The Percentage Decrease Formula, Explained

The formula

percentage decrease = (old − new) ÷ old × 100

Note the order is reversed from percentage increase to keep the result positive when there’s a decrease.

Example: from 80 down to 60

(80 − 60) ÷ 80 × 100
= 20 ÷ 80 × 100
= 25%

A 25% decrease took 80 down to 60.

Applying a decrease

new = old × (1 − P ÷ 100)

For 80 with a 25% decrease: 80 × 0.75 = 60. The factor 0.75 is the “keep” ratio — you keep 75% of the value.

This is how discounts work

A 30% off sale on a $100 item:

$100 × (1 − 0.30) = $100 × 0.70 = $70

The same math, just called “discount” instead of “decrease”.

Maximum possible decrease

100% decrease takes any value to 0. 50% decrease cuts in half. Decreases above 100% don’t physically make sense for most quantities (you’d be subtracting more than you have).