Discount Calculator Guide: Sale Price, Stacking Coupons, and Why '50% Off Twice' Isn't Free
A discount calculator does the simple math behind every sale tag — but the corner cases (stacking coupons, percentage off after a flat dollar coupon, BOGO offers) catch a lot of shoppers off guard. This guide explains every common discount type and the small mistakes that quietly cost real money.
Most sales advertise a single percentage: 25% off, 40% off, etc. The math is straightforward — multiply the original price by (1 minus the percentage as a decimal) to get the sale price. $80 × 0.60 = $48 for a 40%-off shirt. The calculator does this in milliseconds.
What gets confusing is everything that piles on top: 'extra 15% off already-reduced items,' 'use code SAVE20 for 20% off your $50+ order,' '$10 off when you spend $40,' BOGO ('buy one get one free or 50% off'), and the sales-tax-after-discount question. Each is a different formula, and stores design these to be hard to compare.
The two main discount types
A percentage discount is multiplicative — it scales with the price. 20% off saves more on a $200 jacket ($40) than on a $20 t-shirt ($4). A flat-dollar discount (like '$10 off') is additive — it saves the same dollar amount regardless of what you buy. Most stores combine these to maximize psychological appeal: a $10-off coupon feels generous, while a 20% banner makes the whole store look on sale.
When comparing two offers, convert them both to the actual dollar savings on the item you're buying. '$15 off a $60 item' (25%) is a worse deal than '30% off' on the same item ($18 saved). The percentage that sounds smaller can win if the dollar threshold is high.
Common discount stacking rules
When multiple discounts apply at once, the order they're applied changes the final price. Stores choose the order that's worse for you unless you check.
| Scenario | What 'better' order looks like | Example on $100 |
|---|---|---|
| 20% off + $10 coupon | Apply $10 first, then 20% | ($100 - $10) × 0.80 = $72 vs $100 × 0.80 - $10 = $70 — flat-first costs you $2 more here. |
| 20% off + 10% off (stacking) | Multiply both: 0.80 × 0.90 = 0.72 | $72. Note: this is NOT 30% off — that would be $70. |
| 50% off + 50% off | Multiply: 0.50 × 0.50 = 0.25 | $25. Two halves don't make zero — you pay 25% of the original. |
| Tax after discount | Tax applies to the discounted price (most common) | $72 × 1.08 = $77.76 (sales tax 8%) |
Discount types you'll encounter
- Percentage off
- Reduces price by a fixed proportion. 30% off = pay 70% of original.
- Flat dollar off
- Subtracts a fixed amount. '$15 off your purchase'. Better the smaller the original price.
- Tiered discount
- More you spend, more you save: '10% off $50+, 15% off $100+, 20% off $150+'. Designed to push you to the next tier.
- BOGO (Buy One Get One)
- Free or discounted second item. BOGO Free = effectively 50% off if you buy two; BOGO 50% Off = effectively 25% off if you buy two.
- Bundle discount
- Buy multiple specific items together for a flat price. Compare to buying separately to confirm you're saving.
- Threshold-free shipping
- Spend $X+ to skip the shipping fee. Worth it only if you'd buy that much anyway.
- Member-only / first-order
- Discounts for new customers or loyalty members. Often the deepest discount available.
Computing your final price step by step
- 1
Start with the original price
The price before any discount. If multiple items, sum them first.
- 2
Apply percentage discounts
Multiply by (1 - percentage/100). 20% off → multiply by 0.80.
- 3
Apply flat-dollar coupons
Subtract the dollar amount. Most coupons apply after percentage off; check the fine print.
- 4
Apply tax
Tax usually applies to the discounted subtotal, not the original price. Multiply by (1 + tax rate/100).
- 5
Add shipping if any
Shipping fees are usually after tax; tax-on-shipping varies by jurisdiction.
Buyer beware — common pricing tricks
- •'Original price' that's been inflated for the sale. Compare against the same item at competitors before buying.
- •Sale countdowns and 'X people viewing' counters. Most are marketing, not real urgency.
- •Rounding the discount up: '$1.99' looks like '$1' but rounds to '$2' in your head. Always read the actual digits.
- •Bundle 'savings' that include items you didn't want. The $99 bundle with 4 items might be cheaper than the 4 items separately, but if you only wanted 2 of them, the comparison is different.
- •Loyalty cards that require a download or signup. Calculate whether the discount justifies the data you're handing over.
Extended FAQ
Why is '50% off twice' not 100% off?
Each 50% applies to the price at the moment, not the original. After the first 50% you have half left; the second 50% takes half of that, leaving 25% to pay. To get to zero you'd need a 100% off coupon.
Do I add discounts (20% + 10% = 30%)?
Almost never. Stacking discounts are multiplicative: 0.80 × 0.90 = 0.72, which is 28% off, not 30%. Always multiply the remaining percentages.
Should percentage or flat-dollar coupon be applied first?
If you have a choice, applying flat-dollar first saves you more (the percentage discount applies to a smaller subtotal). Most retailers apply percentage first by default — read the cart total to confirm.
Is sales tax applied to the discount?
Tax is usually charged on the discounted price (not the original), which is good for you. But some 'manufacturer coupons' don't reduce the taxable amount — the store recoups the tax. Check the receipt.
Are the prices I enter saved or shared?
No. The calculator runs entirely in your browser. Nothing is uploaded.