Discount Calculator Guide: How to Calculate Any Discount Instantly
Learn how to calculate discounts, stacked promotions, and reverse-calculate original prices. Master the math behind every sale.
Shopping sales can be confusing. Is '30% off plus an extra 10%' the same as 40% off? No — it's actually 37%. And when a store says 'Buy 2 Get 1 Free', what's the actual per-unit discount? Understanding discount math prevents you from overpaying and helps you find genuinely good deals.
Basic Discount Calculation
The fundamental formula is simple: Discounted Price = Original Price × (1 - Discount Rate). For a $80 item at 25% off: $80 × 0.75 = $60. The discount amount is $80 × 0.25 = $20. This works for any single-discount scenario.
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Discount Calculator →Stacked Discounts Are Not Additive
When a store offers '30% off + extra 10% off', many people assume it's 40% total. It's not. You apply each discount sequentially: $100 × 0.70 = $70, then $70 × 0.90 = $63. The total discount is 37%, not 40%. The difference grows with larger discounts — '50% + 20%' is 60%, not 70%.
Common Promotion Types Compared
- Buy 1 Get 1 Free (BOGO): 50% off per unit when buying 2
- Buy 2 Get 1 Free: 33.3% off per unit when buying 3
- Buy 3 Get 2 Free: 40% off per unit when buying 5
- 20% off everything: Straightforward 20% savings regardless of quantity
Reverse-Calculating the Original Price
If you see a sale price and want to know the original: Original Price = Sale Price ÷ (1 - Discount Rate). A shirt marked $42 at 30% off was originally: $42 ÷ 0.70 = $60. This is useful for evaluating whether a 'sale' is truly a good deal.
Real vs. Fake Discounts
Some retailers inflate original prices before applying discounts, making the deal appear better than it is. Compare the 'discounted' price with competitors' regular prices. Track prices over time using price history tools. A '50% off' item that was never sold at full price isn't really half off.
Frequently Asked Questions
Is '30% off + extra 10%' the same as 40% off?
No. Stacked discounts are applied sequentially, not added. 30% off then 10% off equals 37% total discount. The formula: 1 - (0.70 × 0.90) = 0.37 or 37%. The difference increases with larger discount percentages.
How do I calculate the discount percentage from two prices?
Discount % = ((Original - Sale) ÷ Original) × 100. For example, an item reduced from $80 to $52: ((80-52) ÷ 80) × 100 = 35% off.
What's the per-unit discount for Buy 2 Get 1 Free?
You pay for 2 items but get 3. The discount per unit is 1/3 = 33.3%. If each item is $30, you pay $60 for 3 items instead of $90 — saving $30 or 33.3%.
How do I know if a sale price is actually a good deal?
Compare the sale price with regular prices at 2-3 other retailers. Check price history if available. Calculate the actual discount percentage. Be skeptical of 'up to X% off' claims — the best discounts usually apply to unpopular items or sizes.
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Seokjun
Founder of QuickFigure. Building tools that make complex calculations and document tasks simple for everyone.
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