The check digit is 9
An ISBN (International Standard Book Number) is a 10- or 13-digit number used to uniquely identify a book.
The ISBN contains a very important digit called the check digit.
The check digit of an ISBN is very important as it helps in preventing errors in transmission.
When an ISBN is entered into an ISBN reader, the check digit of the ISBN is compared with the check digit calculated from the other digits of the ISBN. If these two digits match, then there is very high chance that the ISBN is correctly entered.
For example, if an ISBN, say, 9781681972718 is entered into the reader,
i. the inputted check digit is the last digit which is 8
ii. the calculated check digit by the reader will also come up with 8.
iii. since the two values match, then the ISBN has actually been entered correctly.
If however, the entered ISBN number was 9781861972718, then,
i. the inputted check digit is the last digit which is 8
ii. the calculated check digit by the reader will also come up with 2.
iii. since the two values do not match, then the ISBN has actually been entered incorrectly.
To calculate the check digit of an ISBN, the steps are;
i. The first 12 digits of the number are taken.
In our case,
the digits are: 978147180930
ii. Multiply the first digit by 1, the second by 3, the third by 1, the fourth by 3 ... in that format until the twelfth digit which will obviously be multiplied by 3.
9 x 1 = 9
7 x 3 = 21
8 x 1 = 8
1 x 3 = 3
4 x 1 = 4
7 x 3 = 21
1 x 1 = 1
8 x 3 = 24
0 x 1 = 0
9 x 3 = 27
3 x 1 = 3
0 x 3 = 0
iii. Find the sum of the answers in (ii) above
9+21+8+3+4+21+1+24+0+27+3+0 = 121
iv. Find the modulo 10 division of the result in (iii). This is just the remainder when the result is divided by 10.
121 mod 10 = 12 remainder 1
v. If the remainder as calculated in (iv) is zero, then the check digit is 0. If otherwise, subtract the remainder from 10 and the result will be the check digit.
The remainder is 1 which is not zero. Therefore, the check digit is
10 - 1 = 9
The check digit is 9 and
the complete ISBN number will be 9781471809309
