How to Calculate Discounts and Find the Best Sale Prices
A "50% off" tag feels like found money. Retailers know that how a discount is framed changes how much you're willing to spend — even when the savings are mathematically identical. Understanding how discounts actually work helps you cut through the psychology and make better purchasing decisions.
The Basic Percent-Off Formula
Sale price = Original price × (1 − discount rate)
- 30% off a $80 item: $80 × 0.70 = $56.00
- 15% off a $249 item: $249 × 0.85 = $211.65
Or use the discount calculator to get the result instantly without mental math.
Finding the Original Price from a Sale Price
Original price = Sale price ÷ (1 − discount rate)
Example: an item is on sale for $68 after a 20% discount. Original price = $68 ÷ 0.80 = $85.00.
Stacking Discounts: Why 20% + 10% Off Is Not 30% Off
Sequential discounts do not add up. Example:
- Start: $100
- After 20% off: $100 × 0.80 = $80
- After additional 10% off: $80 × 0.90 = $72
- Total savings: $28 (not $30) — that's 28% off, not 30%
General formula: Total discount = 1 − (1 − d1) × (1 − d2).
Sales Tax on Top of Discounts
Sales tax is typically applied to the sale price after discounts:
- $120 item, 30% off = $84 sale price
- 8.5% sales tax → $84 × 1.085 = $91.14 total
Are "Big Sale" Events Actually Good Deals?
Research on Black Friday pricing tells a cautionary story. A 2019 analysis by consumer watchdog Which? tracked prices on 83 products advertised as Black Friday deals and found that 95% were available for the same price or cheaper at other points in the year. Signs a discount may be inflated:
- The "original" price was only active for a very short window before the sale
- The sale price matches or exceeds the typical market price from other retailers
- Price history tools (CamelCamelCamel, Keepa) show the "sale" price as the normal going rate
For fast in-store calculations, the discount calculator handles percent off, final price, and original price lookups in one tool.