Two packs, two prices, one question

You are looking at two offers for the same laundry detergent: a 750-millilitre bottle for 5.49 euros and a larger 1.25-litre one for 8.99 euros. At first glance the larger bottle seems more expensive and therefore less appealing, but it may not be at all once you convert the price to the same unit. The unit price, meaning the price per litre, per kilo or per individual item, is what turns two differently sized packs into figures you can actually compare. If a shop does not show this figure directly, you can work it out yourself in a few seconds.

The calculation behind it is always the same

The basic idea is simple: the total price divided by the total quantity gives you the price per unit. For liquids, you divide the price by the number of litres; for food, by the number of kilos; for multipacks, by the number of items inside. This one formula works whether you are looking at laundry detergent, coffee capsules or batteries, and it needs nothing more than the two figures that are on every product page anyway: the price and the pack size.

The example from the start, worked through to the end

The small bottle of detergent costs 5.49 euros for 750 millilitres, which divided by 0.75 litres gives a price per litre of 7.32 euros. The larger bottle costs 8.99 euros for 1.25 litres, which divided out gives a price per litre of 7.19 euros. So the larger bottle really is cheaper per litre, just by a small amount you would not have seen without doing the maths. With a bigger price gap between two pack sizes, the result can also go the other way, which is why the quick calculation is worth doing for every comparison instead of relying on the assumption that bigger is always cheaper.

When comparing by quantity is misleading

The unit price only shows what one unit costs, not whether you actually need that quantity. A large pack with a lower price per litre is only worth it if you really use it up before the expiry date or before you next need it. If half of it goes off because the pack was too big, the lower unit price was not a saving in the end, just a number on paper. This applies especially to food and cosmetics with a limited shelf life.

When units cannot be compared directly

Some comparisons need an extra step before the actual calculation starts. Coffee capsules are often sold per pack with a certain number of capsules, while loose coffee is priced by weight in grams. A direct comparison requires you to first convert the number of capsules into a comparable quantity, such as grams of coffee per capsule, before a price per kilo means anything at all. The same goes for multipacks with different numbers of items, where a comparison only makes sense after dividing by the actual number of items rather than by the number of packs. Especially with toiletries and everyday products, where sensible saving without stockpiling is a topic anyway, this quick conversion regularly pays off.

Where the legal unit price helps, and where it does not

For certain goods, EU law requires shops to show not only the total price but also the price per kilo, litre or another standard unit, known as the unit price. Where this information is shown, it completely replaces your own calculation. This is an EU rule; other countries, such as the UK, the US or Canada, have their own rules on unit pricing, and they may be stricter or looser. In practice, the unit price is missing for some product categories, on special offers or in the mobile view of some shops, or it is printed so small that it is easy to miss. In those cases, your own quick calculation remains the most reliable method.

A short routine for your next purchase

• Note down the price and pack size of both offers before you decide on one.
• Bring both quantities to the same unit, for example by converting millilitres into litres, before you divide.
• Divide the total price by the converted quantity and put the two results side by side.

A look at the relevant category also makes it easier to find several pack sizes of the same product side by side in the first place. For a single comparison, these three steps take less than a minute and stop the supposedly big saving on a bulk pack from being nothing more than an optical illusion created by the higher total figure. If you compare this way more often, over time you get a feel for which pack size really offers better value for which product, without writing out the calculation every time. Our article on shrinkflation and unit prices shows how manufacturers sometimes choose pack sizes in a way that makes comparing even harder.

Why some shops round the unit price

Another stumbling block is rounding. Some shops show the unit price rounded to just two decimal places, which, with very small pack sizes or very cheap products, can make the difference between two offers disappear, even though it is actually there when you calculate more precisely. If you calculate yourself using the unrounded figures, you will see small differences that get lost in the shop's rounded display. On a single purchase, this rarely makes much difference, but for regular repeat purchases of the same product, the small difference can certainly add up over time.

Frequently asked questions

Do I really need to work out the unit price myself if there is a legal unit price?

Not if the shop shows it correctly and clearly. In practice, a quick check of your own is still worthwhile, because the information is not required everywhere and is sometimes shown in places that are easy to miss.

How do I compare products with completely different pack units?

First bring both quantities to the same unit, for example by converting capsules into grams or millilitres into litres, and only then work out the price per unit. Comparing the total prices directly without this extra step can easily lead to the wrong result.

Is a bulk pack always worth it if the unit price is lower?

Only if you actually use up the quantity before it goes off. A lower unit price brings no saving if part of the pack ends up being thrown away.

Does the unit price rule also apply to shops outside the EU?

The EU rule on unit prices applies to shops that are subject to EU law. Countries outside the EU, such as the UK, the US, Canada or Switzerland, have their own rules, which is why it is especially worth checking the price and pack size yourself when no comparable figure is shown.