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, a simple discount: Original price $120, discount 25% Discount amount = $120 × 0.25 = $30 Sale price = $120 − $30 = $90

Example 2, stacked discounts: $150 item, 20% off, then a further 10% at the till. After the first discount: $150 × 0.80 = $120 After the second: $120 × 0.90 = $108 Total saving $42, which is 28% off, not 30%

This is the one people get wrong, and stores know it. Stacked discounts multiply rather than add, because the second reduction comes off an already-reduced price. Put both percentages into the calculator above and it shows the real figure.

Example 3, working backwards: Sale price $84, marked 30% off. What was it before? Original = $84 ÷ (1 − 0.30) = $84 ÷ 0.70 = $120

Switch the dropdown to the second option and the calculator does this one.

Common discount traps:

• “Buy 3 for the price of 2” works out to 33.3% off, which is often less than the headline suggests
• “Up to 70% off” means some items are at 70% and most are nowhere near it
• Advertised “was” prices sometimes reflect a price the item spent very little time at. Several countries now regulate this, requiring the original to have been genuinely charged for a set period before a sale claim is allowed

Black Friday tip: Track prices through October. A useful number of November “discounts” start from a figure quietly raised a few weeks earlier, and a price-history browser extension settles the question in seconds.

Sale price vs markdown: Retailers use “markdown” for the absolute reduction in dollars and “discount” for the percentage. A $30 markdown on $120 is a 25% discount. Same event, two vocabularies, and the one on the sign is whichever sounds larger.