Why "20% off + an extra 10% off" is not 30% off#
Stacked discounts apply one after another to whatever price is left, not to the original price each time — the second discount is a percentage of an already-reduced number, so it removes less in absolute terms than the same percentage would off the original. $100 at 20% off becomes $80; a further 10% off $80 removes $8, not $10, landing at $72 — a 28% total discount, not the 30% that simply adding the two percentages would suggest. This gap gets larger the more discounts are stacked, which is exactly why this tool computes them in sequence instead of summing the percentages.
The order discounts are entered in does not change the final price#
Mathematically, multiplying by (1 − 0.20) and then by (1 − 0.10) gives the identical result regardless of which order those multiplications happen in — so whether a 20% discount or a 10% discount is applied "first" does not change the final price, only the intermediate number shown along the way. What does matter is applying each one as a percentage of the current running price, not the original — that is the actual source of the common miscalculation.
Why the effective discount percentage is worth checking#
The final price is the number that matters at checkout, but the effective total discount (28% in the example above, not the naively-summed 30%) is useful for comparing deals — "30% off" advertised as two stacked discounts is a smaller discount than one flat 30% off the original price, even though both use the number 30 somewhere in the marketing.