in the calculation:

• Step 1 - multiply each of the first nine digits by a weighting factor as follows;




Digit Position (Starting from the left)	Factor
1	10
2	9
3	8
4	7
5	6
6	5
7	4
8	3
9	2




• Step 2 - add the results of each multiplication together
• Step 3 - divide the total by 11 and establish the remainder
• Step 4 - subtract the remainder from 11 to give the check digit

There are two occasions where the check digit calculation process must be modified slightly:

• if the result of step 4 is 11 then a check digit of 0 is used
• if the result of step 4 is 10 then the number is invalid and not used.

Example: Suppose the first nine digits of the number are 401 023 213

• Step 1 - apply weighting factors




Digit Position	Value	Factor	Result
1	4 x	10	= 40
2	0 x	9	= 0
3	1 x	8	= 8
4	0 x	7	= 0
5	2 x	6	= 12
6	3 x	5	= 15
7	2 x	4	= 8
8	1 x	3	= 3
9	3 x	2	= 6




• Step 2 - add the results of each multiplication together
  40 + 0 + 8 + 0 + 12 + 15 + 8 + 3 + 6 = 92
• Step 3 - divide the total by 11
  92 ÷ 11 = 8, remainder 4
• Step 4 - subtract the remainder from 11 to give the check digit
  11 - 4 = 7

The complete NHS Number in this example is therefore: 401 023 2137

Please note there are some restrictions

Constant number strings such as 444 444 4444, 666 666 6666 etc are not issued.

So, although they pass the checksum, they are invalid.

The reason for excluding numbers based on a single repeating digit is to safeguard the integrity of the numbering system and prevent potential mistaken use or abuse.

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