A sign says 40% off. Another says 25% off plus an extra 15%. A third adds tax at the register. Which is the best deal, and what will you actually pay? Discount math is simple once you know the two shortcuts — and one trap that stores are happy for you to miss.
Short answer: To get a sale price fast, multiply by (1 − the discount): 30% off means × 0.70. Stacked discounts multiply, not add — “30% then 20% off” is 44% off, not 50%. The Percentage Calculator does any of it instantly.
The fast way to any sale price
Most people compute the saving, then subtract. Skip a step: multiply by one minus the discount to land on the final price directly.
- 25% off $80: 80 × 0.75 = $60
- 40% off $65: 65 × 0.60 = $39
- 15% off $120: 120 × 0.85 = $102
The trick is just turning “X% off” into “you pay (100 − X)%.” It’s one multiplication instead of a subtraction, and it’s the whole skill.
The stacked-discount trap
Here’s the one that quietly costs shoppers. “Take 30% off, then an extra 20% at checkout” feels like 50% off. It isn’t — the second discount comes off the already-reduced price.
| Step | Price |
|---|---|
| Start | $100 |
| 30% off | $70 |
| Then 20% off $70 | $56 |
You paid $56 — a 44% discount, not 50%. The math is 0.70 × 0.80 = 0.56, and percentages multiply when they stack. This is the exact same reason a value that rises 50% then falls 50% doesn’t return to where it started — see percentage points vs. percent. Stores lean on the intuition that discounts add; they don’t.
To reverse it: to find the single discount equal to stacking two, multiply what you pay (0.70 × 0.80 = 0.56) and subtract from 1 → 0.44, or 44% off. The Percentage Calculator chains them without the mental gymnastics.
Where tax comes in
The last piece is order of operations. In US retail, the discount comes first, then sales tax is applied to the reduced price — so a coupon shaves a little off your tax, too.
- $50 item, 20% off → $40
- 8% tax on $40 → $3.20
- Total: $43.20
If tax were charged first (it isn’t, normally), you’d pay slightly more. Rules vary by state and item type — a manufacturer coupon can be an exception, since the store gets reimbursed and tax falls on the full price — but for an ordinary store discount, “discount, then tax” is the standard you can assume.
Is the bigger-percentage deal always better?
Not necessarily — it depends on the base price. 20% off a $50 shirt saves $10; 50% off a $15 shirt saves $7.50. The bigger percentage isn’t the bigger saving here. When you’re comparing deals across different prices, compute the actual dollars off, not just the headline percent — the Percentage Calculator makes that a two-second check. And for the flip side of everyday money math — tips and splitting a bill — see how to split the bill.
The pocket rules
- Sale price = original × (1 − discount). 30% off → × 0.70.
- Stacked deals multiply. 30% then 20% = 0.7 × 0.8 = 0.56 → 44% off.
- Discount before tax. Coupon first, tax on the remainder.
- Compare dollars, not just percentages. A big percent off a cheap item can save less than a small percent off a dear one.
Learn these four and you’ll price any sale before the assistant finishes ringing it up — and never fall for a “50%” that’s really 44%.