How do I calculate a discount?

Multiply the original price by the discount percentage, then subtract that amount from the original price. Equivalent shortcut: final price = original price × (1 − discount/100).

Is 20% + 30% off the same as 50% off?

No. Stacked discounts multiply: 20% off followed by 30% off leaves you paying 0.8 × 0.7 = 0.56 of the original price, i.e. an effective 44% discount, not 50%.

How do I calculate the original price from a sale price?

Divide the sale price by (1 − discount/100). For example, if $64 is the price after 20% off, the original price was $64 ÷ 0.8 = $80.

How is 'extra 20% off already-reduced items' calculated?

The extra percentage applies to the already-reduced price, not the original tag. A $100 item at 30% off becomes $70, then an extra 20% applies to $70, giving a final price of $56.

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How to Calculate a Discount — Percent Off, Stacked Discounts & Real Savings

Shopping · 5 min read · Last updated September 2026

The basic discount formula

Final price = Original price × (1 − Discount% ÷ 100)
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:

Effective total = 1 − (1 − d₁) × (1 − d₂)

$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:

Total % off = 100 − (100 − a) × (100 − b) ÷ 100

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.

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.