Discount Calculator

A discount calculator finds the sale price after a percentage reduction, or reverse-calculates the original price when you know the sale price and discount percentage.

The formulas:

Sale price from original and discount: Sale price = Original price × (1 − Discount% / 100) Discount amount = Original price × (Discount% / 100)

Original price from sale price and discount: Original price = Sale price / (1 − Discount% / 100)

Worked examples:

Example 1 — Simple discount: Original price: £120, discount: 25% Discount amount = £120 × 0.25 = £30 Sale price = £120 − £30 = £90

Example 2 — Stacked discounts: £150 item, first 20% off, then additional 10% off. After first discount: £150 × 0.80 = £120 After second discount: £120 × 0.90 = £108 Total saving: £42 (28% overall, NOT 30%)

Stacked discounts do not add — they multiply.

Example 3 — Reverse calculation: Sale price £84, was 30% off. What was the original? Original = £84 / (1 − 0.30) = £84 / 0.70 = £120

Common discount traps:

• “Buy 3 for the price of 2” = 33.3% off (not the advertised “save £X”)
• “Half price” must mean at least 28 days at full price (UK ASA rules)
• “Up to 70% off” means only some items are 70% off: most are less

Black Friday tip: Track prices before November — many “Black Friday” discounts start from an artificially inflated price set in the weeks before the sale.

Sale price vs markdown: Retail uses “markdown” to mean the absolute reduction in £. “Discount” typically means the percentage. £30 markdown on £120 = 25% discount.