Discount Calculator: How to Calculate Sale Prices & Double Discounts

September 5, 2026 · 4 min read

Sales and discounts sound simple — until you encounter stacked discounts, coupons on top of clearance prices, and sales tax added afterward. That 50% off sign does not always mean you pay half price. Knowing how to calculate the actual final price prevents surprises at checkout.

Simple Discount Calculation

Original price × (1 - discount rate) = sale price. A $200 item at 30% off: $200 × 0.70 = $140. For quick mental math, find 10% first and work from there. 10% of $200 is $20, so 30% is $60 off, giving you $140.

Stacked Discounts Are Not Additive

An extra 20% off an item already 40% off is NOT 60% off. The first discount brings a $100 item to $60. The second discount takes 20% off $60, giving $48. The total discount is 52%, not 60%. This is how stores make stacked discounts sound bigger than they are.

Adding Sales Tax

Calculate the discount first, then add tax to the discounted price. A $100 item at 25% off with 8% sales tax: discounted price is $75, tax is $75 × 0.08 = $6, final price is $81. In some regions, tax is calculated on the original price before discount — know your local rules.

Is the Sale Actually Good?

Compare the per-unit price, not the discount percentage. A 50% off deal on a $30 item ($15 final) might be worse than a 30% off deal on a $18 item ($12.60 final) for the exact same product at a different store. The percentage means nothing without the starting price.

Try It Now

Our free Discount Calculator handles this instantly — no signup, no limits.

Also useful: our Percentage Calculator for related calculations.