Discount calculator with stacked offers and tax
Work out what a percentage off actually costs you. Enter the list price and the discount, add a second discount if the offer stacks, and add sales tax or VAT if it is applied at the till. The result shows the price at each step, the total saving and the real effective discount, which is never the sum of the two percentages.
How to use the Discount Calculator
- Enter the list price before any discount.
- Enter the headline discount, and a second one if the offer stacks.
- Add sales tax only if it is added at the checkout rather than already in the price.
- Press Calculate discount to see each step and the total saving.
The formulas
One discount: final = price × (1 − d/100). Two stacked discounts apply one after the other: final = price × (1 − d₁/100) × (1 − d₂/100). Tax is then added to the discounted figure: with tax = final × (1 + t/100).
Worked example: a £120 jacket with 25% off is £90. An extra 10% off at the till takes it to £81. With 20% VAT added on top that is £97.20. The saving before tax is £39, and the effective discount is 1 − (0.75 × 0.90) = 32.5%, not 35%.
Why stacked percentages never add up
The second discount is taken from the already-reduced price, so it is worth less in cash than it looks. "50% off, then 50% off again" is 75% off, not free. The general rule for two discounts is d₁ + d₂ − (d₁ × d₂ ÷ 100): for 25% and 10% that is 25 + 10 − 2.5 = 32.5%. The same logic works in reverse for increases, which is why a 20% fall followed by a 20% rise leaves you 4% down.
Discount, tax and the order of operations
Discounts are applied before tax in every system that charges tax on the amount actually paid: UK VAT, EU VAT and US state sales tax all work this way. That means the tax falls as the discount rises, and the advertised "20% off" saves you slightly more than 20% of the shelf price if the shelf price excludes tax. Note the regional difference in how prices are shown: UK and EU prices to consumers must include VAT already, so you would leave the tax field at zero; US prices are usually shown before sales tax, so fill it in.
Working backwards
| Question | Method |
|---|---|
| What was the original price? | Divide the sale price by (1 − d/100). £81 after 32.5% off was £120 |
| What percentage off is this? | (original − sale) ÷ original × 100 |
| Is "3 for 2" better than 30% off? | 3 for 2 is 33.3% off, but only if you want three |
| Is "buy one get one half price" a 50% deal? | No — it is 25% off the pair |
One caution on multi-buy offers: the per-unit price after rounding can differ by a penny or two from the headline, and the total you are charged is what matters. Check the till receipt rather than the sign.
Frequently asked questions
Is 25% off plus 10% off the same as 35% off?
No. The second discount comes off the already-reduced price, so the effective discount is 32.5%. The general rule is d₁ + d₂ − (d₁ × d₂ ÷ 100).
How do I find the original price from a sale price?
Divide the sale price by (1 − discount/100). An item at £81 after 32.5% off was £120 before.
Is the discount applied before or after tax?
Before. Sales tax and VAT are charged on the amount you actually pay, so the tax falls with the discount.
Should I fill in the tax field?
Only if tax is added at the checkout, as with US sales tax. UK and EU consumer prices already include VAT, so leave it at zero.
Privacy
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Last updated 2026-09-23.