20% Off Then Another 10%: Why the Total Discount Is 28%

Imagine an item marked 100.00 with a 20% sale and an additional 10% coupon. The first step saves 20.00. The second saves 10% of 80.00, which is 8.00. Total saving is 20.00 + 8.00 = 28.00. A single 30% reduction would instead save 30.00 and leave 70.00.

The phrase ‘additional 10%’ does not necessarily tell you the merchant's actual rules. This worked example assumes the coupon applies to the entire sale subtotal and can be combined with the sale. Check exclusions, minimum spend and coupon eligibility before comparing the mathematics with a real checkout.

A 20% discount means paying 80%, so multiply by 0.80. A further 10% means paying 90% of what remains, so multiply again by 0.90. The combined multiplier is 0.72. Subtract it from 1 to get the effective discount: 1 − 0.72 = 0.28, or 28%.

For two percentage discounts a and b, written as decimals, final price = original price × (1 − a) × (1 − b). This makes clear why two discounts of 50% leave a quarter of the original price rather than making an item free. Each step acts on what remains.

Enter original price 100 and discount 20 in Discount Calculator. Record the final price, 80. Then enter 80 as the new original price and 10 as the discount. The second result is 72. The tool runs one discount at a time; it does not infer coupon stacking from a store's offer text.

To check the effective rate, subtract the final price from the original and divide by the original: (100 − 72) ÷ 100 × 100 = 28%. For an original price of zero, report a zero monetary saving without dividing by zero to invent an effective percentage.

Two percentage multipliers commute in exact arithmetic: 0.80 × 0.90 equals 0.90 × 0.80. A fixed-amount coupon is subtraction, so it behaves differently. On 100.00, a 20% discount followed by 10.00 off leaves 70.00. Taking 10.00 off first and then 20% leaves 72.00.

That two-unit difference is not a calculator error. It comes from a different base for the percentage. Ask whether the fixed coupon reduces the subtotal before the percentage offer, whether both may be used, and whether any minimum-spend check uses the original or discounted amount. The table does not choose a merchant policy.

Discount Calculator rounds monetary discount amounts to two decimal places at each run. A checkout may round per item, per coupon or on the final basket. Percentage order is interchangeable in exact arithmetic, but cent rounding at intermediate steps can change that conclusion. Keep each intermediate value when investigating a small discrepancy.

Shipping, tax, fees and excluded items require separate lines. A correct discount subtotal does not establish a correct tax total. The companion tax-order guide shows a distinct issue: whether a fixed reduction changes the tax base. Do not mix that assumption into the stacked-percentage formula without saying so.

Compare final prices for the same eligible items, currency and quantity. A larger advertised percentage is not necessarily a better deal if it applies to a smaller subset of the basket. Likewise, a fixed coupon may win for a small subtotal and lose for a larger one.

Write a short comparison record: eligible subtotal, first reduction, second reduction, rounded subtotal, and separately stated extras. If a receipt differs, locate the first line where the two calculations diverge rather than applying more discounts until the totals happen to match.