Annex 2: ISBT 128 check character calculation

Calculation procedure

ISBT 128 donation numbers utilise check characters based on the ISO 7064 modulus 37,2 algorithm. This annex shows how to calculate the check character for a given number. The calculation is based on the donation number string excluding the leading ‘=’ symbol and the flag characters.

To calculate the ISBT check character:

1. For each character in the string, determine its check value as required by ISO 7064 (see Table A2).
2. For each character in the string, determine its weighted check value by multiplying the check value from step 1 by the nth power of 2 (2ⁿ), where n is the position of the character from the right-hand end of the string.
3. Sum the weighted values from step 2.
4. Find the modulus 37 value of the value from step 3.
5. Subtract the value obtained in step 4 from 38.
6. Find the modulus 37 value of the result of step 5. This is the 37,2 checksum.
7. Calculate the barcode check character by adding 60 to the 37,2 checksum. The eye-readable check character can be determined by cross-referencing the 37,2 checksum value to the characters in Table A2.

Table A2: Mapping from characters to ISO 7064 check values

Character	Value
0	0
1	1
2	2
3	3
4	4
5	5
6	6
7	7
8	8
9	9
A	10
B	11
C	12
D	13
E	14
F	15
G	16
H	17
I	18
J	19
K	20
L	21
M	22
N	23
O	24
P	25
Q	26
R	27
S	28
T	29
U	30
V	31
W	32
X	33
Y	34
Z	35
*	36

Calculation example

The following is a worked example for donation number G123 498 654 321.

Step 1 – determine each character's check value

Using Table A2, each character in the donation number has the following check value:

• G = 16
• 1 = 1
• 2 = 2
• 3 = 3
• 4 = 4
• 9 = 9
• 8 = 8
• 6 = 6
• 5 = 5
• 4 = 4
• 3 = 3
• 2 = 2
• 1 = 1

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Step 2 – determine each character's weighted value

Each character's check value is multiplied by 2ⁿ where n is its position from the right of the string:

Character	Check value	Position from right (n)	2ⁿ	Weight value
Character G	Check value 16	Position from right (n) 13	2ⁿ 8192	Weight value 131072
Character 1	Check value 1	Position from right (n) 12	2ⁿ 4096	Weight value 4096
Character 2	Check value 2	Position from right (n) 11	2ⁿ 2048	Weight value 4096
Character 3	Check value 3	Position from right (n) 10	2ⁿ 1024	Weight value 3072
Character 4	Check value 4	Position from right (n) 9	2ⁿ 512	Weight value 2304
Character 9	Check value 9	Position from right (n) 8	2ⁿ 256	Weight value 2304
Character 8	Check value 8	Position from right (n) 7	2ⁿ 128	Weight value 1024
Character 6	Check value 6	Position from right (n) 6	2ⁿ 64	Weight value 384
Character 5	Check value 5	Position from right (n) 5	2ⁿ 32	Weight value 160
Character 4	Check value 4	Position from right (n) 4	2ⁿ 16	Weight value 64
Character 3	Check value 3	Position from right (n) 3	2ⁿ 8	Weight value 24
Character 2	Check value 2	Position from right (n) 2	2ⁿ 4	Weight value 8
Character 1	Check value 1	Position from right (n) 1	2ⁿ 2	Weight value 2

Step 3 – sum the weighted values

Σ (weighted values) = 148354

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Step 4 – find the modulus 37 value of the sum of weighted values

148354 (mod 37) = 21

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Step 5 – subtract the modulus 37 value from 38

38 - 21 = 17

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Step 6 – find the modulus 37 value of the result

17 (mod 37) = 17

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Step 7 – determine the barcode check character by adding 60

17 + 60 = 77