Why 30% and 10% off is not 40% off
2026-11-07
Why adding discount rates gives the wrong answer, how to multiply stacked discounts correctly, and what to check when coupons and points apply.
Adding discount rates gives the wrong answer
It is tempting to think that 30 percent off plus a 10 percent coupon equals 40 percent off. The real checkout price is higher than that. Stacked discounts multiply rather than add.
How to multiply the discounts
Apply the first discount, then apply the next discount to the result. Each discount leaves a share of the price, which is (1 − discount rate). A 30 percent discount leaves 70 percent. A further 10 percent discount keeps 90 percent of what remains.
- Final price = original price × (1 − rate 1) × (1 − rate 2)
For an original price of 10,000 won, 30 percent off gives 7,000 won, and then 10 percent off gives 6,300 won. The saving is 3,700 won, which is 37 percent of the original price, not 40 percent.
The order does not change the result
If you apply the 10 percent first and the 30 percent second, the final price is still 6,300 won. Multiplication gives the same result in any order. What matters is which discounts apply and how many there are.
When coupons and points are involved
Coupons and reward points are often deducted separately at checkout. In that case the final amount can differ from this calculation. Check on the payment page which discount is applied first, and whether points are calculated from the discounted price.
A checklist before you pay
1. Write down the original price and every discount, and check whether they can be combined.
2. Enter the discount rates in order in the calculator to see the final price.
3. Check any conditions for free shipping or a minimum order amount.
4. Compare the result with the total shown on the payment page.
A bigger discount is not always cheaper
A high discount rate does not guarantee the lowest price. The original price may be inflated, or you may buy extra items just to meet the conditions. Compare the final price, and ask whether you really need the item.