Unit Price Comparison Tools That Catch Shrinkflation Online

Last spring a box of dishwasher tablets I buy every six weeks kept its price, its colour and its shelf position. It quietly went from 50 tablets to 44. Nothing on the product page flagged it. The only thing that caught the change was the small grey line under the price that said 0.27 per tablet instead of 0.24.

That line is a unit price, and unit price comparison tools are the most underrated shopping tools you can use. They cost nothing, they rarely need an account, and they answer the one question a discount banner never does: how much am I paying for each gram, litre or sheet? This guide covers where to find unit prices, which tools calculate them for you, and the mistakes that make the numbers lie.

Why Unit Price Comparison Beats the Sticker Price

The sticker price tells you what leaves your account today. The unit price tells you what the product actually costs. Those two numbers drift apart in three common situations: when a pack shrinks, when a bigger pack is priced higher per unit than a smaller one, and when a multi-buy offer hides a weaker base price.

A quick example from a coffee aisle. A 250 g bag at 4.20 works out to 1.68 per 100 g. The 1 kg bag on "special" at 17.50 works out to 1.75 per 100 g. The big bag feels like the bulk deal. It is the worse buy by about 4 percent, and you would never see it by comparing the prices on the tiles.

Where Unit Prices Already Hide on Shopping Sites

Most large grocery and household retailers already show a unit price, but they bury it. Before installing anything, learn where your regular shops put it:

Under the main price in a smaller, lighter font, often as "per kg", "per 100 ml" or "per unit".
Inside the product details tab, next to the pack size, on sites that keep the listing tile clean.
In the sort menu, where some shops offer "price per unit, low to high". This is the single most useful setting on a grocery site and very few people touch it.

Marketplaces are the weak spot. Third party sellers fill in pack sizes by hand, so the unit price is either missing or calculated from a wrong quantity. Treat any marketplace unit price as a hint, not a fact.

Unit Price Calculator Tools Worth Using

When a shop does not show a usable unit price, you need a calculator. You have three realistic options, and none of them requires handing over your shopping history.

Tool type	Best for	Weak point
Phone calculator or notes app	Two items side by side	Slow with more than three options
Simple unit price app	Comparing four to eight packs in one go	Many free ones show heavy ads
Spreadsheet with one formula	Tracking your regular items over months	Needs ten minutes to set up

The spreadsheet is the one I recommend for anything you buy more than once a month. Make four columns: item, price, quantity, and price divided by quantity. Add a date column and you have a shrinkflation log. When a row jumps by more than 5 percent with the same sticker price, the pack has changed.

What about browser extensions?

Some extensions promise to add unit prices across every shop. A few do it well, but most read the pack size from the product title, which breaks the moment a seller writes "Family Size" instead of a number. They also tend to request permission to read every site you visit. If you already worry about comparing prices across sites without getting fooled, adding a tool that guesses quantities does not help.

How to Catch Shrinkflation in Four Steps

Shrinkflation is rarely announced. Brands change the pack, keep the price and redesign the label so the new size looks fresh rather than smaller. A short routine catches it:

Pick the ten items you buy most often. Groceries, cleaning products, nappies and pet food cover most households.
Record the unit price, not the sticker price, the first time you buy each one.
Check the unit price again every second or third order. It takes under a minute if the shop sorts by unit price.
When the unit price rises but the sticker price did not, look at the pack size line. That is your shrink.

Do this for three months and you will know which brands shrink quietly and which raise prices honestly. Most people find three or four regular items were never the cheap option they assumed.

The Bulk Buying Advice That Often Backfires

The standard tip is "buy the biggest pack, it is always cheaper per unit." My own logs say otherwise. In a sample of 40 household items I tracked across two supermarkets, the largest pack had the lowest unit price only 26 times. In the other 14 cases a mid sized pack or a multi-buy of smaller packs won.

There are good reasons for this. Retailers know shoppers assume bulk is cheaper, so the big pack does not need a sharp price to sell. Promotions also land on the sizes that move fastest, usually the middle ones. Then there is waste. A 5 litre tub of yoghurt at a low unit price is expensive if you bin a third of it.

So the better rule is: compare unit prices across every size on the day, and only count the amount you will actually use before it expires. A unit price on food you throw away is fiction.

Mistakes That Make Unit Prices Mislead

Unit prices are only as honest as the units behind them. Watch for these traps:

Mixed units on one page. One washing powder shows price per kg, the liquid next to it shows price per litre, and the capsules show price per wash. Convert everything to price per wash or per use.
Drained weight versus net weight. Tinned fruit and vegetables can list either. A tin with a cheap net weight price may contain mostly liquid.
Concentration. A small bottle of concentrated cleaner can beat a large bottle of a diluted one. Check the dilution ratio on the label.
Sheets, not rolls. Toilet paper and kitchen towels sold as "mega rolls" vary wildly in sheet count. Price per 100 sheets is the only fair comparison.

Unit prices also help against inflated reference prices. A "was 9.99, now 6.99" label means little if the unit price is identical to last month. For more on that trick, see how fake was prices make ordinary prices look like bargains.

FAQ

Are shop unit prices always accurate?

On major grocery sites they are usually right because the system calculates them from stock data. On marketplaces and smaller shops errors are common, so recalculate by hand when the number looks too good.

Which unit should I compare household products by?

Use the unit you actually consume. For detergent that is price per wash, for paper it is price per 100 sheets, and for food it is price per 100 g or per kg of edible product.

Do unit price apps need my personal data?

A basic calculator needs nothing but the numbers you type. Skip any unit price app that asks for an account, your location or your contacts before it will do simple division.

The Practical Takeaway

Before your next order, switch your main shop to sort by unit price and write down the per unit cost of your ten most bought items. Check them again in a month. That one list catches shrinkflation, exposes weak bulk deals and makes fake discounts obvious. More tools for this kind of checking are in our shopping tools section.