Understanding the base in percentage change
Divide by the original value
Percentage change uses the difference between new and old values, divided by the old value, multiplied by 100. A move from 80 to 100 is an increase of 25%.
Direction changes the answer
A move from 100 back to 80 is a decrease of 20%, not 25%. The absolute difference is the same, but the reference value is different.
Handle zero explicitly
If the original value is zero, the usual percentage-change formula cannot divide by it. Report the actual difference or choose a different meaningful comparison rather than using an infinite percentage.
Try the workflow
A change from 80 to 100 is a 25% increase because the difference, 20, is divided by the original value, 80.
Open percentage calculator