Discount Calculator
Find the real price after a discount — including stacked offers, where the headline percentages never simply add up.
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How it works
A single discount is straightforward. Two stacked discounts are where the arithmetic goes wrong: "50% off, then an extra 20% off" is not 70% off. The second discount applies to the already-reduced price, so the true combined figure is 60%.
Tax is applied last, to the discounted price, which is why a sale genuinely reduces the tax you pay as well as the price.
Final = price × (1 − d₁) × (1 − d₂) × (1 + tax)
- d₁ — the first discount, applied to the full price
- d₂ — a second discount, applied to whatever the first one left
- Effective discount — the single discount that would produce the same price
Worked example
A 4,000 item at 50% off, plus a further 20% off, with 18% tax
Inputs
Results
The signage implies 70% off. The real figure is 60% — a 400 difference on this item.
Frequently asked questions
Why do stacked discounts not add up?
Because the second is applied to the reduced price, not the original. 50% then 20% leaves 0.5 × 0.8 = 0.4 of the price, so 60% off rather than 70%. The gap widens as the discounts get larger.
Does the order of the discounts matter?
No. Multiplication commutes, so 50% then 20% gives exactly the same price as 20% then 50%. If a retailer insists the order matters, something other than arithmetic is going on.
Is tax charged before or after the discount?
After, in virtually every system — tax is due on the amount actually paid. That means a discount reduces the tax proportionally, which this calculator reflects.
Is my data sent anywhere?
No. The calculation runs entirely in your browser. Nothing you type is transmitted to us or stored.
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