Discount Calculator

What this calculator does

Discounts look simple and are not, because three separate traps hide inside them. Stacked percentages multiply rather than add, so “20% off, then another 20% off” is 36% off, not 40%. A fixed coupon changes value depending on whether it lands before or after the percentages. And tax is charged on the discounted price, so the saving is slightly bigger than the sticker suggests. This calculator handles all three openly, with a step table that shows the price after every single step, so no figure on the page has to be taken on faith. It also runs backwards: the real discount percentage between two prices, and the original price behind a sale price, and it prices “buy some, get some free” offers as the discounts they are.

How to use it

1. Stacked discounts go in order, in separate fields. Stores apply them in sequence, each one acting on what is left, and the step table mirrors the register. The results call out the difference between the added-up percentages and the effective discount, because on big stacks the gap is real money: 20% + 20% + 20% sounds like 60% off and delivers 48.8%.
2. The coupon order is a choice because stores differ and the buyer wins one way. A percentage discount shrinks whatever it acts on, so applying it first and the coupon second leaves the coupon at full face value: on $100 with 20% off and a $10 coupon, coupon-after costs $70 and coupon-first costs $72. When a retailer lets the coupon land after the percentages, that is the better order, every time, and this page defaults to it.
3. Tax goes on the discounted price. In most US states sales tax is charged on the amount actually paid after store discounts, which is how the calculator applies it. Manufacturer coupons are treated differently in some states (tax on the pre-coupon price); if your receipt shows that, leave the tax field empty and compare against the register total.
4. A coupon bigger than the remaining price floors at zero. The step table says so when it happens. Registers do the same; no store pays you to take the item, whatever the coupon says.
5. Reverse modes answer the two questions shoppers actually ask. “What discount was that?” takes the ticket price and the price paid. “What was the original price?” takes a sale price and the advertised percentage and divides back up, the check to run when a “was” price feels invented: a $72 tag at 20% off implies a $90 original, and if the item never sold at $90, the discount is theatre.
6. Buy-one-get-one is a percentage in disguise. Buy 2 get 1 free is 33.33% off the three items and drops the per-item cost from $30 to $20. Whether it is a good offer depends entirely on whether you wanted three, which is a judgement; the effective percentage and per-item cost this page reports are the facts for making it.

How it is calculated

Each percentage discount acts on the running price: new price = price x (1 – discount/100), applied in order and rounded to the cent at each step the way registers round. Combined effect of stacked discounts = 1 – the product of the (1 – d/100) terms. A coupon subtracts its face value at the chosen point in the sequence, with the price floored at $0.00. Tax = discounted price x tax rate. Reverse percentage: discount = (original – sale) / original x 100. Reverse price: original = sale / (1 – discount/100). Buy-get-free: effective discount = free items / total items received x 100; cost per item received = amount paid / items received.

Worked examples

One clean discount: $120 at 25% off: $90.00, saving $30.00, an effective 25%, the only case where the headline and the truth are the same number.

The stacking lesson: $200 with 20% off and then another 20% off: step one gives $160.00, step two acts on the $160 and gives $128.00. You saved $72.00, an effective 36.00%, not the 40% the two signs invite you to believe. The missing $8 is the second discount being taken on a smaller base.

The coupon, both orders: $100, 20% off, $10 coupon. Coupon after: $80.00 then $70.00, saving 30%. Coupon before: $90.00 then $72.00, saving 28%. Same shelf, same coupon, same discount, $2 apart purely on sequence: the page defaults to the order that favours you and lets you flip it to match a store’s policy.

Tax on the discounted price: the $120 item at 25% off with 8% tax: tax is 8% of $90, which is $7.20, for a total of $97.20. Taxing the original $120 would have charged $9.60; the discount quietly saves tax as well as price.

Working backwards: a jacket marked from $85 to $63.75 was discounted by $21.25, exactly 25.00%. And a $72 sale price at an advertised 20% off implies an original of $90.00 and a saving of $18.00, which the reverse mode prints with its own check line.

The offer in costume: buy 2, get 1 free on a $30 item: pay $60.00, receive three items worth $90.00, effective discount 33.33%, cost per item received $20.00.

Reading your result

The effective total discount is the only percentage that describes the whole deal, so compare offers on it, not on the largest single number in the window. A store offering 40% off competes with one offering 25% plus a further 20% at the register (an effective 40% as well: 0.75 x 0.80 = 0.60) and with one offering 30% plus a $10 coupon, where the winner depends on the price. The step table exists because sequences cannot be judged at a glance; run both offers through it and compare final prices, tax included, which is the only ranking that spends.

Limitations

• Stacking rules are store policy, not law. Many retailers will not combine certain promotions, exclude brands from percentage events, or refuse a coupon on a clearance item. The calculator computes the sequence you enter; whether a register accepts that sequence is the store’s call.
• Rounding can differ by a cent. This page rounds each step to the cent, the common register behaviour. Systems that carry full precision and round once at the end can land a cent away on long stacks. A cent is the honest size of that disagreement.
• “Was” prices are taken as entered. The reverse modes verify arithmetic, not honesty. If an original price was never genuinely charged, the derived saving is fiction with correct maths, which is why regulators in many places require reference prices to be real; the calculator gives you the implied original so you can judge it.
• Tax treatment of manufacturer coupons varies by state. Store discounts reduce the taxable amount in most states; manufacturer coupons may not. The page taxes the discounted price, the common case, and the limitation is stated so a one-line difference on a receipt does not look like an error here.

Frequently asked questions

Why is 20% off plus 20% off not 40% off?

Because the second 20% is taken from the already-discounted price. The first discount removes 20% of $200, which is $40. The second removes 20% of the remaining $160, which is $32. Total removed $72, which is 36% of $200. Percentages of different amounts can never be added; the amounts themselves are what add, and this page’s step table shows each amount as it happens.

Is it better to use the coupon before or after a percentage discount?

After, for the buyer, always. A coupon taken before the percentage gets discounted itself: a $10 coupon inside a 20%-off sequence contributes only $8 of saving. Taken after, it contributes its full $10. Stores that apply coupons first are not making an error; they are making $2. If you get the choice, take the coupon last, and if the store’s order is fixed, this page’s coupon-order switch reproduces it exactly.

Do I pay sales tax on the discounted price or the original price?

Usually the discounted price, when the discount is the store’s own: you are taxed on what you actually pay. The main exception is manufacturer coupons in some states, which are treated as the manufacturer paying part of your bill rather than the price falling. The difference is small per item and large across a year of receipts, which is why the tax field sits in this calculator at all.

How much is “buy one, get one 50% off” really worth?

On two equal items, 25% off the pair: you pay 150% of one price for 200% of the goods. It is the most flattering framing in retail, since the 50% in the headline applies to only half the basket. The buy-get mode on this page converts any such offer to its effective percentage and per-item cost, which are the two numbers worth comparing against a straight discount elsewhere.

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Method: sequential percentage discounts applied multiplicatively to the running price with per-step cent rounding; coupon subtracted at the user-selected point with a $0.00 floor; sales tax applied to the discounted price; reverse percentage and reverse price derived from the same identities; buy-get-free expressed as free items over total items received. Last reviewed: 13 October 2026.