A 16-ounce jar of peanut butter is $3.49. The 28-ounce jar next to it is $5.99. Which is the better price?
The package prices can’t tell you, because they buy different amounts. The unit price can: 21.8 cents an ounce for the small jar, 21.4 for the big one. Almost the same, which is worth knowing before you carry the big one home.
The short answer is: compare unit prices when two products are priced in the same unit and you’ll use all of what you buy. Compare package prices when you’re choosing how much to spend today, not which is cheaper per ounce. The rest of this is about the cases where the unit price looks like a fair comparison and isn’t.
Where the unit price comes from
It’s the small figure on the shelf tag, usually in a corner: so much per ounce, per pound, per count. There is no federal rule requiring it. According to the National Institute of Standards and Technology’s guide to US retail pricing laws, unit pricing is mandatory in Connecticut, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Oregon, Rhode Island, Vermont and Washington, D.C. Elsewhere, stores that show it do so voluntarily, and the rules for how they show it vary.
When the unit prices don’t compare
Different units in the same aisle
NIST’s own example is the juice aisle, where one tag can be priced per gallon, the next per quart, another per pint and another per fluid ounce. Its guidance to stores is to use one unit across a category, but a guide isn’t a guarantee. Before comparing two tags, check the unit printed next to the number. A quart is 32 fluid ounces and a gallon is 128, so divide down to ounces when the tags disagree.
Per count against per weight
Paper towels, trash bags, dishwasher pods and diapers are often priced per count, and sometimes per square foot or per load. Per count only compares like with like: 40 thin bags against 40 heavy ones is the same unit and a different product. Where the tags use different units, there’s nothing to convert. You’re back to deciding what the thing is worth to you.
The tag that ignores the sale
A sale sign can sit beside a shelf tag whose unit price is still worked out from the regular price. NIST’s guidance says sale signs should carry a unit price of their own; not every store does it. When something is on sale, divide the sale price by the size yourself.
“2 for $5”
A multi-buy offer is a price per item in disguise: $2.50 each, against, say, a regular $2.79. Whether you have to buy two to get it depends on the store and is usually in the sign’s small print. Either way, the unit price on the regular tag describes the $2.79, not the $2.50.
The big pack you won’t finish
A unit price assumes every unit gets used. A family-size bag of salad greens at a lower price per ounce is more expensive per ounce eaten if a third of it goes in the trash. The tag can’t know that. You can.
When the package price is the one that matters
The package price is what you pay today, and sometimes that’s the real question: the budget for this trip, or how much you’re willing to risk on a brand you haven’t tried. It’s also the number that stays the same when a package gets smaller, which is exactly why it’s the wrong one to compare over time.
The Bureau of Labor Statistics handles this by converting every price to a standard size before comparing it. Its example: a $5.99 tub of ice cream that goes from 64 ounces to 60 is a 6.7% price increase, even though the price on the tub never changed. Here is the same thing with coffee, using illustrative numbers:
| Package price | Size | Per ounce | |
|---|---|---|---|
| Last year | $8.99 | 12 oz | 74.9¢ |
| This year | $8.99 | 10.5 oz | 85.6¢ |
Same package price, 14.3% more per ounce. Compared by package price, nothing happened.
The third price: what you paid per unit last time
The shelf tag compares the products in front of you, today, at this store. It can’t tell you whether today’s 21.4 cents an ounce is more than you paid for the same jar in the spring. That comparison needs your own past prices, on the same footing: per ounce, per pound, per count.
Receipts are where those live, with a catch. A US receipt usually prints the package price and, for anything weighed, the weight and the price per pound. The size of a packaged item is printed only when it happens to fit in the name, and often it doesn’t. So the per-ounce history of your coffee only exists if someone wrote the size down. That’s what a price book is for, and the same per-unit footing is what makes working out your own grocery inflation honest.
CartEncore keeps the price you paid on every line of every receipt. When a receipt prints a quantity and its numbers add up, it also keeps the price of one, next to the line total. When the receipt prints no size, it doesn’t guess one, and it won’t notice that a package has shrunk. The size on the shelf is still yours to check.
One product’s prices over time: what one product has cost you →
When a weighed line on the receipt doesn’t look right: how to check your receipt →