5 min read · July 28, 2026
How to Calculate Discounts: Formula, Percentages & Sales Tax
When a store shows '30% off,' the final price is obvious enough on a calculator—but in your head? The math is faster than you think, and knowing it keeps you from overspending on items just because they're marked down.
The discount formula
Discount Amount = Original Price × Discount Percentage / 100. Final Price = Original Price − Discount Amount. A $80 shirt at 25% off costs $80 − ($80 × 25/100) = $80 − $20 = $60.
Or combine the steps: Final Price = Original Price × (1 − Discount % / 100). The $80 shirt: $80 × (1 − 0.25) = $80 × 0.75 = $60. This 'multiply by (1 − discount %)', is faster once it clicks.
Mental math for quick calculations
10% off means pay 90%: $50 → $45. 20% off means pay 80%: $50 → $40. 25% off means pay 75%, or divide by 4: $100 → $75. For anything else, take 10% and scale it: 15% off is 10% + half of 10%.
At the register, the gap between 'sounds like a deal' and 'actually saves money' is where discounts deceive. A $40 item at 15% off saves $6—good. A $200 item at 5% off saves $10—less impressive per dollar, though the total is higher.
Stacking discounts
If a store offers 20% off and then an extra 10% off, these don't add to 30%. Apply them in order: original $100, after 20% off = $80, then 10% off $80 = $72. Net discount: 28%, not 30%. (The second discount applies to the reduced price, not the original.)
Always apply discounts sequentially to get the true final price, even though advertisements often quote them as if they add together.
Calculating savings and ROI on spending
Don't buy something just because it's on sale. 30% off a $200 item you don't need still costs $140 more than not buying it. That said, if you were going to buy it anyway, the $60 savings is real money. The hard part is honest about the 'were going to buy it anyway' part.
For repeat purchases (groceries, essentials), tracking the best prices over time beats one-off shopping—buying shampoo at 50% off when it's normally half-priced is not the win it seems.
Put it into practice
The fastest way to learn the math is to play with the numbers.
Open the Discount Calculator