Discount Calculator

The Discount Calculator turns a "40% off" sign into the two numbers you actually want: what you will pay and how much you will save. Enter the original price and the discount - either a percentage such as 25% or a fixed amount such as $15 off - and it returns the sale price, the money saved, and the discount as a share of the original. It removes the checkout-line arithmetic and the guesswork about whether a deal is as good as it looks.

It also handles the parts of real-world pricing that trip people up. Retailers frequently stack an "extra 10% off" coupon on top of an already-reduced price, and that does not add up the way it seems - 20% off followed by an extra 10% off is a 28% effective discount, not 30%. The calculator applies the stacked coupon to the discounted price and shows the true combined percentage. You can also add a sales tax rate to see the final amount at the register.

Everything runs in your browser in plain JavaScript. No prices are sent anywhere, there is no sign-up, and the calculator keeps working with no connection once the page has loaded - handy for comparing offers while you are out shopping. It works just as well on a laptop for pricing a product, planning a promotion, or checking a quote.

Stacked discounts multiply rather than add, and the gap is larger than people expect. Twenty per cent off followed by a further ten per cent is not thirty per cent off, it is twenty-eight, because the second discount comes off an already reduced price. The effective discount is shown for exactly this reason. Order matters when tax is involved too: tax is calculated on the discounted price, so a coupon applied before tax saves slightly more than the same coupon applied after. Fixed-amount and percentage discounts behave differently as the price changes, which is why a fixed five off is generous on a cheap item and negligible on an expensive one. The tool shows both the money saved and the effective rate so the two are comparable.