Unit prices: reading the shelf tag to find the real deal

Every shelf tag in a grocery store answers two questions. The big number tells you what the package costs. The small number, the unit price, tells you what the product costs per ounce, per pound, per count or per load. If you only read one, read the small one. It's the quickest way to compare a big bag with a small one, or a store brand with a name brand, without doing math in the aisle.

What the unit price is

The unit price is the package price divided by the quantity in the package, expressed in a standard unit. The National Institute of Standards and Technology (NIST) publishes a Unit Pricing Guide for retailers and describes unit pricing as a tool for shoppers to judge price and value. NIST's guide recommends that the unit price appear close to the product, that it be easy to read, and that it appear on sale tags as well as regular tags.

That guide is a best practice, not a federal requirement. Whether a store must show unit prices depends on state and local law, and NIST notes that many jurisdictions don't require it. Most large grocers show them anyway, but formats vary.

Steps: reading the tag

1. Find the unit price on the shelf tag. It's usually smaller than the package price, often in a corner, with a label like "per oz" or "per lb."
2. Check the unit. Both items you're comparing need the same unit. One tag in ounces and another in pounds isn't a fair comparison until you convert.
3. Compare. The lower unit price is the lower cost per unit of product.
4. Factor in the coupon yourself. The tag doesn't know about your coupon. Subtract the coupon from the package price and divide by the quantity again.
5. Ask whether you'll use it all. A lower unit price on a bag you'll throw half of away isn't a saving.

Where the tag can mislead

• Mixed units on one shelf. Paper towels can be priced per roll, per sheet or per square foot depending on the tag. Rolls differ in size, so per roll is the least useful.
• Sale tags without a unit price. NIST recommends showing it on sale tags too, but not every store does. Do the division yourself.
• Multi-buy offers. "Two for" prices often show the unit price for one item at the multi-buy rate, or not at all. Read carefully.
• Different product forms. Concentrated and regular detergents may be priced per fluid ounce, but a "per load" figure, if shown, is the fairer comparison.

Pet food: a worked example without numbers

Pet food is a good place to practice because sizes vary so much.

Dry food usually comes in bags of several weights. Compare the per-pound or per-ounce unit price across bag sizes of the same formula. Larger bags are often lower per unit, but not always, especially when a smaller bag is on promotion. If your pet takes a long time to finish a large bag, check the bag's quality date and storage instructions before buying the biggest size.

Wet food comes in cans and pouches of different sizes. Tags are typically per ounce. A case of small cans can cost more per ounce than larger cans of the same food, or less; only the tag tells you.

Treats are where units get messy. Some tags show per ounce, others per count. A per-count price only compares fairly between treats of similar size. When in doubt, use weight.

When there's no tag at all

Online listings, warehouse clubs and some smaller stores may not show a unit price. The math is simple: price divided by quantity. A calculator app on your phone is all you need. Write your results in your price notebook so next time you already know your low for that product, which is the heart of our stockpiling play.

The habit

Glance at the unit price before you reach for anything with more than one size on the shelf. It takes a second and it's the most reliable price comparison a store gives you.