A 20% discount followed by 10% leaves $72 from an original $100, so the effective discount is 28%, not 30%. Stacked offers are sequential: after the first discount, the original price is no longer the base for the second. The intermediate price makes both stages auditable.
Multiply remaining fractions
On $100, a first 20% discount leaves $80. A second 10% discount uses $80 and removes $8, leaving $72. Equivalently, multiply 100 × 0.80 × 0.90 = 72. The two remaining fractions produce 0.72, so the effective discount is 28%, not 30%.
A fixed second coupon makes order visible
If $100 receives 20% off and then a $10 coupon, the stages are $80 and $70. Applying $10 first and then 20% would produce $72. DiscountCalc releases only the first-percentage-then-second-offer sequence, so it never silently chooses between two materially different merchant rules.
Compare offers by final price and effective rate
Effective discount always compares total savings with the original price: (original − final) ÷ original × 100. It is a useful mathematical summary, but not proof that one deal is eligible, cheaper after fees, or better for a basket with excluded items. Preserve the stage breakdown beside the summary percentage.