Determine the number of students in the population at the start of the period of measurement.
Determine the number of students who have left the population, those who have dropped out, during the period of measurement.
Subtract any exceptions from the total number of dropout students. The exceptions you subtract can vary widely depending on the intended use of your data. Because dropout rate is a rough measure at best, most third-party organizations that request attrition data will provide you with guidelines for what exceptions to take into account. Exceptions could include students who leave due to a transfer, a medical emergency, a change of program, a suspension or expulsion.
Divide the number of student dropouts (less exceptions) by the total number of students at the beginning of period of measurement. The resulting total will be a decimal value.
Multiply the decimal value by 100 to arrive at the percentage of student attrition.