What this calculator does
Find out which of two or three offers really costs you the least. Enter what you're buying, then describe each deal. The calculator adds up each offer's final cost, with shipping, tax and rebates, and tells you which one wins and by how much.
Deals come in many shapes: 20% off, $15 off a $75 order, buy one get one half off, free shipping over $50, or money back by rebate. They're hard to compare in your head. Putting the same purchase through each one turns them into plain dollar totals you can line up.
Where it's used
Why this math matters outside the classroom:
- Online shopping. A shopper compares 20% off a $100 cart plus $7.99 shipping, which is $87.99, with $15 off and free shipping, which is $85.
- Big purchases. Someone buying a $600 appliance compares 10% off at checkout, $540, with a $75 rebate, $525 after the money comes back.
- Stocking up. A family buying 6 items at $10 compares buy one, get one free, $30, with 40% off everything, $36.
- Math class. Students find where 20% off and $15 off cost the same by solving 0.2 × x = 15, which gives x = $75.
Grade level and standards: Grade 7; Common Core State Standards for Mathematics 7.RP.A.3. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Enter the price of one item, how many you're buying and the usual shipping cost.
- Add your sales tax rate if you want it included.
- For each offer, pick the kind of deal and fill in its numbers. Add a different item price if one store charges more.
- Read the lowest total, how much it saves and the table that shows each offer's math.
What the inputs mean
- Price of one item
- The regular price of one item. Each offer uses this unless you give it its own price.
- How many you're buying
- How many of the item you're buying. Buy one, get one deals need at least 2 to save anything.
- Shipping cost, when you pay it
- What you'd pay for shipping when an offer doesn't ship free. Use 0 for store pickup.
- Sales tax rate (optional)
- Optional. One rate for all offers, charged on the items after any instant discount. Shipping isn't taxed here.
- Offer A: kind of deal
- Percent off takes a share off the whole order. Dollars off takes a flat amount. Buy one, get one discounts every second item. A rebate is money you get back after you pay.
- Offer A: item price, if different (optional)
- Leave blank to use the item price above.
- Offer A: percent off
- For buy one, get one, this is the discount on every second item (100% means free).
- Offer A: dollar amount
- Dollars off at checkout, or the rebate you get back later.
- Offer A: minimum order for the dollars off (optional)
- Many dollars-off deals only work above a set order size, such as $15 off orders of $75 or more.
- Offer A: free shipping on orders of at least (optional)
- The order size where shipping becomes free. Enter 0 if it's always free. Leave it blank if you always pay shipping.
- Offer B: item price, if different (optional)
- Leave blank to use the item price above.
- Offer C: item price, if different (optional)
- Leave blank to use the item price above.
Formula and method
subtotal = item price × number of items percent off: discount = subtotal × percent ÷ 100 dollars off: discount = the amount, if the subtotal reaches the minimum buy one, get one: discount = (number of items ÷ 2, rounded down) × item price × percent ÷ 100 final cost = subtotal − discount + shipping + tax − rebate
Each offer starts from the same purchase. The calculator takes off any discount you get at checkout, then adds shipping unless the order is big enough to ship free. The free-shipping check uses the price after the discount, which is how most stores do it.
Sales tax, if you add it, is charged on the items after the discount. A rebate isn't taken off at checkout. You pay the full amount first and get the money back later, so it comes off at the very end.
The offer with the lowest final cost is the best deal. The calculator also shows how much it saves compared with the next best one.
Worked example
You're buying 4 items at $25 each, and shipping is $7.99. Offer A is 20% off. Offer B is $15 off orders of $75 or more, with free shipping over $50. Offer C is buy one, get one 50% off.
- Every offer starts at $25 × 4 = $100.
- Offer A: $100 − $20 = $80, plus $7.99 shipping = $87.99.
- Offer B: $100 − $15 = $85. That's over $50, so it ships free: $85.00.
- Offer C: 4 items make 2 pairs, so 2 items get 50% off: 2 × $25 × 50% = $25. Then $100 − $25 = $75, plus $7.99 shipping = $82.99.
- Offer C wins: $85.00 − $82.99 = $2.01 less than Offer B.
Result: Offer C, buy one get one 50% off, is cheapest at $82.99. It saves $2.01 over Offer B and $5.00 over Offer A.
Understanding the result
Shipping often decides the winner. In the example, Offer B has the smallest discount, but free shipping lifts it past Offer A. Always compare the final cost, not the discount alone.
Count a rebate only if you're sure you'll claim it. Rebates often need forms, receipts and a deadline. If there's a chance you'll forget, compare using the price you pay at checkout instead.
A deal that pushes you to buy more isn't a saving if you didn't need the extra items. Set the number of items to what you'd really buy, then compare.
Assumptions and limitations
- All offers are for the same number of the same item.
- Buy one, get one discounts every second item, and all items cost the same within an offer.
- Free shipping is checked against the order after its instant discount.
- Tax is one rate, charged on the items after instant discounts, with shipping not taxed.
- Real offers can have extra rules, such as caps, excluded items or a membership. Check the terms.
- Whether shipping and rebated amounts are taxed depends on where you live, so the tax line is an estimate.
- It compares single offers. To stack two discounts on one price, use the discount calculator first, then enter the combined percent here.
- Prices change. The comparison is only as good as the prices you enter.
Frequently asked questions
Is a percent off or a dollars off deal better?
It depends on how much you spend. 20% off beats $15 off once the order is over $75, because 20% of $75 is $15. Below that, the dollars off saves more. To find the break-even order, divide the dollars off by the percent as a decimal: $15 ÷ 0.20 = $75.
Should I add items to get free shipping?
Only if you'll use them. If shipping is $7.99 and you're $5 short, adding a $5 item you need is a good trade. Adding a $20 item you don't need costs you more than the shipping would have.
How are buy one, get one deals counted?
Every second item gets the discount. With 4 items, 2 are discounted. With 3 items, only 1 is, because the third has no partner. For a closer look at one multi-buy deal, use the BOGO calculator.
Why does the table show two prices for a rebate?
"Paid at checkout" is what leaves your account on the day. "Final cost" is what you've really spent once the rebate comes back. Both matter if money is tight this month.
How this calculator is checked
Method: Percent and fixed-amount discounts, every-second-item multi-buy discount, threshold-based free shipping and post-purchase rebate, each applied to the same purchase and compared by final cost. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-25. Found a problem? Tell us.
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