Why is 20 percent then 10 percent not 30 percent off?

Because the second discount comes off the already reduced price. 100 less 20 percent is 80, and 10 percent off 80 is 8, so you pay 72 and the effective discount is 28 percent. Stacked discounts multiply rather than add, and the gap grows as the discounts get larger.

Does the order of the two discounts matter?

No. Multiplication is commutative, so 20 then 10 gives the same total as 10 then 20. Which is worth knowing when a shop insists a voucher must be applied last.

Can I use it for a price increase?

Not here, since a discount is capped at 100 percent. The percentage calculator does increases, and the margin calculator handles pricing upwards from a cost.

Discount Calculator

Sale price, savings, and stacked discounts done right.

Original price

Discount

Second discount

Sale price, savings, and stacked discounts done right.

How to use it

Enter the price and the discount. Currency symbols and separators are stripped.
Add a second discount if there is one. A voucher on top of a sale price, for instance.
Check the effective discount. Shown whenever there are two, because it is not the sum of them.

When you would use this

A single discount is one multiplication. Two of them are the reason this page exists. Stacked discounts multiply, they do not add. Twenty percent off, then ten percent off with a voucher, is not thirty percent off. The second discount applies to the already reduced price, so 100 becomes 80 and then 72, and the effective discount is 28 percent. The gap widens as the numbers grow: 50 and 50 is 75 percent off rather than free. Whenever there are two discounts the effective figure is shown alongside the sum you might have expected, because the point of the tool is to make that difference obvious rather than to quietly produce the right number. The order does not matter, which surprises people. Multiplication is commutative, so applying the voucher first or last gives exactly the same total.

Questions

Why is 20 percent then 10 percent not 30 percent off?: Because the second discount comes off the already reduced price. 100 less 20 percent is 80, and 10 percent off 80 is 8, so you pay 72 and the effective discount is 28 percent. Stacked discounts multiply rather than add, and the gap grows as the discounts get larger.
Does the order of the two discounts matter?: No. Multiplication is commutative, so 20 then 10 gives the same total as 10 then 20. Which is worth knowing when a shop insists a voucher must be applied last.
Can I use it for a price increase?: Not here, since a discount is capped at 100 percent. The percentage calculator does increases, and the margin calculator handles pricing upwards from a cost.