Compare discounts by the price you will actually pay
The largest number on a sale label is not necessarily the best deal. A useful comparison starts with the final price of the same product, including the costs that sit outside the discount.
Keep the saving separate from the final price
The discount amount is the original price multiplied by the discount rate. Subtract it from the original price to find what remains to pay. Savings are useful only when the reference price itself is meaningful.
“30% off” cannot tell you the cash saving without the original price. A product with a lower starting price can cost less even with a smaller percentage discount.
References: OpenStax — Sales tax, commission and discount applications
Successive discounts do not add together
When a second discount applies to an already reduced price, its base is smaller. Taking 25% off and then another 10% leaves 75% × 90% = 67.5% of the original price: a total reduction of 32.5%, not 35%.
A fixed-value coupon is a different operation. Check the stated order and eligible products; do not assume that every seller applies a coupon before or after the percentage discount in the same way.
Compare equivalent purchases
Match the package size, quantity and specification. If sizes differ, compare the price per unit as well as the sale price.
Include mandatory charges and delivery costs that are actually quoted to you. A discount calculation cannot know the seller’s conditions or final checkout total.
Worked example: another percentage or a fixed coupon?
A fictional item costs 480. A 25% discount takes it to 360. Another 10% off that reduced price produces 324. The saving is 156, which is 32.5% of the original price.
If the second offer were instead a coupon worth 30 deducted after the first discount, the price would be 330. Comparing 324 with 330 is more informative than comparing “10%” with “30” without their starting amounts.