A discount is not the same as a price change
Use the original price
For a 15% discount on 200, the reduction is 30 and the resulting value is 170. This arithmetic does not include taxes, shipping or other charges unless you include them in your inputs.
Do not add successive discounts
A 10% discount followed by another 10% discount gives 81% of the original price, not 80%. Each discount acts on the current value.
Keep rounding appropriate
Round only to the precision your task needs. Currency displays often use two decimal places, while an intermediate calculation may benefit from more precision.
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