Why "30% + 20% Off" Isn't 50% — Discount Stacking Math

A "30% off + 20% extra coupon" sale is not 50% off — it's exactly 44% off. Stacked discounts break consumer intuition in ways marketers exploit. Here's the math, five common traps, and a 3-second mental shortcut.

1. The Stacking Formula

Final price = Original × (1 − x/100) × (1 − y/100) Combined discount = 1 − (1 − x/100) × (1 − y/100)

2. Five Traps Consumers Fall For

① "50% off + 10% extra" feels like 60%

Actually 55%. Marketing copy often exploits the intuition gap.

② "Buy 2 Get 1" ≠ 50% off

It's 33.3% off per unit — and only if you actually want three.

③ "VAT excluded" pricing

Add 10% for personal purchases; businesses can claim input credit.

④ "Up to 70%"

"Up to" usually applies to one or two slow-moving items; average is far lower.

⑤ Card discount + coupon + points

Often exclusive, not additive. Check the total before paying.

3. Stacked Discount Table

4. The 3-Second Mental Shortcut

Combined ≈ (x + y) − (x × y / 100).
E.g., 30% + 20% → 50 − 6 = 44%. Exact.

5. Calculators

• Discount Calculator
• VAT Calculator
• Dutch Pay Splitter