### Calculating the check digit

Excerpted from the ISSN Manual, a publication of the ISSN International Network

The purpose of a check digit is to guard against errors caused by the incorrect transcription of an ISSN. The modulus 11 basis using the weighting factors 8 to 2 for calculating the check digit is one of the most efficient systems for detecting transcription errors.

The procedure for calculating the check digit, which may be carried out automatically in a computer, is as follows:

1. Take the first seven digits of the ISSN (the check digit is the eighth and last digit): 0 3 1 7 8 4 7
2. Take the weighting factors associated with each digit : 8 7 6 5 4 3 2
3. Multiply each digit in turn by its weighting factor: 0 21 6 35 32 12 14
4. Add these numbers together: 0+21+6+35+32+12+14 = 120
5. Divide this sum by the modulus 11: 120:11 =10 remainder 10
6. Substract the remainder from 11: 11-10 = 1
7. Add the remainder, which is the check digit, to the extreme right (low order) position of the base number of the ISSN: 0317-8471

If the remainder is 10, substitute an upper case X in the check digit position. If there is no remainder, put a zero in the check digit position.

It should be noted that the check digit is an essential and inseparable part of the ISSN.

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Library of Congress Help Desk ( February 19, 2010 )