Compare unit price, not the % off
To compare unit price, pick a unit both packages share — ounces, pounds, or count — then divide the shelf price by it. A loud 40% off on a smaller bottle can still cost more per ounce than the boring store brand next to it.
Unit price is shelf price ÷ a unit you can actually use
Unit price is the cost per ounce, pound, count, or whatever measure both items actually share. That's it. If you came here for how to calculate unit price grocery, that's the whole method: pick the unit, then divide.
Shelf price ÷ ounces (or pounds, or count) = the number that matters.
Don't divide by "family size." Don't divide by the % off. Those aren't units you can pour.
Rutgers NJAES fact sheet FS138, Unit Pricing: What It is and How to Use It, puts it simply: unit price is the cost per ounce, pound, count, or other common measure so you can compare different sizes. Bigger packages are not always cheaper — sometimes two smaller cereal boxes win. The sheet also asks whether you'll use it before it spoils, and whether you have storage. That's right. We're going to beat the fact sheet with one labeled aisle example, because a 40% off tag on dish soap is where shoppers actually lose.
The tiny type on grocery shelf tags is often already doing this divide. Trust it only after you check that both tags use the same unit. If one is per ounce and the other is per pound, those two numbers are not a contest yet.
Worked example: 40% off 12-oz vs store-brand 20-oz
Labeled example, not a current ad. Two bottles of dish soap, same shelf.
Labeled example — not a current ad
Brand 12-oz dish soap
Regular $4.99. "40% off" → $2.99.
$2.99 ÷ 12 oz ≈ 25¢ per ounce.
Store-brand 20-oz dish soap
Shelf price $3.49.
$3.49 ÷ 20 oz ≈ 17¢ per ounce.
Store brand wins by about 8¢ an ounce. The 40% off is louder. The unit price is worse.
Run it on your phone: 2.99 ÷ 12, then 3.49 ÷ 20. One column. That's unit price vs sale price. The $2.99 is a real sale. The per-ounce number still loses.
Compare unit prices different sizes the same way online: convert first if the packs don't match, then divide. Price per ounce is the tie-breaker, not the badge.
If you're stacking a promo on that 40% off, divide the final register price, not the first tag. A coupon that only hits the brand bottle can still leave you paying more per ounce.
When the shelf tag lies (mixed oz / lb / count)
The printed unit price is only useful if both tags use the same unit. One bottle per ounce, one jug per pound, one pack per count — those three numbers are not comparable until you convert.
16 ounces = 1 pound. If a tag says $1.92 per lb and the one next to it says 12¢ per oz, convert one of them. $1.92 ÷ 16 = 12¢ per ounce. Tie. Pods-per-count vs fluid ounces are not the same product. Don't race those tags.
When the units don't match, ignore the printed unit prices and divide yourself on ounces both packages share. If you can't name the unit, you can't compare the bottles.
A strikethrough was $X on the same tag is a separate claim. If it was never really $X, the discount is theater. Unit price still wins even when the was-price is honest: a real markdown on a tiny size can still lose to a bigger bottle at regular price.
Bulk only wins if you'll use it
A cheaper price per ounce on a gallon you waste is not a deal. Rutgers asks the storage and spoilage questions on carrots. Same test on soap, cereal, and the warehouse-club jug.
Will you use it before it spoils? Do you have a place to put it? If not, the 17¢ per ounce is a fiction — you pay it on what you dump, too.
Club packs win when they're on the list you already had. They lose when the unit price looks better only because you bought six months of something that expires in six weeks. Two smaller boxes beating one giant box is the same divide, plus will-you-finish-it.
Don't add junk to unlock shipping, and don't add a second gallon to "win" bulk. That's the same impulse in a bigger bottle.
Put it on this week's 3-must-buy list
A long list is how impulse wins. Unit price is check 1. The habit that makes it stick: 3 must-buys you were getting anyway, 2 maybes if the per-ounce number is actually better, ignore the rest.
If the 20-oz store brand wins the divide, it goes on the three. If the 12-oz 40% off loses, it does not get a maybe because the sign is red. That's how to spot a fake 50% off in the soap aisle too — same six checks, same refusal to let the headline % pick the cart.
Save this: the six checks live on a one-pager you can screenshot. Print it. Keep it on your phone. Use it on one cart this week.