An off-by-one error in computer programming is an avoidable error in which a loop iterates one too many or one too few times. Usually this problem arises when a programmer fails to take into account that a sequence starts at zero rather than one, or makes mistakes such as using is less than where is less than or equal to should have been used in a comparison.

For example, in the C programming language, a program loop that iterates five times would be written as follows:

for(int i = 0; i < 5; i++ ) { /* Body of the loop */ }

The loop body is executed first of all with i equal to 0; i then becomes 1, 2, 3, and finally 4 on successive iterations. At that point, i becomes 5, so i < 5 is false and the loop ends. However, if the comparison used were <= (less than or equal to), the loop would be repeated six times: i takes the values 0, 1, 2, 3, 4, and 5. Likewise, if i were initialized to 1 rather than 0, there would only be four iterations: i takes the values 1, 2, 3, and 4. Both of these alternatives can cause off-by-one errors.

Another such error can occur if a do-while loop is used in place of a while loop loop (or vice versa.) A do-while loop is guaranteed to run at least once.

Off-by-one errors are a frequent source of computer bug in computer software.

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