The check letter is one of the simplest ways to test whether a Zimbabwe ID number is internally consistent. This guide explains the logic behind it.

What mod-23 means

The mod-23 calculation is the rule used to produce the check letter. In simple terms, it works like this:

Worked example

Here is a worked example for ID number 12-345678-A-90:

Step 1: Extract registration code and sequence number

Registration code: 12

Sequence number: 345678

Step 2: Combine them

Combined value: 12345678

Step 3: Divide by 23

12345678 ÷ 23 = 536768 remainder 1

Step 4: Map remainder to letter

Remainder 1 = A

The table below shows how remainders map to letters. The letters I and O are not used.

Remainder	Letter	Remainder	Letter
0	Z	12	M
1	A	13	N
2	B	14	P
3	C	15	Q
4	D	16	R
5	E	17	S
6	F	18	T
7	G	19	V
8	H	20	W
9	J	21	X
10	K	22	Y
11	L	-	-

The letters I and O are excluded because they can easily be confused with the numbers 1 and 0.

The check-letter system helps because:

If an ID has an incorrect check letter, it usually means one of the following:

Instead of calculating the letter by hand each time, use the validator to:

Remainder	Letter	Remainder	Letter
0	Z	12	M
1	A	13	N
2	B	14	P
3	C	15	Q
4	D	16	R
5	E	17	S
6	F	18	T
7	G	19	V
8	H	20	W
9	J	21	X
10	K	22	Y
11	L	-	-

Remainder	Letter	Remainder	Letter
0	Z	12	M
1	A	13	N
2	B	14	P
3	C	15	Q
4	D	16	R
5	E	17	S
6	F	18	T
7	G	19	V
8	H	20	W
9	J	21	X
10	K	22	Y
11	L	-	-

Remainder	Letter	Remainder	Letter
0	Z	12	M
1	A	13	N
2	B	14	P
3	C	15	Q
4	D	16	R
5	E	17	S
6	F	18	T
7	G	19	V
8	H	20	W
9	J	21	X
10	K	22	Y
11	L	-	-