If you only change one habit when shopping, make it this: look at the price per kilo or per litre instead of the price on the pack. It's the only number that makes two products comparable, and the only one that exposes a product whose price went up without the label changing. Almost every decision that genuinely saves money is made with this figure.
Why the pack price is useless for comparing
Because it compares different things. Two jars of passata at £1.45 and £1.60 tell you nothing if one holds 350g and the other 500g. The first works out at £4.14/kg and the second at £3.20/kg: the one that looked dearer is 23% cheaper.
This kind of comparison shows up on almost every shelf, and not always by accident: formats exist partly to make comparing hard work.
Where to find the number
On the shelf label, almost always in smaller type than the big price, written as "£/kg", "€/l" or "per unit". In the EU and the UK, price-marking rules require shops to show both the selling price and the price per unit of measurement for goods sold by weight or volume, precisely so shoppers can compare. Online it's usually under the price or on the product page.
On the receipt, though, it almost never appears: you'll see the amount charged and little else. That's why comparing prices from saved receipts means recalculating it by hand.
Shrinkflation
The manoeuvre of holding the price and cutting the contents. A litre becomes 900ml, a 250g pack becomes 220g, a roll has fewer sheets. The pack price doesn't move, so it doesn't register on the shelf and it doesn't register in your memory either.
In price per unit of measurement, though, it's impossible to hide: if the contents drop 10% at the same price, the price per kilo rises 11.1%. That's the number to watch. Signs it has happened: the "redesigned" pack, the box that's suddenly taller and narrower, or the usual product running out sooner without you changing anything.
Four traps in unit pricing
- The big pack isn't always better value. Everyone's intuition, and it fails more often than you'd think, especially when the small size is on promotion. Check it, don't assume it.
- Watch the reference unit. Some categories are priced per item and others per drained weight. Comparing per kilo on a tinned product using net rather than drained weight artificially flatters whichever one carries more liquid.
- Concentrate versus ready-to-use. A concentrated detergent and a regular one aren't comparable per litre; compare per dose or per wash.
- Cheap per kilo but wasted. If the big pack goes off before you finish it, the real price is what you paid divided by what you used.
Turning it into decisions
- Pick the ten or fifteen products you buy most often. That's where a small difference gets multiplied by frequency and turns into real money.
- Record their price per unit of measurement, not the pack price.
- Compare across the shops you actually use. On branded products it's a direct comparison: it is literally the same item, so the gap is money you're leaving on the table for nothing in return.
- Revisit every few months. What you're looking for isn't today's price, it's the change: if something rises per kilo without rising on the label, you know what happened.
Doing this with a calculator in the aisle is possible but tiring, which is why it doesn't last. Grotally does it from the receipt: it normalises every line to price per unit of measurement and compares the same product across shops and over time, so the decision arrives already made. The full background is in why your shop rises faster than the CPI, and the method for breaking down your spending is in where your grocery money actually goes.