Discount Calculator – Sale Prices, Stacked Coupons and Multi-Buy Deals
A price tag that says "25% off" is easy. Things get harder with an extra 10% for members, a fixed coupon, a "buy 2 get 1 free" shelf, or two shops with different shipping. This discount calculator covers all of these. It works out the sale price and your checkout total with tax, finds the original price from what you paid, shows how stacked discounts really combine, prices multi-buy deals, and ranks competing offers for the quantity you need.
How to calculate a percentage discount
Multiply the original price by what is left after the discount: sale price = original × (1 − discount). For 25% off 80, that is 80 × 0.75 = 60, so you save 20. A fixed amount off works the same way from the other direction. 15 off 49.99 leaves 34.99, which is an effective discount of 30.01%. That figure lets you compare a money-off coupon with a percentage sale.
Why stacked discounts multiply instead of adding
"20% off, then an extra 10% off" is not 30% off. The second discount applies to the already-reduced price: 200 × 0.8 = 160, then 160 × 0.9 = 144. That is 28% off, and you pay 144, not 140. With percentages only, the order makes no difference, because multiplication doesn't care about order. Add a fixed coupon and the order starts to matter. Taking 15 off after both percentages gives 129.00. Taking it off first gives 133.20. The Stacked tab shows both ends of that range and a step-down chart of each reduction.
Working backwards to the original price
To find what something cost before the sale, divide instead of multiplying: original = sale price ÷ (1 − discount). If you paid 59.99 after 40% off, the original price was 59.99 ÷ 0.6 = 99.98. A common mistake is to add 40% back on (59.99 × 1.4 = 83.99), which gives the wrong answer. If your receipt total includes sales tax, remove the tax first by dividing by 1 + tax rate.
The maths of multi-buy deals
"Buy 2 get 1 free" is the same as "3 for the price of 2". That is 33.33% off, but only in multiples of three. Buy 7 items at 12 each and just 2 are free, so you pay 60 instead of 84, which is 28.57% off. Buying 9 gets you back to 33.33%. The Multi-buy tab tells you when one more item would be free or would unlock another free one. It also handles carts where items have different prices, because stores differ on which items they discount. "3 for 10" deals are covered too, including shops that charge the deal rate per item.
Sales tax before or after a discount
With a store discount, tax is usually charged on the reduced price. On 3 items at 80 with 25% off and 8.25% tax, that means 180 × 0.0825 = 14.85 in tax. Some coupons, often manufacturer coupons in the US, are taxed on the price before the coupon, which would make the tax 19.80. Rules differ from place to place, so the calculator lets you pick either basis. Where shelf prices already include VAT or GST, leave the tax rate at zero.
Comparing deals
The best deal depends on how many you actually need. At a quantity of 3, "buy 2 get 1 free" beats 30% off, but at a quantity of 4, 30% off wins again. The Compare tab plots total cost against quantity for up to four offers, including shipping, so you can see where the lines cross.