How to Factor Coupons, Cashback and Rewards Into Unit Price
Subtract every discount you will actually get from the shelf price first, then divide by the size. A $1 coupon on a $4.99 box makes it a $3.99 box — and that can change which product is cheaper per unit. Discounts that apply to everything in your cart can be left out, because they don’t change the winner.
Last updated: October 11, 2026
Worked example 1: a coupon flips the result
Brand A cereal is 18 oz for $4.99 and you have a $1.00 coupon for it. Brand B is 24 oz for $5.49 with no coupon.
| Brand A (18 oz) | Brand B (24 oz) | |
|---|---|---|
| Shelf price | $4.99 | $5.49 |
| Per oz, before coupon | $0.277 | $0.229 |
| Price you pay | $4.99 − $1.00 = $3.99 | $5.49 |
| Per oz, after coupon | $3.99 ÷ 18 ≈ $0.222 | $5.49 ÷ 24 ≈ $0.229 |
Before the coupon, B is clearly cheaper per ounce. After it, A is about 3% cheaper. The coupon didn’t just save a dollar — it changed the answer.
Worked example 2: cashback on one product
Paper towels: Pack A is 6 rolls × 120 sheets for $9.49. Pack B is 8 rolls × 110 sheets for $11.99, and a cashback app offers $1.50 back on Pack B.
- Pack A: 720 sheets, $9.49 ÷ 720 ≈ $0.0132 per sheet.
- Pack B before cashback: 880 sheets, $11.99 ÷ 880 ≈ $0.0136 per sheet — A is about 3% cheaper.
- Pack B after cashback: $11.99 − $1.50 = $10.49, so $10.49 ÷ 880 ≈ $0.0119 per sheet.
- With the cashback, B is about 10% cheaper per sheet.
Enter the price after the discount in the calculator below. For sheets and rolls, the paper towel calculator is ready to use.
Any currency works — just use the same one for both products.
Enter two products to compare
Which discounts to subtract
- Paper or digital coupons for that exact product and size.
- Store-card or loyalty prices shown on the shelf tag, if you have the card.
- Instant rebates or “price drops” at checkout.
- Cashback offers tied to one product (from an app or the store), if you will actually claim them.
- Reward points — but only at the value you will really redeem them for.
Which discounts to leave out
- A percentage back on your whole purchase (for example, 2% cashback on a credit card). It lowers both products by the same percentage, so the cheaper one stays cheaper.
- A “spend $50, get $10” offer on the whole basket — it doesn’t make one product cheaper than another.
- Coupons with conditions you won’t meet, such as a minimum quantity you don’t need.
- Rewards you know you won’t redeem before they expire.
Coupons that require more than one item
“Save $2 when you buy 2” is a discount on the pair. Work it out per item: two jars at $3.49 cost $6.98, minus $2.00 is $4.98, so $2.49 per jar. Enter one jar at $2.49 — or two jars as a multipack at $4.98. Only count it if you need both jars; buy-more deals are covered in BOGO vs. percent off.
Watch out: coupons often target the small size
A coupon on the smaller size can make it look like the better buy without actually beating the big one. A $1 coupon on a 12 oz jar at $3.99 gives $2.99, or $0.249 per oz. If the 24 oz jar is $5.49, that is $0.229 per oz — still cheaper without any coupon. Always compare after the coupon, against every size on the shelf.
Quick checklist
- Subtract only discounts tied to that product, that you will really get.
- Ignore discounts that apply equally to the whole basket.
- Divide the final price by the size (or total size for multipacks).
- Compare against every size and brand, not just the one with the coupon.
If you want to know whether a discounted price is actually good, compare it with your own baseline in a grocery price book.
Worked example 3: a store-card price on the smaller size
A 100-load bottle of detergent is $19.99, with $3.00 off for store-card holders. The 150-load bottle is $27.99 with no discount.
- Small bottle with card: $16.99 ÷ 100 ≈ $0.170 per load.
- Large bottle: $27.99 ÷ 150 ≈ $0.187 per load.
- With the card price, the smaller bottle is about 9% cheaper per load.
Without the card it would be $0.200 per load and lose. A shelf tag often shows only the card price, so make sure you are entering the price you will actually pay.
Frequently asked questions
Should I subtract the coupon before comparing unit prices?
Yes. Subtract the coupon from the shelf price, then divide by the size. That is the price per unit you actually pay.
Does credit card cashback change which product is cheaper?
No. A percentage back on everything lowers both prices by the same percentage, so the cheaper product stays cheaper.
How do I handle “$2 off when you buy 2”?
Subtract the discount from the price of both items and divide by two to get the price per item — but only if you need both.
Can I use a coupon on a sale item?
Often yes, but it depends on the store’s coupon policy. If you can, subtract both the sale and the coupon before working out the price per unit.