How to use unit prices to actually find the cheapest groceries

The tiny number that saves you the most money

Most people walk straight past unit prices on their groceries every single shop. They're the small numbers on supermarket shelf labels: the cost per kilogram, per 100 grams, or per litre. Australian supermarkets are required to display them under the Unit Pricing Code, and they're the fastest way to compare products that come in different sizes.

Here's a common example. You're choosing between a 400g tin of tomatoes for $1.20 and an 800g tin for $2.80. The bigger tin looks like better value because, well, it's bigger. But check the unit price: the small tin is $3.00/kg and the big one is $3.50/kg. You're paying more per kilo for the larger size.

This happens more often than you'd expect. And once you start checking, you'll notice it everywhere.

Where to find unit prices (and why they're sometimes confusing)

In Woolworths and Coles, unit prices sit on the shelf label in smaller print below the main price. ALDI prints them too, though the layout varies by store. The catch is that not every product uses the same unit. Olive oil might show $/100mL in one store and $/L in another. Cheese might be $/kg in one aisle and $/100g around the corner.

This inconsistency is one of the reasons people ignore unit prices entirely. It takes an extra second to convert, and when you're wrangling 30 items with a toddler in the trolley, that second feels like a lot.

The good news: you don't need to check every item. Focus on the products where you're comparing sizes or brands, and skip the ones where you already know what you want.

Three traps that catch most shoppers

Once you understand unit prices, these traps become obvious. Until then, they quietly cost you money every week.

1. The "bulk is always cheaper" assumption. Sometimes smaller packs have a better unit price, especially during specials. A 1kg bag of rice on special at $2.00/kg beats a 5kg bag at $2.40/kg. Check before you default to the big bag.
2. The format switch. One brand shows $/100g, the one next to it shows $/kg. You think you're comparing, but you're comparing different scales. If one says $1.20/100g and another says $9.00/kg, they're almost the same price ($12.00/kg vs $9.00/kg). Without converting, you might pick the wrong one.
3. The multipack illusion. A four-pack of yoghurt for $6 seems cheaper than individual tubs at $1.80 each. Do the maths: $6 divided by 4 is $1.50 per tub. In this case the multipack wins. But it doesn't always. Some multipacks carry a premium for the convenience of the packaging.

How to compare across stores without visiting three shops

Unit prices help you compare products within the same aisle. But what about comparing across stores?

In Discount Trolley, you can search a product or scan the barcode and see prices across Coles, Woolworths, and ALDI where data is available. That gives you cross-store price context without leaving the shop you're already in. You can check whether the 800g tin of tomatoes at Woolworths is actually cheaper than the equivalent at Coles, or if ALDI's version undercuts both.

Price history adds another layer. If a product is marked as a special, you can check whether the price genuinely dropped or if it's been sitting at that level for weeks. Combine that with your unit price knowledge and you've got a pretty clear picture of what's actually good value.

None of this requires spreadsheets or three separate store visits. Just a quick search or scan while you're already doing your shop.

Quick reference: common unit price formats on shelf labels

Here are the units you'll see most often at Australian supermarkets and what they're typically used for:

• $/kg: fresh produce, meat, cheese, rice, cereals, flour, pasta
• $/100g: snacks, deli items, chocolate, smaller packaged goods
• $/L: milk, juice, soft drinks, cleaning products, cooking oils
• $/100mL: sauces, condiments, smaller bottles, personal care
• $/each or $/unit: eggs, bread rolls, nappies, individual items

The one conversion worth memorising

If you remember nothing else from this post, remember this: to convert $/100g to $/kg, multiply by 10. To convert $/kg to $/100g, divide by 10.

So if Brand A shows $1.50/100g and Brand B shows $12.00/kg, Brand A is actually $15.00/kg. Brand B is cheaper by $3 per kilo. That kind of difference adds up over a weekly shop, and it takes about two seconds to check once you know the trick.

For liquids, the same logic applies between $/100mL and $/L. Multiply or divide by 10 and you're comparing like for like.

Questions shoppers still ask