A percentage decrease works the same way as a percentage increase. Here's the formula again: Keep in mind Percentage Increase = (Original Number - New Number) ÷ (Original Number) × 100 Say you have been eyeing a special anniversary edition of your favourite graphic novel at a bookstore. It usually costs $39 but today it is marked down to $35. You can calculate the discount off like this: Keep in mind $39 - $35 = $4 $4 ÷ $39 = 0.1026 0.1026 × 100 = 10.26% percentage decrease 10.26% off your graphic novel is a nice deal, to be sure. But knowing that percentage you might decide that you're patient and better off waiting for next month's 20% sale.