Why stacked discounts multiply
A single discount is easy: take the percentage off and pay the rest. Stacked discounts are where shops earn their money, because the second one applies to the already reduced price, not the original. The discounts multiply rather than add, which is why "30% + 10%" is 37% off, and retailers know most people read it as 40. That gap isn't an accident. It's the whole point of splitting one discount into two.
The calculator shows the price after the first discount as well as the final one, so you can see exactly what base the second discount worked on. If the second box is 0, it behaves as a plain single-discount calculator.
Worked example
With the defaults and an extra 10% typed into the second box: 30% off $79.99 means paying $55.99 and saving $24.00; stacking an extra 10% brings it to $50.39, an effective 37.0% off, not 40%, because the second discount applies to the already reduced price.
Frequently asked questions
Why don't stacked discounts add up?
Because the second discount is taken off the already reduced price, not the original. 30% off followed by 10% off leaves you paying 70% then 90% of that, which is 63% of the original price, a 37% total discount. The discounts multiply, they don't add.
How do I compare a "$15 off" deal with a "20% off" deal?
Convert them into the same units at your price. On a $60 item, 20% off is $12, so $15 off wins. On a $100 item, 20% off is $20, so the percentage wins. Neither format is better in general; it always depends on the price of the thing you're actually buying.
Is a bigger percentage always the better deal?
Only when it's applied to the same base price. 40% off an inflated "was" price can cost you more than 25% off an honest one. Compare the final prices you'd actually pay, not the size of the percentage on the sticker.