Unit price: the only number that really compares two products

Two packs side by side. One is bigger and more expensive, the other smaller and cheaper. Which one is the better deal? The question sounds simple, yet almost nobody does the maths — because it means doing division in your head in the middle of an aisle while pushing a trolley.

The number that answers it exists, and in many countries stores are required to show it, but it's printed tiny and all but disappears on online grocery sites. Here's how to read it, how to calculate it, and where it makes the biggest difference.

Why the shelf price compares nothing

A shelf price means nothing on its own: it depends on the quantity. Comparing €3.45 and €4.20 only makes sense if both products contain the same amount, which almost never happens. Manufacturers know this, and the variety of pack sizes is no accident.

There's even a name for when the quantity drops but the price doesn't: shrinkflation. The pack looks the same size, goes from 500 to 450 grams, and the price tag stays identical. Only the unit price reveals what happened.

Working out a unit price in three seconds

The formula is a single division: price ÷ quantity in kilos (or litres).

Two shortcuts cover 90% of cases:

• Products sold in grams: divide the price by the quantity and multiply by 1,000. For 500 g it's even easier: double the price.
• Products sold in centilitres: for 50 cl, double the price. For 33 cl, multiply by three.

In this example, the biggest pack isn't the cheapest. That's common — and it's exactly what the packaging is designed to hide.

What the rules require on the shelf

In the European Union and the United Kingdom, stores must display the price per unit of measure for products sold by weight or volume. It's the small figure at the bottom of the shelf label, often in a tiny font, sometimes in light grey on white. In the United States, the rules vary by state, and many retailers show it voluntarily.

Two practical limits: it isn't always expressed in the same unit from one product to the next in the same aisle — one per kilo, another per 100 grams — and it isn't always updated at the same time as promotions. Check the unit before comparing two labels.

The five aisles with the biggest gaps

Some aisles concentrate the gaps, because pack formats vary the most there and buying habits are the most automatic.

1. Coffee — pods, beans, ground: price-per-kilo gaps between formats often exceed three to one.
2. Laundry detergent and cleaning products — pods versus bottles, with dosages that make comparison even harder.
3. Cereal and biscuits — “trial size” packs are almost always the most expensive per kilo.
4. Baby products — nappies and formula, where frequent purchases turn small gaps into a large yearly difference.
5. Drinks — multipacks, large bottles and single units, with gaps that shift with promotions.

Bigger isn't always cheaper

The assumption runs deep: the bigger the pack, the lower the price per kilo. That's often true, not always, and the exceptions are rarely accidental.

The large pack is sometimes more expensive per kilo when it's positioned as a “family” or “convenience” product, when it includes an accessory, or when the mid-size pack is on promotion. Automatically grabbing the biggest pack costs you money several times a year.

The problem with online grocery shopping

On grocery websites, the unit price is often there but relegated to small print under the main price, sometimes hidden behind a hover, and it frequently disappears from search results — which is exactly where you compare.

The result: online shopping, supposedly easier for comparing, is in practice harder than the store aisle. That's the problem Nexily UnitWise solves, by displaying the price per kilo directly on each product on major grocery sites, including in product lists.

When unit price isn't enough

This number compares quantities, not quality. It says nothing about ingredients, origin, sugar content or real yield in use.

Three cases where it misleads:

• Concentrates — a concentrate looks expensive per kilo but makes three times as much.
• Laundry detergent — compare the price per wash, not per litre, since doses can vary by a factor of two.
• Products with a lot of added water — ham, fish, ready meals, where the drained net weight is the real figure.

Unit price is still the best starting point: it eliminates the most obvious fake bargains in seconds. For promotions and struck-through prices, a different mechanism is at play, covered in our article on checking prices before you buy.