Sale Discount Stack Calculator: Calculate Stacked Discounts
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Stacked discounts are more common than ever: retailers run site-wide sales, then accept coupon codes on top, sometimes alongside cashback portals, loyalty points, and credit card rewards. Understanding how stacked discounts actually work prevents both under- and over-estimating your savings. The critical insight is that stacked discounts are not additive: a 20% off sale followed by an additional 10% off coupon does not give you 30% off. Instead, the 10% coupon is applied to the already-discounted price, yielding a total saving of 28%. This is because each discount is applied sequentially to the running price, not to the original price. The mathematical formula for total discount from stacked percentages is: total discount = 1 minus the product of (1 minus each individual discount). For three discounts of 20%, 10%, and 5%, the total is 1 minus (0.80 times 0.90 times 0.95) which equals 31.6%, not 35%. This calculator takes up to three discount percentages plus sales tax, applies them in sequence, and shows the price at each step.
Why stacked discounts are not additive
When you see "20% off, plus an extra 10% off with code SAVE10", it is natural to assume the total saving is 30%. It is actually 28%. Each discount is applied to the running price, not the original. After 20% off a $100 item, the price is $80. The 10% coupon then takes 10% of $80, saving $8, for a final price of $72. That is a $28 saving on a $100 item: 28%, not 30%.
The formula for any number of stacked percentage discounts is: total discount = 1 minus the product of (1 minus each discount expressed as a decimal). For discounts d1, d2, d3:
Total discount = 1 − ((1 − d1) × (1 − d2) × (1 − d3))
This calculator applies the correct sequential formula and compares the actual total discount to the naive additive sum so you can see the difference clearly.
Sales tax on discounted prices
In most US states, sales tax is calculated on the final discounted price you pay, not the original price. If you buy a $100 item with 30% off, you pay $70, and sales tax applies to $70. Some promotional pricing structures (such as manufacturer coupons vs retailer coupons) can affect the taxable base; check your state's revenue department guidance for specific cases.
This calculator applies sales tax to the price after all discounts are taken, which is the standard treatment for retailer discounts and coupon codes in most jurisdictions.
Stacked discounts: frequently asked questions
Why are stacked discounts not additive?
Each successive discount is applied to the already-reduced price, not the original. So the dollar saving from each layer is smaller than if applied to the original price. A 20% discount followed by a 10% discount gives 28% total, not 30%, because the 10% applies to 80% of the original price.
What is the formula for stacked discounts?
Total discount % = 1 minus the product of (1 minus each discount expressed as a decimal). Example: 20% and 10% gives 1 minus (0.80 times 0.90) = 0.28, or 28%. For three discounts of 20%, 10%, and 5%: 1 minus (0.80 times 0.90 times 0.95) = 31.6%.
Does the order of discounts matter?
No. Because multiplication is commutative, applying 20% then 10% gives the same result as 10% then 20%. The final price is identical regardless of order. The order only affects what the intermediate step prices look like.
When is stacking discounts worth the effort?
Always, if each discount applies to something you were going to buy anyway. Even small additional discounts add up significantly over many purchases. A 5% loyalty discount stacked on a 20% sale saves an extra 4% (20% of 5%) vs taking only the sale price.
Do cashback portals stack with sale prices and coupon codes?
In most cases, yes. Cashback is typically calculated on the amount you pay, so it applies after other discounts are taken, adding to your overall saving. Check the cashback portal's terms to confirm stacking is permitted for the retailer.
Official sources
- FTC pricing guidance and consumer resources: ftc.gov.
- Consumer education resources: consumer.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.