Discount Calculator

The Discount Calculator turns a "40% off" sign into the two numbers you 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. That last figure makes it a discount percentage calculator as well: $15 off a $60 item is a 25% discount, which is easier to compare against a rival "20% off" offer. 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. Add a sales tax rate and it works as a discount calculator with tax, showing the final amount at the register: a $60 sale price with 8% tax comes to $64.80.

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. Tax is calculated on the discounted price, and whether the coupon comes before or after tax depends on its type. A percentage coupon gives the same total either way, since 10% off and 8% tax multiply in any order. A fixed-amount coupon does not: $5 off before 8% tax saves $5.40 at the register, while $5 off the taxed total saves $5. 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.