Unit prices: the shelf label that does the maths for you, and when it quietly doesn't

Every supermarket shopper eventually stands in front of two jars of peanut butter, one bigger and one on offer, trying to work out which is cheaper. The answer is usually printed on the shelf already, in smaller type than the price: the unit price. It’s one of the most useful numbers in the shop and one of the least read.

What the unit price tells you

The unit price is the cost per standard amount, such as per ounce, per pound, per 100 grams or per item, so you can compare packs of different sizes directly. A 16-ounce jar at one price and a 28-ounce jar at another stop being a mental arithmetic problem: the lower unit price is the cheaper peanut butter, ounce for ounce.

The National Institute of Standards and Technology, which supports the weights-and-measures officials who inspect shops in the United States, publishes a best-practice guide for retailers on unit pricing. Its central recommendations are simple: show the unit price clearly on the shelf label, and use a consistent unit of measure for products shoppers compare. Unit pricing rules aren’t the same everywhere. Some states require it and many shops do it voluntarily, so how well it’s done varies from one chain to the next.

When it quietly fails

The weak spot is consistency. When one brand of kitchen roll is priced per roll and another per 100 sheets, or one cereal per ounce and another per pound, the unit prices stop being comparable at a glance. That’s exactly the problem the NIST guide asks retailers to avoid. When you see it, you have to convert one of them yourself, or fall back to the price and the pack size.

A few other things to watch:

• Offer tags. A promotional price is sometimes shown with the old unit price beside it, or with no unit price at all. Work it out from the offer price and the pack size.
• Multi-buys. “Three for” deals usually need you to buy all three for the unit price to apply.
• Different products under one tag. Check that the label on the shelf edge belongs to the pack above it.
• Count versus weight. Kitchen roll, nappies, dishwasher tablets and coffee pods are best compared per sheet, per item or per wash, not per pack.

The big pack isn’t always the cheap pack

Bulk usually wins per unit, which is the whole point of buying it. The big pack is usually cheaper per ounce, which is exactly why it’s worth the two seconds it takes to check the times it isn’t. A smaller pack on offer can beat a large one at full price, and a mid-size pack is sometimes the best value on the shelf.

And a lower unit price only saves money if you use all of it. A bargain-sized bag of salad that half goes in the bin costs more per portion actually eaten than the small one. Our rule: buy big on things that keep, such as rice, pasta, tins, cleaning products and frozen food, and buy the size you’ll finish for anything fresh.

Shrinking packs

Unit prices also catch something the headline price hides: packs that get smaller while the price stays the same. If a product you buy every week costs the same but the unit price has gone up, the pack has shrunk. The shelf label is the easiest place to notice it.

What we’d actually do

Read the unit price before the price. Check that the two products you’re comparing use the same unit, and convert if they don’t. Work out the unit price yourself on offers and multi-buys. Buy in bulk only what you’ll use before it spoils. It takes a few seconds per item, and on a weekly shop the savings add up over a year.