Discounts feel like the easiest maths in the world, take a percentage off and pay less, which is exactly why so many people get them subtly wrong. Stacked offers, tax interactions, and reversed calculations all hide small errors that add up across a basket or a shopping season. Knowing where the traps are turns you from someone who hopes the till is right into someone who knows.
This guide covers the most common discount mistakes: adding percentages that should be multiplied, confusing the order of discounts and tax, misreading which price a percentage applies to, and being fooled by discounts that sound better than they are. None is hard to avoid once you have seen it.
Stacking discounts by adding them
The classic error is treating two discounts as one big one. A 25% discount followed by a further 10% off is not 35% off. The second discount applies to the already-reduced price, not the original, so the true saving is smaller.
The correct way is to multiply the remaining fractions. After 25% off you pay 75%; after a further 10% you pay 90% of that, so 0.75 × 0.90 = 0.675, you pay 67.5% of the original, a 32.5% discount, not 35%. The gap looks small on one item but compounds across a full basket.
combined price = original × (1 − d₁) × (1 − d₂)Getting the order of tax and discount wrong
When a discount and a sales tax both apply, the order can change the final figure depending on local rules. Tax calculated on the discounted price is not the same as a discount applied to the taxed price, and assuming one when the other is used produces a small but real error.
For most everyday purchases the difference is minor, but on large amounts it matters. If you are checking a receipt or comparing offers, find out which is applied first, usually the discount, then tax on the reduced total, and calculate in that order rather than guessing.
Misreading which price the percentage applies to
Percentages always attach to a particular base, and losing track of which one causes trouble. A discount is a percentage of the original price; a reverse calculation to find the original from the sale price is a division, not the discount added back.
If a price after 20% off is $48, the original was $48 ÷ 0.8 = $60, not $48 plus 20%, which would give $57.60. The discount was taken from the larger original, so recovering it requires dividing. A reverse-percentage calculator handles this cleanly when the numbers are not round.
A worked example
A jacket is marked $120, reduced by 25%, and then a checkout coupon takes a further 10% off. The instinct is to call it 35% off, or $78. The real price is $120 × 0.75 × 0.90 = $81, a 32.5% discount, not 35%.
The $3 gap looks trivial here, but scale it across a full basket or a season of promotions and the difference between assumed and actual savings grows into real money. Multiplying the remaining fractions, rather than adding the percentages, keeps you accurate every time.
Discounts that only sound generous
Some offers are designed to feel bigger than they are. A percentage off a price that was quietly inflated first is not the bargain it appears, and multi-buy deals often bundle items you would not otherwise buy, so the headline saving overstates the real one.
The defence is to anchor on the price you actually pay, not the percentage advertised. Work out the final cost, compare it to what you would pay elsewhere or for only what you need, and judge the deal on that. A large percentage off a bad starting price is still a bad price.
Most discount mistakes come from adding percentages that should be multiplied, mishandling the order of tax and discount, or reversing a calculation by adding when you should divide. Anchor on the final price you actually pay, multiply the remaining fractions for stacked offers, and check which base each percentage applies to. Do that and no promotion can quietly overstate its savings.
Frequently asked questions
How do I combine two discounts correctly?
Multiply the fractions you still pay, do not add the discounts. After 25% off you pay 0.75; a further 10% off means 0.75 × 0.90 = 0.675, so you pay 67.5% of the original, a 32.5% discount, not 35%.
Does it matter whether discount or tax comes first?
It can. Tax on a discounted price differs from a discount on a taxed price. The difference is usually small but real; on larger purchases, check which order applies locally and calculate in that sequence.
How do I find the original price before a discount?
Divide the sale price by one minus the discount. A $48 price after 20% off came from $48 ÷ 0.8 = $60. Adding the percentage back gives the wrong answer because the discount was taken from the larger original.