The price on the label is the price of the pack. And packs differ: 400 grams, 500, 750, a kilogram. If you compare the figures on labels, you compare apples with pears. You need a common measure: price per kilogram or per litre. Knowing how to work it out is the most useful habit in the shop, and you do not need a calculator.

The one-line rule

The price per kilogram is the price of the pack divided by its weight in kilograms. A 500-gram pack is half a kilogram, so you multiply the price by two. A 250-gram pack is a quarter, the multiplier is four. Litres work the same way: a 750-millilitre bottle is three quarters of a litre, so you divide the price by three and multiply by four.

Remember a few multipliers and you will count almost instantly:

  • 100 grams: multiply by ten.
  • 200 grams: multiply by five.
  • 250 grams: multiply by four.
  • 400 grams: multiply by two and a half.
  • 500 grams: multiply by two.
  • 1.5 litres: divide by one and a half.

An example with 400 grams and a kilogram

Say a 400-gram pack and a kilogram pack of the same product sit side by side. You do not need to know the prices to get the idea. Four hundred grams is exactly two fifths of a kilogram. So if the small pack costs more than two fifths of the big one, the kilogram in the small pack is dearer. If less, it is cheaper.

That means the big pack is not bound to be the better buy. It is better when its price is lower than the small pack's price multiplied by two and a half. Do not guess: check.

For oil, where bottles come in all sorts of volumes, this works especially well. Here is how the sunflower oil offers look:

Where it is cheaper: sunflower oil

All offers

Bring everything to one weight

The simplest method: bring everything to one weight. Take two labels, recalculate both per kilogram and note them in your phone. It takes half a minute, and then you know which product in which shop is your baseline.

For grains it is very handy, because they are sold in bags of 800, 900 grams or a kilogram. The weight in the bag is not necessarily round. As an example, rice on offer:

In the catalogues now: rice

All offers

When the kilogram says little

There are products where the price per kilogram misleads. First: goods with waste. Chicken with bones and boneless fillet cost differently, but they also give different amounts of meat. We wrote about this in the article on chicken.

Second: goods in liquid. A jar of peas states the net weight and the drained weight. You pay for it all together, but you make dinner from the solid part. Compare the drained weight.

Third: counted goods. Eggs in a ten-pack and in a pack of six are compared by price per piece, not per kilogram. And remember the size grade: eggs can differ in it.

Here are eggs you can compare per piece:

In the catalogues now: eggs

This is not in the current catalogues right now: eggs. As soon as it shows up in a promotion, the cards will appear here.

Search the catalogues for something similar

Price per portion

Sometimes it is better to count not the kilogram but the portion. Dry grain or pasta per portion is about a hundred grams. So a kilogram gives ten portions, and the price per portion is a tenth of the price of the kilogram. Chicken per portion is about two hundred grams on the bone.

That is how you see that a pack that looks cheap can be expensive on the plate, and meat that seems dear can suit a family dinner just fine. More about planning in the article how to save on groceries.

What to do at the shelf and in the app

Many shelf labels already print the price per kilogram or litre in small type. Do not skip it: it is made for you. Compare those figures with each other, not the big ones. If there is no such line, take out your phone and count by the rule above.

Remember units too: grams and millilitres are not the same, but for water, milk and juice you may treat them as close. For oil and honey it is approximate, so compare oil with oil, in litres.

A short reminder

  • Find the weight or volume on the pack.
  • Bring the price to a kilogram or a litre.
  • Compare only such prices: kilograms with kilograms.
  • For tins take the drained weight.
  • For meat count the bones.
  • For eggs count the pieces.
  • For flyers with offers do not forget the dates. How to read them is in the article on flyers.