A shopkeeper friend of mine once advertised "20% off, plus an extra 10% off" during a sale and had a customer at the counter insist that meant 30% off the marked price. He wasn't trying to shortchange anyone — he genuinely believed it too, until I worked the actual numbers out on his calculator. The real discount was 28%, not 30%. Two percentage points doesn't sound like much, but on a ₹50,000 purchase that's a ₹1,000 difference, and it's the kind of thing that causes arguments at the billing counter.
Flat discount — the simple case
A flat discount subtracts a fixed rupee amount, regardless of the price. "₹500 off" means ₹500 off whether the item costs ₹2,000 or ₹20,000. Simple, predictable, easy to verify — there's no math trick to get wrong here.
Percentage discount — also simple, on its own
A percentage discount subtracts a percentage of the price. 20% off a ₹1,000 item is ₹200 off, bringing it to ₹800. Also straightforward — the confusion only starts when you stack more than one.
The successive discount trap
Here's the part that trips people up. When two percentage discounts are applied one after another — not added together — they don't simply sum. The second discount applies to the already-discounted price, not the original price.
Take ₹100 with a 20% discount followed by a 10% discount:
Step 1: ₹100 × (1 − 0.20) = ₹80
Step 2: ₹80 × (1 − 0.10) = ₹72
Final price: ₹72. That's a total of 28% off the original ₹100 — not 30%. The reason is that the second 10% is calculated on ₹80, not on the original ₹100, so it only removes ₹8, not ₹10.
The general formula for two successive discounts d1 and d2 (as decimals) is:
Effective discount = 1 − [(1 − d1) × (1 − d2)]
For 20% and 10%: 1 − (0.8 × 0.9) = 1 − 0.72 = 0.28, i.e. 28%. The more discounts you stack, the bigger this gap between "what it looks like" and "what it actually is" becomes.
A retail example
A clothing store runs "flat 30% off, extra 15% off on checkout" during a festival sale. A shirt marked at ₹2,000:
After 30%: ₹2,000 × 0.70 = ₹1,400
After extra 15%: ₹1,400 × 0.85 = ₹1,190
Final price: ₹1,190. That's a total discount of 40.5%, not 45%. Customers who expect 45% off and see ₹1,190 on the receipt (versus the ₹1,100 that 45% flat would give) often assume they've been shortchanged — when the math is actually correct, just not intuitive.
A B2B example — trade discount plus cash discount
This stacking also shows up constantly in B2B billing. A distributor offers a 12% trade discount on a ₹1,00,000 order, and an additional 2% cash discount for payment within 7 days:
After trade discount: ₹1,00,000 × 0.88 = ₹88,000
After cash discount: ₹88,000 × 0.98 = ₹86,240
Effective total discount: 13.76%, not the 14% you'd get by simply adding 12% + 2%. On high-value B2B orders, that 0.24 percentage point difference can be a real number — worth getting right on both sides of the invoice, especially since GST is typically calculated on the post-discount taxable value, so an error here can also throw off your tax calculation.
The quick rule to remember
Never add successive percentage discounts together. Always apply them one at a time, each to the result of the previous step. It's a small habit that avoids both customer disputes and pricing errors on your own invoices.
Work out exact successive-discount pricing — including flat, percentage, and multi-step discounts — with our Discount Calculator.