The big box isn't always the better buy. Here's how to read the price-per-ounce tag, do the math when it's missing, and catch a shrunken package.
Two jars of peanut butter, two sizes, two prices. Which one is cheaper? The sticker price can't tell you, because you're not buying the same amount. The number that answers it is the unit price: what you pay per ounce, per sheet, per count or per load. Once you start reading it, you'll notice how often the “family size” or the item on sale isn't the cheaper choice.
The National Institute of Standards and Technology (NIST), which publishes model unit pricing rules that several states have adopted, defines it simply: unit pricing is “the cost per unit of measure of goods, such as the cost per milliliter or per ounce.” (NIST)
In stores that show it, the unit price is the small number on the shelf tag, usually in a corner, next to a unit like “per oz” or “per 100 ct”. The big number is the price of the package. The small number is the one that lets you compare.
Not every store shows unit prices. The U.S. Government Accountability Office found that as of November 2024, only 16 states, the District of Columbia and two territories had unit pricing laws or regulations. (GAO) So sometimes you'll need your phone's calculator:
The numbers above are made-up examples to show the math, not real store prices.
Buying in bulk usually lowers the unit price, but not always. Smaller sizes sometimes go on sale while the large size doesn't, and some large packs are priced higher per unit than the medium size next to them. The only way to know is to compare the unit prices, every time. And a lower unit price only saves money if you finish the product before it goes stale or expires.
A sale sign often shows just the deal price in big type. NIST's guide says unit prices should appear on sale signs, including club-card promotions, as well as on the regular shelf label, and that a multi-buy offer like “buy 2 for $0.75” should get its own unit price. (NIST) In practice, some sale signs leave it off. When that happens, divide the sale price by the amount yourself and compare it with the unit price of the other sizes on the shelf — the sale size isn't automatically the cheapest one. Our guide on whether a sale is actually a good deal covers the other tricks.
“Shrinkflation” — a package getting smaller while the price stays the same — is exactly what a unit price exposes. The GAO reported that from 2021 to 2024, product downsizing was three times more common than upsizing. Across all products its effect on inflation was small, but in some categories it was bigger: downsizing added 3.0 percentage points to price increases for household paper products and 1.6 points for cereal between 2019 and 2024. (GAO)
Unit pricing helps. The GAO cites a study of canned tuna that found shoppers in states with unit pricing rules cut back on downsized products by 30 to 84 percent more than shoppers in states without them. (GAO) If a product you buy every week suddenly has a higher unit price at the same shelf price, check the net weight or count on the package.
Roll counts are the hardest to compare, because “1 mega roll = 4 regular rolls” claims differ by brand. Ignore the roll count and compare the total number of sheets or the total square feet printed on the package — NIST lists area, such as square feet, as a unit for products like these. (NIST) Sheet sizes vary too, so square feet is the fairer comparison when it's printed.
Online listings make unit prices harder to see: a “pack of 6” and a “pack of 24” of the same item can sit side by side at prices that look unrelated. Divide each by its count before deciding. The same rule applies: compare the same product, the same size per unit, and the same condition.
Look at the small number, not the big one. If the unit prices use different units, convert them. If there's no unit price, divide. And buy the bigger size only when its unit price is lower and you'll actually use it.
Price AI identifies a product from a photo or barcode, pulls current prices from major retailers, and gives you a Deal Score from 1–100. When listings come in different pack sizes, it compares them per unit and shows the price each, so a 3-pack and a 24-pack aren't judged by the total. When it can't identify something, it says so instead of guessing.
Divide the price by the amount in the package, using the same unit for every product you compare. For example, $3.49 for 16 ounces is about $0.22 per ounce.
Usually, but not always. Sales on smaller sizes and pricing differences between sizes can make the smaller package cheaper per unit, so compare the unit prices each time.
Unit pricing isn't required everywhere. The GAO found that as of November 2024, 16 states, the District of Columbia and two territories had unit pricing laws or regulations.