How to Calculate a Percent Change Using Python

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To calculate a percent change, you can use the following formula:

pc = (new – old) / old * 100%

where old and new are the values before and after change respectively.

Here’s how we can implement it in Python:

>>> def percent_change(old, new):
...     pc = round((new - old) / abs(old) * 100, 2)
...     print(f"from {old} to {new}   -> {pc}% change")
...

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Watch the built-in abs function inside the percent_change function. Without it you would get wrong results if the old value was negative.

Here’s how it works:

>>> percent_change(60, 60)
from 60 to 60   -> 0.0% change
>>> percent_change(100, 120)
from 100 to 120   -> 20.0% change
>>> percent_change(100, 80)
from 100 to 80   -> -20.0% change
>>> percent_change(50, -50)
from 50 to -50   -> -200.0% change
>>> percent_change(-20, 20)
from -20 to 20   -> 200.0% change
>>> percent_change(-20, -50)
from -20 to -50   -> -150.0% change