How to Calculate a Discount (Percent Off & Stacked)
The basic discount formula
Savings = Original price − Final price
Shortcut for mental math: a 20% discount means you pay 80% of the price — move the decimal once and multiply.
Worked examples
Simple: $120 item at 25% off → 120 × 0.75 = $90 (you save $30)
Reverse: sale price $64 after 20% off → 64 ÷ 0.80 = $80 original
The stacked-discount trap
"30% + 20% off" is not 50% off. The second percentage applies after the first one is already applied:
$100 at 30% off → $70. Extra 20% off applies to $70 → 70 × 0.80 = $56.
Effective discount = 44%, not 50%. The $6 difference is why stores love stacked offers.
Quick stacked-discount table
| Stack | Effective discount |
|---|---|
| 10% + 10% | 19% |
| 20% + 20% | 36% |
| 30% + 20% | 44% |
| 50% + 20% | 60% |
| 50% + 50% | 75% |
Discount vs cashback vs coupon codes
- Instant discount reduces the price before tax — usually the most predictable saving.
- Cashback arrives later and may be capped or expire; treat as partial value.
- Loyalty points have uncertain value — compare offers on final cash price, not headline percentages.
Common mistakes
- Adding stacked percentages instead of multiplying them.
- Applying an "extra %" to the original price instead of the reduced price.
- Comparing percentage-off across items with very different base prices — absolute savings matter too.
- Forgetting that tax is applied to the discounted price in most jurisdictions.
The successive-discount formula
Two stacked discounts combine by multiplication, not addition. The equivalent single discount is:
Example: 30% plus 20% off gives 100 − (70 × 80) ÷ 100 = 100 − 56 = 44%. On a $1,000 order that is $560 to pay, not the $500 that "50% off" suggests. The same formula works for three or more discounts — keep multiplying the remaining-price factors. Our stacked discounts explainer walks through the checkout math step by step, and the Discount Calculator handles the arithmetic for any combination.
Worked example: Discount Calculator at its default inputs
Open Discount Calculator and press calculate without editing a field. What comes back is the table and result below, so the discount numbers here are checkable in one click.
| Input | Default |
|---|---|
| Original Price | 100 |
| Discount (%) | 20 |
Result: Final Price: $80.00
The steps behind that answer
- Step 1: Discount = $100 × 20% = $20.00
- Step 2: Final price = $100 - $20.00 = $80.00
Original Price: half, as given, and half again
Same calculator, same fields, one change: original price.
| Original Price | Result |
|---|---|
| 50 | Final Price: $40.00 |
| 100 | Final Price: $80.00 |
| 150 | Final Price: $120.00 |
Halve original price and the result moves one way; add half again and it moves the other. Watching that gap is quicker than guessing which field matters most.
Related guides
Stacked discounts multiply, they do not add
Two successive 20% discounts are not 40% off. The second discount applies to the already-reduced price, so the total reduction is 36%. Three 20% discounts give 48.8%. The gap grows with every additional layer.
Retailers sometimes present the sum of the percentages because it sounds larger. The honest comparison is the final price, and that is the number to calculate before deciding.
Discount, markup and margin run in different directions
A 25% discount can be undone by a 33% markup, not a 25% one, because the two percentages are taken on different bases. Confusing them is how a sale price gets set below cost while looking reasonable on a spreadsheet.
If you know the cost and the target margin, price from the margin formula rather than adding a discount-reversal percentage. The arithmetic is short and the error is expensive.
Tax, coupons and the order of operations
Whether tax is applied before or after a discount changes the amount, and jurisdictions differ. A coupon that reduces the taxable base lowers the tax too; a coupon applied after tax does not.
Check the order on the receipt before disputing a total. The discrepancy is usually sequencing rather than an overcharge, and knowing which one it is tells you who to ask.
Returns complicate it further. Refunding a discounted item at its original price is a common dispute, and the outcome depends on the store's written policy rather than on the arithmetic. Keep the receipt that shows the price actually paid.
FAQ
How do I calculate 15% off quickly?
Take 10% (move the decimal one place left), halve it for 5%, and subtract both from the price. On $80: 10% is $8, 5% is $4, so $80 − $12 = $68.
Is a bigger percentage always better?
No — 50% off a $20 item saves $10, while 20% off a $200 item saves $40. Compare absolute savings and what you actually need.
How does buy-one-get-one compare to percent off?
BOGO on identical items equals 50% off per unit only if both items cost the same. On unequal prices it's usually worse.
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