How to Calculate Discounts: Percent Off, Stacked Deals & Sales Tax

How Discounts Work

A discount is a reduction from the original price. Discounts come in two main forms: Percentage off (20% off, 50% off) where savings scale with price, and fixed amount off ($10 off, $25 off) where savings are the same regardless of price.

The Discount Formula (Step-by-Step)

Step 1: Discount Amount = Original Price × (Discount % ÷ 100). Step 2: Final Price = Original Price - Discount Amount. Shortcut: Final Price = Original Price × (100% - Discount%) ÷ 100. Example: 30% off $80: $80 × 0.70 = $56.

Finding the Original Price from a Sale Price

Original Price = Sale Price ÷ (1 - Discount% ÷ 100). Example: A shirt is on sale for $36 with 20% off. Original = $36 ÷ 0.80 = $45. Common mistake: Don't just add 20% to $36 — that gives $43.20, which is wrong.

Stacked Discounts: They Don't Add Up

Stacked discounts are applied sequentially, not cumulatively. $100 item with 20% off + 10% off: $100 × 0.80 = $80, then $80 × 0.90 = $72. Total savings: $28 (28%, not 30%). Formula: multiply remaining percentages — 0.80 × 0.90 = 0.72.

How Sales Tax Affects Your Savings

Discounts are applied before sales tax. A 25% discount on $100 with 8% tax: you save $25 on the price plus $2 on the lower tax. Total savings: $27, not just $25.

Percentage Off vs Fixed Amount: Which Is Better?

$20 off is better on a $50 item (40% savings). 25% off is better on a $500 item ($125 savings). The crossover: Fixed Amount ÷ Percentage as decimal. Below that price, fixed is better. Above it, percentage is better.

Real-World Shopping Examples

Black Friday: A laptop at $1,200 with 20% off + extra 10% + $50 rebate: $1,200 × 0.80 = $960, × 0.90 = $864, - $50 = $814. Total savings: $386 (32.2%). BOGO 50% off: Two $30 items cost $30 + $15 = $45 — that's 25% off per item on average.

Common Mistakes to Avoid

Adding stacked percentages — 20% + 10% = 28%, not 30%
Reversing the formula — finding original price requires division, not addition
Forgetting sales tax — your total savings include the lower tax amount
Ignoring unit prices — a bigger discount on a more expensive item may cost more
Falling for anchor pricing — "Was $200, now $80!" may not reflect true original price

Key Takeaways

Quick formula: Final Price = Original × (1 - Discount/100)
Find original price: Sale Price ÷ (1 - Discount/100)
Stacked discounts multiply — 20% + 10% = 28% total, not 30%
Tax is calculated after discounts — you save on tax too
Compare final prices, not just percentages, when shopping

Ready to calculate your savings? Use our free Discount Calculator to instantly see discounts, stacked deals, and final prices with tax.

FAQs

How do you calculate 20% off a price?

Multiply the original price by 0.20 (which is 20/100). For a $50 item: $50 × 0.20 = $10 discount. Final price = $50 - $10 = $40. Or multiply by 0.80 directly: $50 × 0.80 = $40.

How do you find the original price from a sale price?

Divide the sale price by (1 - discount percentage). If a shirt costs $30 after 25% off: $30 ÷ 0.75 = $40 original price. The formula is: Original Price = Sale Price ÷ (1 - Discount/100).

Are stacked discounts cumulative?

No. Stacked discounts are applied sequentially, not added together. 20% off + 10% off equals 28% total savings (not 30%). The second discount applies to the already-reduced price.

Should I calculate discount before or after tax?

Discounts are always applied before sales tax. The merchant calculates the discounted price first, then adds tax to the reduced amount. You save on both the discount and the lower tax.

What's the difference between a percentage discount and a fixed discount?

A percentage discount scales with price (25% off $200 = $50 savings), while a fixed discount is the same regardless of price ($10 off is always $10). Percentage discounts are better for expensive items; fixed discounts are better for cheap items.