How to Calculate a Discount (and Check the Math on a Sale)

The exact math for finding a sale price from a discount, working out what percent off a deal really is, and reversing a sale price back to the original.

"Calculate a discount" can mean three different problems depending on which two numbers you already have. Mixing them up is the most common reason discount math comes out wrong.

Problem 1: Find the sale price

You know the original price and the discount percent. Sale price = original × (1 − discount %). A $50 item at 30% off: $50 × 0.70 = $35.00.

Problem 2: Find the discount percent

You know both prices and want to check the real percent off. Discount % = (original − sale) ÷ original. A $50 item marked down to $35 saved $15, which is $15 ÷ $50 = 30% off — useful for checking whether a store's advertised percentage matches the actual price drop.

Problem 3: Find the original price

You know the sale price and the discount percent, and want to know what it cost before. Original price = sale price ÷ (1 − discount %). A $35 sale price after 30% off: $35 ÷ 0.70 = $50.00.

The trap: stacking two discounts

If a second discount applies on top of the first, it applies to the already-reduced price, not the original — 20% off then 10% off is a 28% total discount, not 30%. Adding percentages directly only works for a single discount, never for two or more stacked together.

The Discount / Sale Price Calculator solves all three problems, plus correctly stacks multiple discounts in sequence.