The basic percent-off formula

To find a sale price: Sale price = Original price × (1 − discount ÷ 100). A $80 item at 25% off is $80 × (1 − 0.25) = $80 × 0.75 = $60. The amount you save is just the original price minus that result — in this case, $20.

Finding the original price from a sale price

This is the direction people actually get stuck on: you know the sale price and the discount percentage, and want the original. Rearranging the formula above: Original price = Sale price ÷ (1 − discount ÷ 100). If something costs $60 after a 25% discount, the original price was $60 ÷ 0.75 = $80.

Stacking multiple discounts (the part people get wrong)

"30% off, plus an extra 10% off" is not 40% off. Each discount applies to the price that's already been reduced by the previous one, not to the original price. A $100 item at 30% off becomes $70; a further 10% off that $70 is $63 — a combined discount of 37%, not 40%. This is why stacked "extra X% off" promotions are always a little less generous than they sound at first glance, and it's worth actually calculating rather than assuming the percentages simply add.

Discount then tax — order matters

Sales tax is calculated on whatever the final price actually is at checkout, so the sequence is: apply the discount first, then apply tax to the discounted price — never apply tax to the original price and then subtract a discount calculated on that larger number. A $50 item at 20% off with 8% sales tax: $50 × 0.80 = $40, then $40 × 1.08 = $43.20. Doing it the other way around would overstate what you actually owe.

Quick mental math shortcuts

Try it yourself

ToolFlight's Discount Calculator does all of this instantly — sale price, amount saved, and working backward from a sale price to the original — without the mental math.

Open the free Discount Calculator →