Discount Calculator
Work out a sale price properly when more than one discount applies. Add each discount in turn, see the combined saving, and optionally apply sales tax on the discounted subtotal — which is where most quick calculations go wrong.
Why Stacked Discounts Do Not Add Up
Two successive discounts of 20 percent and 10 percent do not produce 30 percent off. The second discount applies to the price that is already reduced, so it removes a smaller amount in cash than the first. This is the single most common error in retail arithmetic, it always favours the seller rather than the buyer, and it is the reason this page applies discounts sequentially instead of summing percentages.
- The original price is the starting point, and each discount you add is applied to the running total rather than to the original figure.
- The page accepts up to six stacked discounts, which covers the layered promotions that retail and e-commerce pricing tends to produce.
- The saving is reported as a combined effective percentage rather than the sum of the individual rates, so the figure you see is the one that describes what actually happened.
- Sales tax is calculated on the discounted subtotal. Taxing the original price after applying a discount is a real and expensive mistake, and it is easy to make when the two steps are done separately.
Calculating a Real Sale Price
- Enter the original price. Prices with decimals are accepted, so a value like 49.99 works directly — and the quick-select buttons include exactly that case because it is where rounding errors tend to show up.
- Enter the first discount percentage. If there is only one, stop here; the result is already complete.
- Press Add Discount for each additional discount and enter its percentage. Order matters for the intermediate subtotals but not for the final result, since multiplication is commutative — 20 then 10 gives the same answer as 10 then 20.
- Remove a discount with the control beside it if you are testing variations. The control is hidden when only one row remains, because a price calculation needs at least one discount to be a discount calculation.
- Tick Add sales tax and enter the rate if tax applies. The field stays disabled until the box is ticked, so it cannot be filled in accidentally and ignored.
- Read the final price and the combined saving, then check the breakdown below to see exactly how each step contributed.
Three Cases Where the Arithmetic Surprises People
The figures below are exactly what the page returns.
- A 100 item with 20 percent and then 10 percent comes to 72.00, a saving of 28.00. The combined saving is 28 percent, not the 30 percent that adding the two rates would suggest. The difference is the 2 percent that the second discount did not apply to, because the price had already fallen.
- The same 100 item with 8 percent sales tax added on top of the discounts comes to 77.76. The tax is charged on 72, not on 100, which is worth 2.24 compared with taxing the original price. On larger sums the same mistake is proportionally larger.
- A 200 item with three consecutive discounts of 30, 20 and 10 percent lands at 100.80 — a combined saving of 49.6 percent, close to half price. The naive sum of the three rates would suggest a 60 percent saving, which is overstating the reduction by more than ten percentage points of the original price.
Where Discounts Appear in Practice
| Alternative | Notes |
|---|
| Sale price plus a coupon code | The most common real case. Retailers usually apply the coupon to the already-reduced price, which is why a 30 percent sale plus a 20 percent coupon gives 44 percent off rather than 50. |
| Member or loyalty discount | Frequently applied after a promotional reduction. Worth checking the terms, because whether a member discount stacks on a sale price is a commercial decision the retailer makes, not a mathematical one. |
| Volume and trade discounts | On invoices these are often applied in sequence, with each percentage taken from the running total. Quotation systems usually show the intermediate subtotals for exactly this reason. |
| Tax after discount | The order here is fixed in most jurisdictions: tax is assessed on the price actually paid. Applying tax first and discounting afterwards produces a different and incorrect figure. |
Rounding and the Numbers You Will Actually Be Charged
The page computes at full precision and rounds only for display, which is the right behaviour but not always what a till does.
- Retail systems frequently round each line to the nearest cent before summing. On a single item the difference is at most a cent, but on a multi-line order it can add up.
- Some jurisdictions require rounding to the nearest five cents on cash transactions. The page does not model this, so a cash total may differ slightly from the figure shown.
- Displayed values are rounded to two decimal places. Three successive discounts on a price with decimals can produce a fraction of a cent that the page rounds away, which is why the breakdown is worth reading rather than only the headline figure.
- The page assumes a single tax rate applied to the discounted subtotal. Real tax systems often have different rates for different categories of goods, and a mixed basket cannot be represented here.
Discount Questions
Why is 20 percent off then 10 percent off not 30 percent off?
Because the second discount is applied to a smaller number. On a 100 item the first discount removes 20, leaving 80, and the second removes 10 percent of 80 — which is 8, not 10. The total reduction is 28. This is not a rounding artefact or a quirk of this tool; it is how successive percentage reductions always behave, and any calculation that simply adds the rates is wrong.
Does the order of the discounts change the result?
No, and that is worth knowing because it feels like it should. Multiplying by 0.8 and then by 0.9 gives the same product as multiplying by 0.9 and then by 0.8. The intermediate subtotals differ, which is what the breakdown shows, but the final price is identical. The order only matters if a later discount has a minimum spend condition tied to the running total.
Should tax be applied before or after the discount?
After. Tax is levied on the amount actually paid, so the discount reduces the taxable base. Applying tax to the full price and then discounting gives a higher figure than any till will charge you. The page applies tax to the discounted subtotal, which is the correct order.
Can I save a calculation and come back to it?
No, and nothing is stored. The calculation exists only in the page, so reloading clears it, and the figures you enter are never transmitted or written to your device. If you need a record of a particular combination, note the values down before leaving.
Last verified: October 3, 2026
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Maintained by Huang Bin · Last updated October 3, 2026