To find a percentage increase, divide the difference between the new and old value by the old value, explains TutorVista.com. The result is then multiplied by 100 to give the percentage.
When calculating percentage increases, the old value is first subtracted from the new value to get the difference between the two values. Since the percentage increase of the new value from the old value is to be determined, the difference is divided by the old value. Multiplying this result by 100 converts the difference to percentage increase. Thus, the formula for calculating percentage increase is ((New value - Old value)/ Old value) x 100.
For example, if a person worked for 20 hours last week and increased it to 30 hours in the current week, then the difference in the number of hours worked between the two weeks is 10 hours, says TutorVista.com. When divided by the old value, or 20 hours, and then multiplied by 100, the percentage increase in working hours comes to 50 percent.
If the old value is more than the new value, the difference between the two becomes negative. In such a case, the result obtained reflects the percentage decrease rather than percentage increase, states TutorVista.com.