Discount Calculator

About the Discount Calculator

The discount calculator works with exact decimal arithmetic, so there are no floating point surprises such as 0.1 plus 0.2. The sale price is the original price times one minus the discount. Take 25% off 80 and you pay 80 x 0.75 = 60. When you add a second discount, it is taken off the price that is already reduced, so 20% off followed by 10% off is not 30% off. On 100 it gives 80 and then 72, which is 28% off in total, and the tool shows that total. Sales tax is worked out on the discounted price, and the discounted price is rounded to the cent first, as a till would show it, then the tax is rounded to the cent too. Halves round up, so 0.125 becomes 0.13. Because each step is rounded, swapping the order of two discounts can change the last cent. The percent off between two prices is the difference divided by the original price, shown with up to two decimals, and the tool says when the new price is higher. To find the original price from a sale price, it divides by one minus the discount, rounds to the cent and then checks the answer by applying the discount again, and tells you if rounding leaves the check one cent away. A discount of 100% or more cannot be worked back. The results were compared with the Decimal type of Python on 30,000 random sales, with up to three discounts and tax, and match. Everything runs in your browser and nothing is uploaded.