Two 20% discount codes feel like 40% off. They aren't — they compound. And most stores don't let you stack at all. Here's the math and the fine print.
The compounding math
$100 item. 20% off → $80. Another 20% off → $64. Total saved: $36, or 36% — not 40%. Add a third 20% code: $64 → $51.20, total 48.8% — not 60%. The gap grows with each stack.
Order matters when units differ
Percentages compound the same way regardless of order. But combining a percentage with a dollar amount flips the answer:
- $10 off, then 20% off: $100 → $90 → $72.
- 20% off, then $10 off: $100 → $80 → $70.
Difference: $2. In most stores, percentage-off applies first, then dollar amounts. Check terms.
Most stores don't allow stacking
"One discount per order" is the default in checkout systems. When stacking works, it's usually: one percentage off sale + a free-shipping code + a dollar off coupon. Percentage + percentage is rare.
Gift cards and cash back
Neither stacks with discounts because they're not discounts. Gift cards are payment tender — they apply to the final total. Cash back is a post-purchase rebate — it cuts your effective price but doesn't compound with the sale.
Single-discount math. For stacks, run the tool multiple times in sequence.