How to Use the Discount Formula for Shopping Deals
Black Friday, flash sales, coupon stacking, BOGO offers — discounts are everywhere, but figuring out the actual price you pay is harder than it looks. Is 30% off followed by an extra 20% off the same as 50% off? (Spoiler: it is not.)
This guide covers the standard discount formula, shows you how stacked discounts really work, explains BOGO and rebate math, and gives you mental shortcuts for quick in-store decisions.
✨Key takeaways
- Discount price = Original × (1 − discount rate). For 25% off a $80 item: $80 × 0.75 = $60.
- Stacked discounts multiply, not add. 30% off + 20% off = 44% total discount, not 50%.
- BOGO 50% off = 25% off each item when buying two.
- Always calculate the unit price or per-item cost to compare deals accurately.
The basic discount formula
Sale Price = Original Price × (1 − Discount Rate). Discount Amount = Original Price × Discount Rate.
Example: a $120 jacket is 35% off. Discount = $120 × 0.35 = $42. Sale price = $120 − $42 = $78. Or directly: $120 × 0.65 = $78.
Use the Discount Calculator to compute any discount instantly, including tax.
Stacked discounts: why 30% + 20% ≠ 50%
When a store offers 30% off "plus an extra 20% off," the second discount applies to the already-reduced price, not the original.
Example: $100 item. First discount: $100 × 0.70 = $70. Second discount: $70 × 0.80 = $56. Total discount: $100 − $56 = $44, or 44% — not 50%.
The general formula for two stacked discounts: Final = Original × (1 − d1) × (1 − d2). The order does not matter mathematically, but the result is always less than the sum of the two rates.
BOGO math decoded
BOGO Free: buy one at full price, get one free. Per-item cost = original price ÷ 2. A $50 shirt BOGO free = $25 each.
BOGO 50% Off: buy one full price, get the second at half off. Total for two = 1.5 × price. Per-item = 0.75 × price. A $40 pair of shoes BOGO 50% off = $30 each.
BOGO deals are only good if you actually want two. Buying something you do not need at 50% off is still 50% wasted.
Rebates: the hidden discount
A rebate is a post-purchase refund. You pay full price now and get money back later. The effective discount is the same as a percentage off, but the timing is different — and many people forget to submit the rebate.
Studies show that 40–60% of rebates go unredeemed, which is why companies love them. If you decide to chase a rebate, submit it immediately and set a calendar reminder to follow up.
Mental shortcuts for in-store math
10% off: move the decimal one place left. 10% of $79 = $7.90.
25% off: divide by 4. $80 ÷ 4 = $20 off → $60.
33% off: divide by 3. $90 ÷ 3 = $30 off → $60.
50% off: halve it. No explanation needed.
For odd percentages like 35%: compute 30% (move decimal, multiply by 3) + 5% (half of 10%). 30% of $80 = $24, 5% = $4. Total discount = $28. Sale price = $52.
Try the calculators referenced in this guide
Put the maths into practice — every calculator is free and runs entirely in your browser.
Frequently Asked Questions
Is it better to get a dollar-amount discount or a percentage discount?
It depends on the price. "$20 off" on a $50 item (40% off) is better than "30% off" ($15 off). Always calculate both to compare.
Do I calculate the discount before or after tax?
Discounts are applied before tax. Tax is calculated on the discounted price. So a $100 item at 25% off with 8% tax: $75 × 1.08 = $81, not $100 × 1.08 × 0.75 = $81. (Same result in this case, because multiplication is commutative.)
How do I know if a sale is actually a good deal?
Track the item's price history using price-tracking tools or browser extensions. "50% off" is meaningless if the original price was inflated. Compare the sale price to what you have seen the item sell for previously.
The Precision Calculator Editorial Team
The editorial team at Get Precision Calculator writes practical, formula-driven guides that explain the maths behind every calculator on this site. All content is reviewed for accuracy before publishing.
Continue Reading
How to Calculate Your Daily Water Intake
How much water do you actually need? A weight-based formula, activity adjustments, climate factors, and signs of dehydration explained with real examples.
Read guide →How to Calculate Electricity Costs for Home Appliances
The kWh formula explained with real examples for common appliances — AC, fridge, TV, washer — plus five tips to cut your electricity bill immediately.
Read guide →Mental Math Tricks for Everyday Calculations
Ten mental math techniques for tips, discounts, multiplication, division, and quick estimation — with worked examples and practice tips.
Read guide →